33
6.2.5.5 Double or combined dormancy 65 

Warm-moist stratification 65 

Combination treatments 65 

Challenging species 65

6.2.5.6 New seed treatments 66 

Chemical soaks 66

Priming 67

6.2.6 Presowing Treatments To Facilitate

Seed Handling 68

6.2.6.1 Coating 68

6.2.6.2 Pelletizing 68

6.2.7 Seed Cleansing and Disinfection 69

6.2.7.1 Aerated water rinse 69

6.2.7.2 Chemical sterilants 69

6.2.7.3 Hot water 71

6.2.7.4 Pesticides 71

6.2.8 Seed Sowing 72

6.2.8.1 Direct seeding 72

Determining sowing rate per container 72

Hand sowing 74

Seeding equipment 74

6.2.8.2 Planting germinants from stratification 76

6.2.8.3 Transplanting emergents from seed trays or 

"pricking out" 76

6.2.8.4 New sowing techniques 78

Miniplug transplants 78 

Fluid drilling 80 

Single- seed sowing 80

6.2.9 Seed Coverings 81

6.2.9.1 Types of coverings 81

Texture 81

Heat buildup 81

Sterility 81

pH 81

Weight 81

6.2.9.2 Proper and uniform depth 81

6.2.10 Summary 83

6.2.11 References 84

6.2.11.1 Literature cited 84

6.2.11.2 General seed propagation references 89

Contents

6.2.1 Introduction 35

6.2.1.1 Basic seed biology 35

6.2.1 .2 Importance of seed source 40

6.2.1 .3 Genetically improved seeds 40

6.2.2 Obtaining High-Quality Seeds 41

6.2.2.1 Collecting and processing seeds 41

6.2.2.2 Considerations when purchasing 

seeds 42

6.2.2.3 Upgrading seed quality 43

Recleaning 43

Sizing 43

6.2.3 Seed Testing 45

6.2.3.1 Sampling 45

6.2.3.2 Measuring physical attributes 47

Moisture content 47

Purity 48

Weight 49

6.2.3.3 Testing seed viability and germination 49

Cut tests 49

Tetrazolium tests 49

X-ray analysis 50

Germination tests 51

New tests 53

6.2.3.4 Interpreting germination tests 53

6.2.3.5 Computing pure live seed 54

6.2.4 Seed Storage 55

6.2.4.1 Classes of seed storability 55

6.2.4.2 Critical storage conditions: moisture 

content and temperature 55

6.2.4.3 Storage containers 55

6.2.4.4 Seed longevity in storage 56

6.2.5 Presowing Treatments To Overcome

Seed Dormancy 58

6.2.5.1 Types of dormancy 58

6.2.5.2 Seeds without dormancy 58

6.2.5.3 Seedcoat dormancy 58

Hot water soaks 58

Fire or smoke 59

Scarification 59

6.2.5.4 Embryo or morphological dormancy 61

Cold-moist stratification 61

Stratification before seed storage 64

34
6.2.1 Introduction

By far, the majority of forest and conservation species are 

propagated from seed. On a number-of-seedlings basis, 95% would 

be a conservative estimate due to the preponderance of commercial 

forest trees grown from seed. On a number-of-species basis, 

however, the percentage would be somewhat lower because many 

native species can only be produced vegetatively.

There are both biological and economic reasons for the popularity of 

seed propagation. First, and most importantly, it preserves the wide 

genetic adaptation which is critical to successful seedling 

establishment and growth in the natural environment. Most natural 

resource outplantings have the objective of preserving the natural 

variation of a plant species, making propagation from seed the 

logical choice. In addition, seed propagation is almost always the 

least expensive propagation method; typical seed costs for forest 

and conservation species ranged from less than 25 cents to 35 

dollars for a thousand seeds in 1995 (table 6.2.1). Seeds also are 

easy to ship and are not restricted by the phytosanitary restrictions 

of vegetative propagation material (Macdonald 1986). Finally, seed 

propagation is advantageous because seeds of most species can 

be stored for several years.

The primary objective of seed propagation is to promote rapid and 

complete germination and establishment in the growth container. To 

achieve this successfully and consistently, growers must have a 

basic understanding of seed biology.

6.2.1.1 Basic seed biology

Novice nursery managers must understand some basic biology 

of how fruits and seeds develop to be certain that seeds that 

they collect or buy are of high quality. First, some definitions 

(Bonner and others 1994):

Fruit = A ripened ovary that develops and surrounds the seed 

after fertilization. Although this term is most commonly associated 

with angiosperms, the word fruit can refer to any seed-bearing 

structure, including cones.

Seed = A ripened ovule consisting of an embryo, its stored 

food supply, and protective coverings.

Plants grown in forest and conservation nurseries have many 

different types of fruits and seeds (figures 6.2.16.2.6), and nursery 

managers must be familiar with characteristics of both. When 

asked to propagate an unfamiliar plant, inexperienced growers 

should consult one of the basic references listed in section 6.2.11.

In particular, Seeds of Woody Plants in the United States 

(Schopmeyer 1974; Bonner in press), the Tree Seed Technology 

Training Course Manuals (Bonner and others 1994), A Guide to the 

Biology and Use of Forest Tree Seeds (Leadem 1996), and

Anatomy and Morphology of Conifer Tree Seed (Kolotelo 1997) 

provide excellent coverage of the basic principles of seed biology.

Nature has designed fruits and seeds to assure the wide 

dissemination and successful establishment of seedlings, but 

many of these ecologically useful adaptations are actually a 

hindrance to easy seed propagation. In fact, for propagation 

purposes, fruits have no use and some

35
36
37
38
39

Seeds of different plant species vary considerably in size, 

appearance, and internal anatomy, but each consists of three basic 

parts: embryo, food storage tissues, and seedcoat. The embryo is 

the miniature new plant that will eventually develop into the 

seedling. The seeds of some species have undeveloped embryos, 

whereas seeds of other species, such as Douglas-fir, have embryos 

that look like miniature seedlings with seed leaves (cotyledons), 

and an undifferentiated stem-root axis consisting of the hypocotyl 

and the radicle (figure 6.2.1 C). Food storage tissues provide a 

source of nutrition for seed germination and seedling establishment. 

The seed coat provides mechanical protection for the embryo and 

prevents desiccation.

Growers must become familiar with the seed anatomy of plants that

they wish to propagate for several reasons:

1 . They must be able to determine if seeds are mature

and viable when they collect fruits or buy cleaned

seeds.

2. The type of seed coat and condition of the embryo

may indicate whether presowing treatment is

needed.

3. The seed structure gives a hint about what type of

care will be required after germination.

For example, a mature Douglas-fir seed has an embryo which is at 

least three-quarters of the length of the seed cavity and is 

surrounded by a firm white megagameto phyte ("endosperm") 

(figure 6.2.1 C). Seeds with a shorter embryo or with a 

megagametophyte that is soft and milky were collected when cones 

were immature. By contrast, a mature seed of Prunus spp. (figure 

6.2.3B) has a tiny embryo with huge cotyledons, no endosperm at 

all, and a thick seed coat that requires mechanical or chemical 

pretreatment before sowing. Aspen seed has no

actually serve the ecologically useful, but horticulturally frustrating, 

function of delaying seed germination (table 6.2.2). For example, 

chamisso sedge apparently has a chemical germination inhibitor in 

the papery hulls surrounding the seed, and germination of some 

seedlots has increased 70% by simply removing the hull (Trindle 

1995).

Nursery managers collecting their own seeds will have to separate 

the viable seeds from their fruits and clean them. Before buying from 

a seed dealer or accepting seeds from a potential customer, 

managers should make a careful inspection to make certain that 

they have been properly identified and cleaned. For conifers this 

inspection is relatively easy but becomes more critical when dealing 

with seeds of other forest and conservation plants that are less 

commonly cultivated. It would be very frustrating to discover right 

before sowing that a new customer has collected the wrong part of 

the flower or that the fruits still need to be processed. For example, 

a nursery that was asked to grow some alder plants from seeds 

supplied by the customer discovered that male catkins had been 

collected instead of female cones!

Typical fruits of forest and conservation plants are of three general 

classes, and each requires different collection and processing 

procedures (Krugman and others 1974):

1. Multiple-seeded dry fruits ("cones") that release their

seeds at maturity, for example, Douglas-fir (figure

6.2.1 A).

2. Single- seeded dry fruits that separate from the parent

plant at maturity, for example, Carya spp. (figure

6.2.2A).

3. Fleshy fruits that separate from the parent plant at

maturity with their seeds enclosed, for example,

Prunus spp. (figure 6.2.3A).
substantial food storage organs and a thin seedcoat (figure 

6.2.513), which means the resultant germinant will be relatively 

weak and require careful culture. A good discussion about the 

anatomy and morphology of commercial conifer seeds can be 

found in Kolotelo (1997), which also includes excellent color 

illustrations.

The genetics of a typical conifer seed contains a mixture of three 

distinct genomes. The embryo contains a mix ture of male and 

female genes, but the megagametophyte and the seed coat are 

both strictly female. This has operational significance because the 

seed coat can be the cause of seed dormancy if it inhibits the 

penetration of water and oxygen and thereby delays germination. 

The female-derived megagametophyte also has a strong influence 

on seed germination and seedling establishment (EIKassaby and 

others 1993a).

6.2.1.2 Importance of seed source

The concepts of seed source or provenance are of paramount 

importance when propagating forest and conservation plant species 

from seeds, and refer to the geographical area (seed zone) in which 

the seed was collected. Seed zones for some species are quite 

large, whereas those for other species can be very small. In 

mountainous areas, the seed source must also be identified by elevational zones that can be as narrow as 152 m (500 ft) (see figure 

1.1 AA in volume one of this series).

Seed source affects seedling performance in 2 ways: growth rate 

and cold tolerance. In general, seedlings grown from seeds 

collected from higher latitudes or elevations will grow slower but 

tend to be more cold hardy during the winter than those grown from 

seeds from lower elevations or more southern latitudes. For example, white ash seedlings grown from seed collected from northern 

Michigan became cold hardy earlier in the year and hardened to an 

average of -38 °C (-36 °F), compared to seed collected from 

southern Mississippi which hardened much slower and only 

reached a maximum hardiness of -27 °C (-16 °F) (Alexander and 

others 1984). So, unless research has shown otherwise, seed 

should always be collected from the same seed zone in which 

the seedlings are to be outplanted.

Research on genetic adaptability may prove that one seed source 

is superior for a particular geographic region. For example, the 

Niobrara River, NE, source of ponderosa pine was tested at 

locations all over the Great Plains and was found to exhibit 

significantly better sur40

vival, growth, and resistance to pine shoot moth (Read 1983). 

Likewise, genetic research has shown that loblolly pine seeds from 

the Livingston Parish region of Louisiana grow faster and have 

more disease resistance when planted all over the South (Wells and 

Wakeley 1966). More intensive research can identify specific "plus" 

or "elite" trees that have proven genetic superiority and eventually 

develop seed orchards from repeated selection and field testing. 

For most forest and conservation species, however, the degree and 

range of adaptation is unknown, so seedlings should always be 

outplanted back into their seed zone of origin. Rudolf (1974) has 

brought together the available information on recommended seed 

collection zones for a number of important northern and western 

coniferous species. Government forestry agencies also have 

published seed transfer guidelines (see section 1.1.1 .1 for more 

discussion on the importance of seed zones in nursery culture).

6.2.1.3 Genetically improved seeds

Customers may specify that their seedlings be grown from seeds 

specially selected for fast growth rate, disease resistance or other 

genetically controlled factors. This special seed can be considerably 

more expensive so single-seed sowing may be requested. The 

basics of producing genetically improved seed and managing seed 

orchards is explained in detail in Rudolf and others (1974).
6.2.2 Obtaining High-Quality Seeds

The key to producing seedlings of consistently high quality is 

prompt and uniform seed germination and vigorous early seedling 

growth, and therefore the importance of insisting on quality seeds 

cannot be overemphasized. Even the best nursery practices 

cannot overcome the cultural problems that result from poor 

seed quality. In fact, the development of container nurseries has 

highlighted the importance of seed quality. In standard bareroot 

seedbeds, seed germination is hidden from view so the high 

number of seeds that are lost to pathogenic organisms or weather 

problems is not evident. By contrast, seeds sown in sterile growing 

media in containers and germinated under ideal conditions can be 

easily observed, thus making poor seed quality much more 

apparent (figure 6.2.7). Nursery managers who sow cheap seeds of 

unknown quality will pay for their false economy. Seed costs are 

only about 1 % of total seedling production. Thus, the cost of 

purchasing high-quality seeds can easily be justified when 

compared to the costs of maintaining a seedlot with many empty 

cells and weak seedlings for the remainder of the growing season. 

(Sample production costs are given in the Nursery Management 

chapter in volume one of this series.)

The term seedlot should be discussed at this point. A seedlot is 

usually defined as a quantity of seeds from the same species that 

was collected from a specific seed zone or source and that is of 

reasonably uniform quality. The term seedlot carries through into 

nursery culture, where it signifies the batch of seedlings grown 

from that particular batch of seeds.

6.2.2.1 Collecting and processing seeds

Some forest and conservation nursery managers prefer to collect 

and process their own seeds because then they have complete 

control of seed source and seed quality. It can also be more 

economical to collect and process seeds if collection coincides with 

times when the nursery crew needs additional work. For some 

conservation species with wide areas of adaptation, seed 

production areas (figure 6.2.8A) can be established right in the 

nursery and, in some cases, windbreaks can be planted with 

species of the desired seed source. Other nurseries have access to 

seed orchards (figure 6.2.813). For most species, however, seeds 

must be collected near the outplanting site.

41
Seed collection and processing requires technical knowledge and 

specialized equipment, not to mention a considerable time 

commitment. Many forest and conservation species do not produce 

seeds every year, and crops of species growing in the same plant 

community do not necessarily produce crops in the same year 

(figure 6.2.9). Even if they do, crops can be widely scattered and 

may not occur in the desired seed zone. Seed quality also can vary 

considerably between years. Although some Douglas-fir seeds are 

produced every year, abundant cone crops occur from 2 to 10 years 

apart. Other trees, such as maples and ash, produce some seeds 

almost every year. Seeds of certain species always are hard to find. 

For example, western larch seeds are always at a premium 

because even when good cone crops occur, problems with 

pollination and insect predation make seed collection almost 

impossible (Danielson and Riley 1995). In good seed years, 

collecting and processing can take several weeks or months to 

complete, and then seeds must be stored properly until they can be 

sown.

Seeds of some species store so poorly that nurseries must collect 

fresh seeds for each crop. For example, blue oak acorns must be 

harvested fresh-directly from the branches- because those on the 

ground already have desiccated, which greatly reduces seed 

quality. This species has an unusually wide 2-month collection window, however. As long as acorns were collected properly and not 

allowed to dry out, germination percentage was consistently over 

90% throughout the period (McCreary

and Koukoura 1990). All of which re-emphasizes the 

importance of knowing the basic biology of a species before 

attempting to propagate it.

State and federal government nurseries usually do their own seed 

collection, processing, and storage and so are excellent sources 

of information for the novice nursery manager. (The subject of 

seed collection and processing is too complicated to cover in more 

detail here and so the reader is referred to the literature sources in 

section 6.2.11.)

6.2.2.2 Considerations when purchasing seeds

Many nursery managers grow seedlings from seeds have 

purchased from commercial dealers, or seeds often are supplied by 

the customer. As mentioned earlier, it is imperative to insist on the 

proper seed source when purchasing seeds and it is a good idea 

to ask seed dealers which seed sources of a particular species 

they have in stock rather than specify which source you want to 

purchase. Some unscrupulous dealers always seem to be able to 

come up with whatever seed source is needed if they want to make 

a sale badly enough. If you are a novice grower, then it might be a 

good idea to call other local nurseries to find out the names of 

reputable seed dealers. The USDA Forest Service's National Tree 

Seed Laboratory has published a handy reference called 

"Commercial suppliers of tree and shrub seed in the United States" 

(USDA Forest Service 1995). It is available in either hard copy or 

can be accessed on the Seedlings, Nurseries, and Tree 

Improvement home page on the World Wide Web:

» willow.ncfes.umn.edu/snti/snti.htm «

Next, always make sure that seeds have been tested for quality. In 

particular, a germination test, percentage purity, and seed weight 

will be needed to properly calculate seed sowing rate. If test data 

are unavailable on the desired source, then lower the price 

accordingly and adjust the projected sowing date until tests can be 

done. Tetrazolium tests are often done on forest and conservation 

species but are less accurate than standard germination tests. 

While it is possible to make an educated guess about seed viability 

from cut tests and oversow enough to compensate for the estimated 

variation, this is never recommended. Production space in container 

nurseries is just too valuable to gamble on seeds of unknown 

quality. There are no bargains-high quality seeds always are worth 

the price and th e effort to locate them. (Seed testing is discussed in 

detail in section 6.2.3.)

42
43

dure needs to be checked carefully with a cut test, however, as 

some filled seeds will float if water bubbles are trapped on the 

seed coat and empty seeds may sink if they are dirty.

A promising new technique for upgrading pine and spruce seedlots 

is the IDS (Incubation-drying-separation) method, which separates 

filled nonviable seeds from filled, viable ones (Simak 1984). Seeds 

are soaked in water at 15 °C (59 °F) to obtain full imbibition, then 

dried at 25 °C (77 °F) to create differences in seed moisture 

content. During drying, viable seeds will retain more moisture than 

nonviable ones, so this difference in weight can be used to separate 

the two fractions by flotation in water (figure 6.2.10). The IDS 

method is still being evaluated and developed, but has promise as a 

simple technique to upgrade the quality of seedlots (Bergsten 

1993). Operational work has shown that a lodgepole pine seedlot 

could be upgraded from 85% to 95% germination capacity by this 

technique. Sometimes, the poorer the seedlot, the greater the gain 

through IDS upgrading: a Douglas-fir seedlot that had been 

damaged during processing was raised from a germination capacity 

of 17% to 96% (Edwards 1993).

Sizing. Some species and seedlots show considerable variation in 

seed size and so there could be several advantages to sizing 

seeds, either with screens or by gravity, before sowing. The 

potential nursery benefits of seed sizing are (1) easier sowing with 

mechanical sowing equipment, (2) faster and more complete 

germination, and (3) more uniform seedling density, which 

translates into higher seed-use efficiency (Belcher and others 

1984). A review of the literature found that seed size was 

significantly correlated with germination rate 50% of the

The amount of seeds needed to grow a given number of seedlings 

will depend on the information from the seed tests (see section 

6.2.8.1 for sowing calculations).

6.2.2.3 Upgrading seed quality

Nursery managers who purchase seeds should expect clean, 

pure seeds of high quality, but those who collect and process 

their own seeds may need to upgrade certain seedlots. Although 

it cannot directly improve the quality of individual seeds, 

upgrading improves the potential performance of a seedlot by 

removing empty, damaged, weak, and immature seeds.

Cleaning and upgrading seeds is a highly specialized art that 

cannot be discussed in adequate detail here. The most 

comprehensive information on woody plant seed collection and 

processing can be found in Seeds of Woody Plants in the United 

States (Schopmeyer 1974; Bonner, in press) and Methods and 

Procedures for Testing Trees Seeds in Canada (Edwards 1987). 

For a discussion of basic concepts and available equipment for 

processing conifer seeds, the reader is referred to the Bareroot 

Nursery Equipment Catalog (Lowman and oth ers 1992) or the Tree 

Seed Technology Training Course: Instructor's Manual (Bonner and 

others 1994). More information on the collection and processing of 

broadleaf trees and shrubs is included in the Hardwood Nursery 

Guide (Williams and Hanks 1994) and Collecting, Processing, and 

Germinating Seeds of Western Wildland Plants (USDA Agricultural 

Research Service 1981) covers other native plants.

If nurseries are given seeds that need upgrading before they can 

be sown, the following techniques are useful.

Recleaning. Nurseries may have to reclean some seedlots before 

they can be sown, especially if precision seeders will be used. 

Although most nurseries typically do not have the full range of 

seed-cleaning equipment, there are some simple procedures to 

upgrade seedlots. The basic seed-cleaning machine is the 

air-screen cleaner, which uses a combination of screening and air 

flow to remove debris. A smaller "office version" is ideal for small 

upgrading jobs. Air-screen cleaners are relatively inexpensive and 

can be used to upgrade seedlots by 3 physical properties: size, 

shape, and density (Bonner and others 1994). Another simple and 

quick way to remove emp ty seeds for some species is by 

flotation-the principle is that heavier, filled seeds will sink in water 

and the lighter, empty or damaged seeds will float. This proce-
44

time and with seedling size 84% of the time (Bonner 1987). Some 

southern container nurseries routinely size their pine seeds and 

have found that medium to medium-large seeds produce the best 

seedlings (Barnett and McL.emore 1984). Trials with other tree 

species have not shown sizing to be operationally useful, however. 

Seed size was found to have no significant effect on seedling 

attributes of Douglas-fir (El-Kassaby and others 1992) or

Sitka spruce (Chaisurisri and others 1994). Dumroese and Wenny 

(1987) found that, although larger western white pine seeds 

germinated faster and more completely, their final seedling heights 

and stem calipers were not significantly different than seedlings 

grown from bulk seed (table 6.2.3). Therefore, for the majority of 

forest and conservation species, seed sizing as a separate process 

is probably not worthwhile.
6.2.3 Seed Testing

High seed quality is essential to consistently produce crops of 

superior seedlings, and the only way to determine seed quality is by 

testing. Although large container nurseries may perform some seed 

tests in-house, most nurseries use the services of commercial seed 

labs. Often, seed companies will have their seeds tested before 

offering them for sale, but in other cases, nursery managers may 

have to have the seeds tested themselves. To ensure that results 

are comparable, seed testing labs around the world follow the 

standard International Seed Testing Association procedures (ISTA 

1985). These rules must be followed whenever seeds are 

exchanged or sold, because sale of tree seeds is regulated by law 

in some states and many foreign countries (Hartmann and others 

1997). Nursery managers should note, however, that these highly 

regulated seed testing procedures may not apply to operational 

nursery conditions and should be prepared to modify them when 

appropriate. For example, the standard germination test is done 

under ideal conditions in a growth chamber. This may give a good 

indication of seed performance under fully controlled growing 

environments but may be poorly correlated with performance in an 

open growing compound.

Seed testing takes time, usually from 4 to 6 weeks. Therefore, 

managers should ask customers if their seeds have already been 

tested or must allow time to have the tests done before they can 

prepare them for sowing. Even if the nursery purchases all its 

seeds, managers should have a basic knowledge of the various 

steps of seed testing and understand the technical terminology so 

that they can communicate easily with seed suppliers and seed 

laboratory personnel. Make certain that labs are members of the 

Association of Official Seed Analysts (AOSA) to ensure that their 

testing procedures meet established guidelines. The National Tree 

Seed Laboratory can perform all types of seed tests or recommend 

another reliable laboratory:

USDA Forest Service National Tree 

Seed Laboratory 5156 Riggins Mill 

Road Dry Branch, GA 31020-9696 

TEL: 912/751-3552 FAX: 

912/751-3554 E-MAIL:

» seedlab@ix.netcom.com « 

website:

» http-//willow.ncfes.umn.edu/seed-lab/ntsl-ol.htm «

The seed testing process can be divided into three 

sequential operations:

1. The seedlot must be sampled in a statistically valid manner so 

that tests reflect the true nature of the entire lot. 2. Tests of 

physical properties (moisture content, thousand seed weight, 

and purity) should be done before seeds are stored, and also 

are necessary to compute sowing rates. 3. Seeds should be 

tested for viability to give an estimate of how quickly and 

completely they will germinate, and how well they will grow after 

they are sown.

The actual testing can be divided into physical seed tests and 

viability tests. Physical seed tests are normally done only once, 

when the seedlot is first collected and processed, because the 

results do not normally change over time. Viability tests also should 

be conducted after seed is collected but, because seed quality will 

change during storage, viability of stored seedlots should be tested 

at intervals of every 2 or 3 years to assure that seed quality has not 

decreased significantly. Seeds of some species can be stored 

successfully for many years, whereas other must be sown 

immediately. (Seed storage is discussed in detail in section 6.2.4 of 

this volume.) Any seed test is only as good as the sampling 

technique that was used to select the test sample, and therefore 

the seedlot must be properly sampled.

6.2.3.1 Sampling

Collecting seed samples is extremely important for two reasons. 

First, the sample must be representative of the entire seedlot and 

second, seed testing laboratories require a certain amount of seeds 

to perform each type of test. The best seed-quality test can only 

apply to the seed sample submitted for analysis and therefore, if a 

sample does not truly reflect the characteristics of the entire 

seedlot, the test results are useless. A seedlot is defined as the 

entire quantity of seed of one species and ecotype from a particular 

location at a single collection date. Even within the same seed 

zone, seed quality changes between different locations and seed 

maturity changes over time, even from week to week. (See section 

6.2.2 for further discussion of these terms.)

The definition of a seedlot will also depend on your objectives as 

well as the type of test that you are doing. During initial seed 

processing, the seedlot will consist of all the containers from that 

collection and so each con45
46

shapes and those with wings, which includes many native shrubs, 

are usually sampled by hand. Insert your hand with open palm and 

fingers tight together and then scoop an equal amount of seeds 

from several locations in the container (Stein and others 1986). 

Large-seeded species, such as oaks and walnuts, are particularly 

diffi cult to sample so it may be necessary to pour them out of their 

storage container to obtain a representative sample. If the seedlot is 

stored in several containers, samples should be drawn from each 

container in at least five different positions in both the horizontal 

and vertical planes (Gordon 1992).

Seed analysts use standard terminology for the stages in the 

sampling and testing process and nursery managers should be 

familiar with these terms (Bonner and others 1994). "Primary 

samples" are collected from the entire seedlot and then mixed to 

form a "composite sample," from which a "submitted sample" will be 

sent to the testing laboratory (figure 6.2.12). The amount of seeds 

required for the full range of tests varies from 2,500 to

tainer must be sampled. After seeds have been stored, however, 

the sampling population may consist of only a portion of the total 

seedlot, depending on the objectives of the test. Most physical 

attribute tests, such as purity and weight, will not change over time 

and therefore the sampling population will always be the entire 

seedlot. However, as mentioned in the previous section, seed viability will change over time, so the sampling population must change 

with the situation. In the case where you have a large quantity of a 

particular seedlot in storage, you will only want a germination test on 

the amount of seed that you will be sowing for your current crop. For 

example, if you have 25 kg (55 lb) of ponderosa pine seed in 

storage but will be sowing only 5 kg (11 lb) this year, then you will 

treat this amount as a seedlot and test it separately. The remaining 

20 kg (44 lb) of seed in storage will now constitute a separate lot

and will have to be tested for viability again the next time it is used. 

Moisture content also changes when a batch of seed is removed 

from refrigerated storage for any amount of time. The purity and 

seed weight will remain the same for both seedlots, however.

Once the seedlot has been defined, it must be sampled 

systematically with appropriate techniques and tools. The proper 

sampling tools and procedures depend on the type of seeds and 

size of the seedlot. Free-flowing seeds, such as pines and 

chokecherry, are generally sampled with a hollow, partitioned probe 

called a "trier" (figure 6.2.11). The closed trier is inserted into the 

seed container on a diagonal plane and then opened to admit seeds 

simultaneously from different levels. Seeds with irregular
5,000 seeds except for large seeded species for which 500 is 

sufficient. On a weight basis, the submitted sample will range 

from 25 to 240 g (0.9 to 8.5 oz) for most conifers but can range as 

high as 1 kg (2.2 Ibs) for large seeded species (Cordon 1992). 

The amount of seed to submit also will depend on seed size and 

density and some typical submitted samples for a variety of forest 

and conservation species are provided in table 6.2.4. 

Requirements may vary between testing laboratories, however, 

and therefore it is always a good idea to contact them before 

sampling just to be certain.

All seed samples should be packaged in rigid containers to protect 

them during shipping, and sent to the testing lab by the quickest 

means possible. If moisture content will be taken, then seeds 

should be placed in a plastic bag to retard moisture loss. Species 

with high moisture contents may heat-up or mold during shipment, 

however, so samples should be kept out of direct sunlight and 

shipped promptly (Cordon 1992). Each seed sample must be 

accompanied by the following information:

• Full name, address, phone, and FAX number of sender

• Date of sampling and of original collection

• Species, seed source and any other specific identification information

• Quantity of seed submitted and of total seedlot

• Type of storage and storage temperature

• Time available for testing

if time is short before sowing, then the lab may recommend a quick 

estimate of viability instead of the standard germination tests, which 

will take at least 4 to 6 weeks. With species such as western white 

pine that have long stratification requirements, the tests can take 4 

to 6 months.

Once it reaches the testing laboratory, the "submitted sample" is 

further divided into a "working sample" that is used for the various 

tests of seed physical characteristics and quality (figure 6.2.12).

6.2.3.2 Measuring physical attributes

The first measurements taken when the laboratory receives 

the seeds are moisture content, purity, and weight. Because 

seed moisture content can change rapidly, it is measured 

immediately.

Moisture content. Seed moisture content provide several types of 

valuable information to the nursery manager. It can indicate seed 

maturity, determine if the seedlot must be treated before storage, or 

indicate the type of presowing treatment to assure rapid and 

complete germination (Bonner and others 1994). In particular, 

nurseries must know the moisture content of seeds that they are 

going to store because seed quality can be rapidly lost if moisture 

content is outside the narrow range of 5 to 8% (table 6.2.5).

47
A microwave oven can be used to dry larger seeds if the procedure 

is done properly (table 6.2.6). If not, it can kill the seeds. Cover the 

seed sample during the entire procedure to make sure that seeds 

do not gain or lose moisture to the atmosphere. The moisture 

content (%) is calculated from the following formula (Edwards and 

Wang 1995):

moisture content = M2 M3 x 100

M2 – M1

where: M1 = weight of the empty conta iner and cover

M2 = weight of container, cover, and seeds before 

drying

M3 = weight of container, cover, and seeds after drying

Note that seed moisture content is expressed as a percentage of 

water loss compared to the original fresh weight of seeds, rather 

than of the ovendry weight, which is the standard when the 

moisture content of soils is calculated (Stein and others 1986).

The next phase of the seed testing procedure involves seed 

that is sent to the lab for a "complete analysis," where one 

submitted sample is tested for purity, thousand-seed weight, 

and germination (figure 6.2.12).

Purity. The purity test determines what proportion by weight of the 

sample is pure seed of the designated species and how much is 

impurities. The test is done by weighing enough seeds to make a 

working sample of approximately 2,500 seeds of most species, or 

500 seeds of large-seeded species (Gordon 1992). The seeds are 

visually inspected and pure seeds are separated from 3 classes of 

impurities: seeds of other species, weed seeds, and inert matter. 

Some seeds of forest and conservation species retain all or part of 

a wing or integument after processing, and these require special 

attention. For example, the wings of Douglas-fir and ponderosa 

pine seeds should be removed during normal processing, whereas 

those of maples or mahogany should not. Nursery managers 

should realize that the pure seed component can still contain 

damaged seeds as long as they are larger than half the size of a 

complete seed (Bonner and others 1994). Thus, pure seed does 

not necessarily mean that a seedlot is also of high quality.

Pure seeds and each category of impurity is weighed and 

expressed as a percentage by weight of the total working sample. 

Good seed processing should always produce

48

The standard laboratory test for seed moisture content involves 

drying the seed sample in an oven and then calculating the weight 

of water lost. However, nurseries often want to obtain a rapid 

estimate of seed moisture content and so, for these operational 

tests, electronic moisture meters (figure 6.2.13) are used for small 

seeds of most commercial tree species. Several brands of seed 

grain meters can estimate tree seed moisture content within 1 % if 

they are calibrated against an ovendry standard. A list of suppliers 

of moisture meters can be found in Lowman and others (1992) and 

the National Tree Seed Laboratory can provide moisture charts for 

the major brands (see address in section 6.2.3).
95% purity for most commercial conifer seeds and higher purity 

(>99%) should be mandatory for seeds grown in container 

nurseries, especially those using precision seeders. Purity is much 

more difficult for many native plant seeds because of irregular seed 

shapes, sizes and appendages.

Weight. Seed weight is traditionally determined by weighing 1,000 

seeds of the seedlot. A working sample for the thousand-seed 

weight analysis is taken from the same working sample as the 

purity test (figure 6.2.12). Most laboratories have seed-counting 

machines that can automatically count out and weigh 1,000 seeds. 

The analysis can also be done by hand: several replications of 100 

seeds each are counted, weighed, and an average used to 

calculate the thousand- seed weight.

Seed weight is a function of seed size, moisture content, and 

proportion of full seed in a given lot, and so also gives an indication 

of seed quality. The thousand-seed weight is required for 

calculating sowing rates in bareroot nurseries but is less important 

in container facilities because sowing is done on a numerical 

basis-one or more seeds per container. The thousand-seed weight 

can easily be converted to seeds per kilogram (or seeds per 

pound) which traditionally is used to describe seedlots (table 

6.2.4).

6.2.3.3 Testing seed viability and germination

Although the two terms are often used interchangeably, nursery 

managers must understand the difference between viability tests, 

which estimate the potential to germinate and grow, and 

germination tests, which measure their actual achievement. 

Viability can be estimated by several means, including the standard 

cut test, the tetrazolium test, and the excised embryo procedure. All 

except the latter are routinely used for nursery testing purposes. 

However, germination tests are the standard measure of seed 

quality in forest and conservation nurseries.

All the following tests assume that seed dormancy requirements 

have been met, but this often is not the case in actual practice. So, 

nursery managers must be familiar with the dormancy 

characteristics of the species that they wish to propagate, and 

either require that the seed dealer perform the necessary 

pretreatment, or allow time to do it themselves. (Presowing seed 

treatments are discussed in detail in section 6.2.5 in this volume.)

Cut tests. This traditional test should not be overlooked because it 

is a simple and quick way to estimate the viability of a seedlot. 

Using a sharp knife or single-edged razor, a sample of seeds is cut 

in half and the contents inspected under a hand lens. Cut tests 

reveal seeds that are empty, damaged, or insect-infested and also 

reveal abnormal morphology. Immature seed structures often 

identify seeds that have been collected too early. Of course, the 

normal structure of seeds of the crop species must be known for 

comparison (figures 6.2.1 to 6.2.6). Like all viability tests, however, 

a cut test will not show whether the seed lot will germinate because 

dormant seeds have normal morphology. The process of using cut 

tests to assess the quality of commercial conifer seeds is well 

described in Anatomy and Morphology of Conifer Tree Seed

(Kolotelo 1997). This publication also includes color photographs of 

common seed problems and a key to classify seeds during cut 

tests.

Cut tests also can be used during and after presowing seed 

treatments to determine their effectiveness. For example, cut tests 

can be used to time cold-moist stratification. When the cotyledons 

of conifer seeds enlarge and turn light green, the seeds are near 

germination and should be dried down and stored under 

refrigeration until time to sow (Banerjee 1994).

Tetrazolium tests. The tetrazolium (TZ) test is the most popular 

seed viability test and can take only hours to complete. The 

chemical principle behind the test is that living seed tissue will 

stain bright red when treated with

49
50

properly. Seed analysts consider several factors when 

evaluating a TZ test (Vankus 1997):

1. Amount of area that is stained

2. Intensity of staining

3. Pattern of staining

4. Turgidity of tissues

5. Presence and condition of essential seed

components

Experience has shown that, with most seeds, tetrazolium results 

will often overestimate the actual germination percentage. There 

are also species differences, however. Oak acorns contain 

chemicals that inhibit the tetrazolium reaction, so these tests tend 

to underestimate the germination percentage (Gordon 1992). Other 

studies have reported excellent correlation between TZ and 

standard germination tests and that for some species, including 

maples and bitterbrush, the TZ test is a better reflection of actual 

nursery performance (Hardin 1981).

From a grower's standpoint, tetrazolium tests are commonly used 

when they are asked to sow a seedlot before a standard 

germination test can be completed. The TZ test is especially useful 

for determining the viability of very dormant seeds that would 

otherwise require long pretreatments before germination tests could 

be conducted (Stein and others 1986). TZ tests also are used to 

determine the viability of ungerminated seeds at the end of 

germination tests (Vankus 1997).

Whether to do TZ tests at the nursery or send them to a testing 

laboratory depends on several factors. Closely examining seed 

structures and learning about seeds of different species would be 

most valuable to any nursery worker. Vankus (1997) provides a 

complete listing of the equipment and supplies needed to do TZ 

tests and also provides addresses for obtaining training in the TZ 

procedure. However, the exact procedure for conducting TZ tests 

varies by family of plant (AOSA 1995) and interpreting the results 

requires experience. So, unless a large number of samples need to 

be tested, it usually is more cost-effective to work with testing 

laboratories.

X-ray analysis. Many seed testing laboratories and some nurseries 

now use X-ray radiography routinely, and interestingly enough, this 

is one of the few technologies that originated with tree seeds 

(Bonner and others 1994). The procedure consists of placing a 

sample of pure seeds on a tray and exposing them in the X-ray 

machine. The X-rays pass through the seeds, strike a photographic 

film,

the normally colorless solution of triphenyl tertrazolium chloride 

(figure 6.2.14). The TZ solution reacts with dehydrogenase 

enzymes that are present in all living tissue and form a reddish, 

insoluble compound called formazan.

The testing procedure is easy to do-a sample of seeds is cut in half 

lengthwise, TZ solution is applied, and the seeds examined with a 

hand lens. Because the staining reaction is affected by pH, 

temperature, time, and concentration of the TZ solution, standard 

laboratory conditions must be maintained during the test. Deeply 

dormant seeds require longer staining times, more concentrated TZ 

solutions, and special preconditioning treatments (AOSA 1995).

Although the TZ staining procedure is relatively easy to perform, the 

stained seeds require experience to interpret
and produce a black-and white photograph called a radiograph 

(figure 6.2.15). Many aspects of seed quality can be determined 

from analyzing these radiographs, including seed structure, embryo 

maturity, empty seeds, insect infestation, and seed coat injuries that 

are invisible to the naked eye (Stein and others 1986). Although it 

cannot measure viability, the X-ray procedure is a valuable adjunct 

to germination tests.

Germination tests. In most forest and conservation nurseries, the 

germination test is the best performance test of a seedlot and 

therefore is used as the operational test of seed viability. In a 

standard germination test, four replicates of 100 seeds each are 

placed on a moisture-retentive substrate under optimal 

environmental conditions of light and temperature. Seed testing 

laboratories conduct the tests under sterile conditions in special germination chambers (figure 6.2.16A/B), and count the number of 

seeds that have germinated on a weekly basis for 4 weeks (table 

6.2.7). Because of differences in seed dormancy, not all forest and 

conservation seeds can be tested in the same manner, and some 

require pretreatment. A list of recommended procedures for tree 

and shrub seeds can be found in the international Rules for Seed 

Testing (ISTA 1985). For example, maple seed embryos must be 

excised and honeylocust seeds must be scarified before testing. For 

species with complex dormancy requirements, such as rose, the 

tetrazolium test is prescribed instead of a germination test. (See 

section 6.2.5 for a full discussion of seed treatments.)

The evaluation procedure for germination tests is also 

standardized. To be counted, seeds must germinate normally and 

exceed some standard growth stage such as "having a radicle four 

times as long as the seed coat." The average number of seeds 

from the four replications that germinate normally is reported as the 

germination percentage or germination capacity. Seeds that 

failed to germinate are usually given a cut test to make sure that 

they are filled and healthy. These cut test data are report51
52

Peak germination = the specific time at which germination is 

highest.

Germination energy = the proportion of germination that has 

occurred up to the time of peak germination.

Other terms such as germination value and peak value (Czabator 

1962) are mathematical expressions that are sometimes reported 

in research reports but are not normally used in operational

nursery practice. A complete discussion of terms used by seed 

testing laboratories and good photographs of normal and abnormal 

germinants for many tree seeds can be found in Edwards and 

Wang (1995) and Kolotelo (1997).

Most nurseries grow a variety of species and seedlots, so that 

running a complete battery of germination tests each year can be 

relatively expensive (figure 6.2.12). Inexperienced nursery 

managers, or growers who have waited until the last minute, are 

often tempted to skip germination tests and oversow instead, but 

this is not a good idea. There is just too much variation in the performance of seeds of forest and conservation species and the waste of 

seed, the high labor costs of thinning, and the growth loss of 

overcrowded seedlings will far exceed any perceived cost savings 

(figure 6.2.17). We recommend that growers always insist on 

current seed germi nation tests when first contacted to grow a crop 

and require them in contract specifications. Allowing enough time in 

the crop schedule for seed pretreatments and testing is the mark of 

an experienced nursery manager.

ed in addition to the germination percentage (table 6.2.7) and they 

give the nursery manager an indication of how well the seedlot was 

processed and the degree of seed dormancy (Gordon 1992).

Speed of germination is another important result of the germination 

test, and so many testing laboratories report germination results by 

week. By plotting the germination percentage over time, nursery 

managers can observe the rate of germination, a good indication of 

vigor. Species or seedlots that show significant variation in 

germination rate may need different presowing treatments. For 

example, the standard 3-week cold-moist stratification treatment 

was shown to be inadequate for a mix ture of Douglas-fir seedlots 

(Edwards and El-Kassaby 1995).

Many germination indices have been developed and some have 

practical application in container nurseries (Bonner and others 

1994):
New tests. Probably the most promising is a nondestructive te st 

termed leachate conductivity. Seeds are soaked in water for 24 

hours at room temperature and then the electrical conductivity of 

the leachate is measured and related to seed quality (Barnett 

1985a; Bonner and Vozzo 1983). The increase in conductivity of the 

leachate results from leakage of cellular substances as the cell 

walls deteriorate or are damaged. Commercial equipment is now 

available to speed and standardize the testing, and preliminary 

evaluations show considerable potential with southern pine seeds 

(Bonner and Vozzo 1986). Advantages are speed, ease of 

operation, and objective evaluation, but the equipment is costly and 

there still are unknown factors that affect measurements. Further 

research is needed to clarify these problems.

6.2.3.4 Interpreting germination tests

Standard laboratory germination tests must be properly interpreted 

before they can be applied in the nursery. Many forest and 

conservation species have dormant seeds, which must be treated 

prior to testing, and so the germination test may not reveal the true 

potential of the seedlot. The degree of dormancy can vary 

considerably within a species; for example, in pines, the length of 

the cold stratification treatment between varieties, ecotypes, or 

even seedlots (Krugman and Jenkinson 1974). Some laboratories 

conduct paired germination tests of both normal and cold stratified 

(prechilled) seeds (table 6.2.7). For seedlots that have unusually 

low germination percentages, nursery managers should request 

tetrazolium tests or X-ray analysis to try and determine the cause of 

the poor performance. (See section 6.2.5 for a complete discussion 

of dormancy.)

Remember that a germination test is an average of potential 

seed viability and that actual performance in the nursery may 

vary considerably. This has been well illustrated in research trials 

but also is evident to nursery managers in operational sowings, 

sometimes tragically (figure 6.2.17). Although presowing treatments 

can reduce it, some inherent variation in germination will always be

present in forest and conservation seedlots (figure 6.2.18) and is 

actually desirable from a genetic standpoint (Campbell and 

Sorensen 1984). Nursery managers must realize that maintaining 

or even increasing biodiversity is a primary objective of many forest 

and conservation outplanting projects, and therefore they must 

expect some variation in germination performance. This is in 

marked contrast to most horticultural seeds

53

which are hybrids that have been bred to germinate quickly 

and uniformly (Hartmann and others 1997).

Germination conditions in the nursery can be considerably different 

from laboratory tests (table 6.2.8). This is especially true in 

minimally controlled propagation environments such as open 

growing compounds. In these situations, the rate of germination 

should be given more attention. Frequently, even if seed testing 

shows that total germination is excellent, a relatively slow 

germination rate indicates that there may be a problem with nursery
54

Nursery managers should always insist on seed of the highest 

quality. Although seed dealers may offer seedlots with poor PLS 

values at bargain prices, this is a false economy that will be 

reflected in operational problems in the nursery. If the low-quality 

seeds belong to the nursery, then they should be sown immediately 

or destroyed because these seedlots deteriorate over time and 

have poor storage potential even under the best conditions. 

Conversely, high-quality seedlots will perform well over a wide 

range of environmental conditions and many can be stored for 

future seedling production (Edwards and Wang 1995).

performance. For many dormant species, a 30-day cold-moist 

treatment is recommended, but even longer periods of treatment 

may increase the speed and uniformity of germination under less 

than ideal conditions (Dunlap and Barnett 1982). Nursery 

managers should also utilize the experience of seed analysts at 

the testing laboratory to answer questions about test results 

because such efforts will be rewarded with marked improvements 

in seedling establishment and growth.

6.2.3.5 Computing pure live seed

A seed quality index called pure live seed (PLS) is often 

calculated after seed testing, especially when seed is to be sold. 

The PLS is a commonly accepted basis for comparing and pricing 

seedlots that differ in purity and viability (Stein and others 1986):

germination (viability) X % purity

% pure live seed = 100
6.2.4 Seed Storage

Most container nurseries only need to store seeds for short periods 

of time, such as from the time of collection or purchase until 

sowing. Sometimes, however, more seeds are collected that can 

be immediately used or it may be more economical to buy larger 

quantities of a popular seedlot and store it from one year to the 

next. Successful storage requires knowledge of the seed characteristics of different species, high initial seed quality, and proper 

seed moisture content. Storage temperature, container and method 

also are important. The subject of seed storage is discussed in 

detail in Gordon (1992) and Bonner (1990).

6.2.4.1 Classes of seed storability

Longevity of seeds is a characteristic of the plant species, and 

there are four classes of seeds based on their genetics and 

composition (Bonner and others 1994):

1. True orthodox---seeds that tolerate desiccation and

therefore store easily for relatively long time periods.

2. Suborthodox---seeds that require the same storage

conditions as true orthodox seeds but have limitations due to seed composition and genetic origin.

3. Temperate-recalcitrant---seeds that are intolerant of

desiccation so must be stored above freezing, and

also must not be stored in airtight containers.

4. Tropical---recalcitrant-seeds that have the same

moisture and gas exchange requirements as temper

ate-recalcitrant species but in addition are very sensitive to low temperatures.

Other seed characteristics also affect storability. Thick or hard 

seedcoats restrict moisture loss, and seeds containing oils tend to 

be harder to store than those containing more starch. Seeds 

collected before they are mature or those that were under 

environmental stress during matu ration store poorly.

Seed handling before storage will also affect storage potential. 

Exposing seeds to direct sunlight or high temperatures, especially at 

high seed moisture contents, is damaging. The care with which 

fruits and seeds are handled and stored during collection, shipping, 

and processing is important. Careless processing can cause cracks 

in the seedcoat or even bruise sensitive seed tissues. A cracked 

seedcoat allows moisture to escape and provides an entry for fungal 

pathogens.

6.2.4.2 Critical storage conditions: moisture content

and temperature

Of all the factors that influence seed storage, moisture content is by 

far the most important. The recommended seed moisture contents 

for storing true orthodox and suborthodox species is 6 to 10% (table 

6.2.9). For recalcitrant species, however, the moisture content must 

be maintained in the 30 to 45% range for storage periods of even a 

few years. Seed moisture contents also can affect the amount of 

undesirable secondary dormancy that develops during storage. 

Studies have indicated that moisture contents of 10 to 18% 

(dry-weight basis) during a 1-year storage period increased the 

degree of dormancy compared to that of seeds with moisture 

contents below 10% or above 18% (McLemore and Barnett 1968).

Temperature is the other important consideration when storing 

seeds. In general, the lower the temperature, the slower the seed 

deterioration rate, although the recommended range is considerably 

different between orthodox/suborthodox and recalcitrant species. 

Most seeds can be stored at temperatures at or slightly below freezing for short time periods except for those in the tropicalrecalcitrant 

category. These seeds are very sensitive to chilling injury which 

occurs at relatively cool temperatures of 12 to 20 °C (54 to 68 °F) 

(Bonner and others 1994).

Refrigerated storage is recommended for seeds of all but 

tropical-recalcitrant species whenever it can be economically 

justified and a reliable power source is available. Prefabricated 

modular refrigeration units are commercially available and make 

ideal seed storage structures. They can be free-standing or installed 

within existing buildings. Humidity control is not needed for orthodox 

and suborthodox seeds but is desirable for recalcitrant species. 

Because frost-free refrigeration units constantly remove moisture 

from the storage environment, all types of seeds must be packaged 

properly to maintain the proper moisture content. (See section on 

refrigerated storage structures in section 1.3.5.4 in volume one of 

this series.)

6.2.4.3 Storage containers

Seed storage containers must be rigid enough to provide physical 

protection during storage and handling. Containers that retard 

moisture loss should be used for storage of true orthodox and 

suborthodox seeds to pre55
vent desiccation. Plastic bottles with screw tops (figure 6.2.19), and 

fiberboard drums or cardboard boxes lined with plastic bags have 

been used. Although rectangular containers are more space 

efficient, round ones assure necessary air spaces. Cardboard or 

fiberboard containers should be lined with plastic bags. Bags of 

0.102 to 0.152 mm (4 to 6 mils) thickness are sufficient except in 

humid environments where thicker ones are used to maintain the 

desired seed moisture content. The recalcitrant species must have 

constant gas exchange, however, and so should be stored in 

containers lined with an unsealed 0.102-mm (4-mil) plastic bag 

(table 6.2.9).

6.2.4.4 Seed longevity in storage

Under natural conditions, most conifer and hardwood seeds have a 

life span of less than 3 years. When conditions are carefully 

regulated in refrigerated storage, however, viability of orthodox and 

suborthodox species can be maintained at high levels for much 

longer (table 6.2.10). Operationally, commercial conifer seeds are 

stored from 10 to 20 years with little appreciable loss in viability. The 

potential longevity of pine seeds under optimum moisture and 

temperature conditions is unknown, but it could approach 100 years 

(Bonner 1989).

Recalcitrant species can be stored for a much shorter period, 

ranging from a few months to a couple of years (table 6.2.10). The 

actual length of storage can vary with56
57

in a genus, however, and oaks are an interesting example. When 

some white oak acorns are collected, the radicles are already 

emerging whereas some red oak acorns can be kept for up to 5 

years when handled and

stored properly. Some cultural treatments can extend the storage 

of acorns, such as soaking them in water for 48 hours at 2 °C (36 

°F) before storing them in loosely tied plastic bags (Gosling 1988).
6.2.5 Presowing Treatments To Overcome

Seed Dormancy

Unlike seeds of horti cultural crops, which have been bred to 

germinate immediately after sowing, those of many forest and 

conservation species become dormant after they mature. Seed 

dormancy refers to a physiological state in which otherwise viable 

seeds will not germi nate even when placed in growth-conducive 

environments (Bonner and others 1994). Dormancy is an ecological 

adaptation that ensures that seeds will only germinate when 

weather conditions, especially moisture and temperature, are 

favorable to the survival of the seedling. The type of dormancy is 

genetically controlled and so is usually the same for a given species 

or even genus. As with most things in nature, however, there are 

always exceptions and oaks are a good example. Most species in 

the red oak group have immature embryos that benefit from 

stratification, whereas most species in the white oak group do not. 

Within each group, though, there are oak species that are 

exceptions to the rule (Schopmeyer 1974). The degree of 

dormancy varies between ecotypes of a species, seedlots collected 

in different years, or even between individual seeds from a given 

plant. This variation is an adaptation that ensures that not all seeds 

will germinate at the same time but that some will germinate each 

year over an extended period.

6.2.5.1 Types of dormancy

Seed dormancy can be caused by several different factors, and 

there is no universal agreement on the best terminology for the 

types of dormancy. For it to be relevant to nursery managers, a 

dormancy classification system should be both logical and 

operationally useful (table 6.2.11). The major types of dormancy 

can be overcome with the following seed treatments. In the case of 

secondary dormancy, the best solution is preventing the condition 

in the first place by proper seed handling and storage.

6.2.5.2 Seeds without dormancy

Not all forest and conservation seeds are dormant; however, even 

nondormant seeds still need some treatment before they can be 

sown. Seeds that have just finished being processed or that have 

been in storage have lower moisture contents than is ideal for 

germination (table 6.2.5). Imbibition is the physiological process by 

which seeds absorb the water necessary to start the metabolic 

reactions that lead to germination. In nursery practice, this is

achieved by soaking seeds in water for 24 to 48 hours. A running 

water rinse is recommended to keep the dissolved oxygen content 

of the water high and avoid

58

stagnated conditions. Some nurseries even use a bubbler in the 

soaking tank. In addition to achieving imbibition, this treatment also 

softens the seedcoat and cleans it, removing possible chemical 

inhibitors or pathogens. (Seed cleansing and disinfection are 

discussed in section 6.2.7.)

6.2.5.3 Seedcoat dormancy

This condition is often called external dormancy because the 

restricting factor is the tissue surrounding the embryo (table 6.2.11). 

The degree of seedcoat hardness varies between species but also 

depends on the ecotype and weather conditions during the seed 

ripening process (Macdonald 1986). Several treatments can be 

used to soften the seedcoat but the best choice will depend on the 

cost and availability of materials and labor. Working with 

honeylocust and black locust seeds, the best scarification treatment 

was found to depend on the size and economic resources of the 

nursery (Singh and others 1991). The objective is to increase the 

permeability of the seedcoat to water and gases. Overly severe 

treatments may injure the embryo, so the gentlest method should 

be tried first, and then more severe treatments can follow until the 

seedcoat is permeable (Bonner and others 1994). Keeping 

accurate and detailed notes of the treatment method and 

timing allows nurseries to develop a seed treatment guide for 

each species or ecotype.

Hot water soaks. This is the traditional treatment for many legume 

seeds or those with waxy seedcoats. A volume of water that is 

approximately 4 to 6 times the volume of dry seeds should be 

brought to a boil. Then, the seeds are immersed for a few minutes 

and the container is removed from the heat and allowed to cool. 

The embryo of some seeds can be damaged by high temperatures 

and so for these species, the water should be heated to only 65 to 

70 °C (149 to 158 °F). Seeds can be removed and dried when they 

swell and become gelatinous to the touch. With some species, such 

as boxwood, imbibed seeds sink to the bottom of the container and 

floaters are removed and retreated. Although some growers use a 

standard treatment time for the hot water soak, it is better to 

experiment with each species and seedlot because of the variation 

in seedcoat thickness. Treated seeds are subject to bacteria and 

fungus infection and so should be sown within a few days. One 

problem with hot-water-treated seeds is that they stick together, 

making them difficult to use in mechanical seeders. One remedy for 

this is to place treated seeds in moist peat moss for a few days 

(Macdonald 1986).
Soaking seeds in aerated water at 16 °C (60 °F) is a simple 

treatment for overcoming the mild seed dormancy of southern 

pines. It is particularly useful when there is not enough time for the 

traditional cold-moist stratification treatment (Barnett 1971).

Fire or Smoke. Dry-heat treatments are seldom recommended for 

overcoming seed dormancy because of the danger of embryo 

damage. A fire treatment, however, has been used on the seeds of 

some woody shrubs (for example, manzanita) from fire-dependent 

plant communities like the chapparral of southern California (Emery 

1988). Obviously, this treatment should be done outside in a safe 

area. Eucalyptus seeds are sown in flats and covered with 6 mm 

(0.25 in.) of soil, topped with a layer of straw. The straw is then 

ignited and allowed to completely burn, and the dry heat breaks 

down the seed coat (Macdonald 1986). This fire treatment is not an 

exact procedure, however, because the amount and duration of the 

heat that reaches the seed cannot be accurately controlled.

The use of smoke to stimulate seed germination of fire-adapted 

native plants was discovered less than 10 years ago in Australia. 

Since then, researchers have demonstrated that smoke from the combustion of plant materials could 

positively affect the germination of 170 different species 

representing 37 plant families and 88 genera (Roche and others 

1997). Both direct exposure to smoke and imbibition in dilute 

aqueous smoke solutions were effective but exposure times varied 

considerably between species. Some plants responded only after 

traditional seed treatments such as mechanical or acid scarification 

and others needed to have their seed stored in soil prior to 

treatment. Although the practical application of this technology still 

needs to be developed for North American species, the use of 

smoke as a pre-sowing seed treatment holds considerable promise 

for plants of fire-adapted plant communities such as the chaparral 

of Southern California.

Scarification. The process of scarification involves weakening 

the hard seedcoat just enough to allow imbibition, and several 

techniques are effective.

Mechanical abrasion. Seedcoats of small quantities of relatively 

large seeds can be treated by hand: nicked with a triangular file 

or sharp knife (figure 6.2.20), rubbed against coarse sandpaper, 

or burned with an electric soldering iron or wood-burning tool. 

This last

59
"hot wire" technique is particularly handy because it makes a small 

hole in the seedcoat allowing the treated seeds to be shipped or 

even stored without lost viability (Bonner and others 1994). Workers 

should always wear protective gloves and can hold small seeds 

with tweezers. Mechanical scarifiers can be homemade or commercially purchased. To treat large seedlots, a rotating drum lined with 

sandpaper or a cement mixer filled with grav el has been used. 

Bunchberry seeds can be scarified by placing them in a rock 

tumbler with 120-grit sand for 5 to 7 days (Date 1994). Macerating 

berries in a modified household blender can be an effective 

mechanical scarification technique. Dull the blades with a file and 

blend the fruit in 2 to 5 volumes of water. This treatment lightly nicks 

the seed coat, improving imbibition and germination (Trindle 1995). 

Whatever technique is used, it is important to regularly check the 

seedcoats to make sure that the treatment has not gone too far.

Acid soaks. Another scarification method is to soak the seeds in a 

strong acid solution. Concentrated sulfuric acid (specific gravity of 

1.84) is preferred but growers must be aware that this is an 

extremely caustic material and that worker safety always must be a 

foremost consideration (table 6.2.12). When properly done, acid 

scarification is a very effective way to remove hard seedcoats and 

stimulate quick germination (figure 6.2.21).

Seeds should be clean and dry. They should not be used directly 

out of cold storage because they may be covered with condensed 

moisture. After placing seeds in the treatment container, the acid is 

slowly poured over them, and they are left to soak for a period of 15 

to 120 minutes. Because the treatment time will vary considerably 

between species and seedlots, it is a good idea to con60

duct some small-scale trials first by removing a few seeds at regular 

time intervals and cutting them to assess the thickness of the 

seedcoat. Another testing procedure is to divide the seed sample 

into several smaller batches, and then treat each for an increasing 

amount of time. The scarified seeds are soaked in water for several 

days and examined. The test batch with the highest proportion of
undamaged swollen seeds indicates the optimum treatment time for 

that seedlot and species (Schopmeyer 1974). With Arctostaphylos 

nevadensis seeds, Trindle (1996) found that seed coat thickness 

was not a reliable guide for determining the optimum acid 

scarification timing and recommended checking the appearance of 

the endosperm itself. A glassy, water-soaked appearance indicates 

that the treatment has gone too far.

Macdonald (1986) presents a good operational procedure for 

acid scarification:

1 . Place one volume of seed in a clean, dry container,

and carefully add twice that volume of acid. Some

growers put the treatment vessel in another container

of cold water to reduce the resultant heat buildup.

2. Stir the seeds at regular intervals to prevent them from

sticking to one another and ameliorate the heat

buildup in any one location.

3. While the seeds are soaking, fill another container of

equal volume with a 5% solution of sodium bicarbonate (baking soda) to use as an acid neutralizer bath

4. When the treatment time is completed, carefully

pour off the acid, move the seeds to the neutralizing

bath and stir.

5. Remove the seeds from the neutralizer and rinse

them thoroughly in clear, cold water.

Although acid scarified seeds can be stored for a few days, it is 

best if they are sown immediately. Scarification of some species 

can be satisfied by either hot water or acid treatment (table 6.2.13). 

The best scarification treatment will depend on the requirements of 

the species and the skill and experience of the grower.

6.2.5.4 Embryo or morphological dormancy

These "internal" types of dormancy can have two different causes 

(table 6.2.11), but in both, the cultural treatment must overcome a 

physiological or morphological condition within the seed itself. As 

was the case with seedcoat dormancy, the degree of dormancy 

can very considerably from species to species and between ecotypes, and so again, the need to try different treatments and keep 

accurate and detailed records cannot be overemphasized.

Cold-moist stratification. For commercial forest tree species, 

stratifying seed under cold and moist conditions is the most 

common treatment to overcome seed dormancy. Cold-moist 

stratification originated from the historical practice of placing layers 

of seeds between alternating layers ("strata") of moist peat or 

sand. The current procedure of placing imbibed seed in plastic 

bags under refrigeration can be more accurately described as 

moist

61
prechilling or naked stratification. Although prechilling is the term 

preferred by seed scientists, stratification will be used in this book 

because it is simple, definitive, and deeply ingrained in operational 

nursery vernacular.

Cold-moist stratification satisfies several important physiological 

functions, including activating enzyme systems and converting 

starches to sugars for quick metabolism. Although the exact 

mechanism is unknown, stratification also changes the balance 

between chemical inhibitors and promoters and so acts as a 

"switch" to chemically stimulate germination. Even in species that

do not exhibit true dormancy, cold-moist stratification produces 

faster, and more complete germination (Moreno 1985). Cold-moist 

stratification also benefits biodiversity because it allows more 

individual seeds to germinate at the same time. For example, a 

mixture of unstratified Douglas-fir seeds showed highly significant 

variation in date of germination compared to those that had undergone cold-moist stratification (figure 6.2.22A) Seeds of some pines 

will germinate without stratification when freshly collected but 

dormancy will develop after they have been stored (Krugman and 

Jenkinson 1974). Another not widely appreciated cultural benefit of 

cold-moist stratification is that early germination is slowed, creating 

an even flush of germination when the sown containers are placed 

in a warm propagation environment (Bonner and others 1994).

The traditional practice of mixing seeds with a moist medium is still 

used for some forest and conservation species. Some nurseries mix 

seeds with damp Sphagnum moss in a plastic bag and place it in a 

refrigerator (figure 6.2.22B/C). The condition of the seeds is 

checked weekly, and they are sown after the prescribed 

stratification period or planted as germinants (see section 6.2.8). 

Naked stratification involves soaking seeds in water to obtain full 

imbibition, draining off excess water, and placing the seeds in 

polyethylene bags in refrigerated storage where the temperature is 

held slightly above freezing. Running water rinses are preferred to 

standing soaks because the bubbling water keeps dissolved oxygen 

levels high and also cleanses the seedcoat of pathogenic 

organisms. In a study with Douglas-fir (Axelrood and others 1995), 

the percentage of seeds contaminated with the Fusarium fungus 

increased during stratification after a standing water soak; seedlots 

treated with running water imbibition showed significantly decreased 

levels of post-stratification contamination.

62

Successful stratification requires that four conditions be met:

Moisture and aeration-Operationally, these 2 factors must be 

considered together because they can be inversely related in the 

stratification environment. Effective stratification requires that seeds 

be fully imbibed and not allowed to dry out for the entire treatment 

period. The moisture contents at which full imbibition occurs varies 

among coniferous species although soaking seed in running water 

at room temperatures for 24 to 48 hours is usually adequate 

(Barnett 1981). If seeds are not fully imbibed, stratification will be 

less effective and will be reflected in slow or irregular germination. 

Seeds of some species are damaged by soaking for too long, 

however. For example, the viability of longleaf pine seeds is 

reduced by soaking for more than 8 hours (Barnett and Pesacreta 

1993). After imbibition, seeds are drained and placed in 

polyethylene bags. The volume of seeds per stratification bag 

should be kept relatively small to ensure good aeration throughout, 

and the bags should be made of plastic no thicker than 0.102 mm (4

mil). This thickness of plastic allows some oxygen and carbon 

dioxide exchange- remember that seeds are alive and 

"breathing"! Some nurseries insert a small hollow tube or straw in 

the top of the bag to increase aeration (figure 6.2.22D). Placing the 

bags on wire mesh racks ensures air exchange under the bag, and 

some nurseries hang the stratification bags from hooks. It is also a 

good practice to have someone move and massage the bags 

weekly to move seeds in the interior to the outside and ensure that 

no anaerobic conditions exist. It is a good idea to check for mold 

development at this time also.

Temperature---The best temperature for cold-moist strati fication is 

dependent on the species and ecotype but most trees and shrubs 

from colder climates need temperatures slightly above freezing. 

The optimum temperature range for most temperate zone species 

is 1 to 5 °C (34 to 41 °F). Growers should make certain that their 

refrigeration units are functioning properly and that temperaturemonitoring equipment is accurate because freezing desiccates the 

seeds and stops the stratification process (Macdonald 1986).

Duration of treatment---The prescribed length of the cold-moist 

stratification treatment can vary from 4 to 20 weeks, depending on 

species, variety, and ecotype (table 6.2.13). Longer stratification 

periods erase the inherent differences within a seedlot and so 

improve the speed
63
The stratification-redry technique can even be applied to unused 

stratified seeds that are left over after sowing. However, when 

already-stratified seeds are dried to about 10% moisture content 

(ovendry basis) and stored, many of the benefits of stratification 

are lost (Barnett 1972, Danielson and Tanaka 1978). Recent tests 

have shown that such seeds can be retreated in what is call 

"double prechilling." Boyer and others (1985) found that a 30-day 

treatment, followed by drying, storage, and an additional 30-day 

treatment a year later, increased the speed of germination over 

that of conventional cold-moist stratification.

and uniformity of germination, resulting in a more uniform crop of 

seedlings (Barnett 1986). This is especially important when 

germination conditions are not optimum (Tanaka and others 1986). 

However, less-dormant species may be adversely affected by long 

stratification periods (Boyer and others 1985). Therefore, the length 

of cold-moist stratification should be based on paired germination 

tests or operational nursery trials whenever possible.

Stratification before seed storage. In recent years, new ideas 

have been emerging about cold-moist stratification, primarily in 

terms of the moisture content of the seeds and the timing of the 

treatment. Most of the early work on this "stratification-redry"

technique was done on western conifers (Edwards 1986) and 

southern pines (Barnett 1985c), and it has recently been used for 

hardwoods. Danielson and Tanaka (1978) found that after a brief 

period of air-drying, cold-moist stratified ponderosa pine and 

Douglas-fir seeds could be stored for several months at 2 °C (34 °F) 

without loss of either viability or the benefits of stratification. 

McLemore and Barnett (1968) found no significant decrease in 

germinability using loblolly pine seeds that were treated and then 

stored for 1 year at different moisture contents and temperatures. 

Seeds stored at just above freezing temperatures and moisture 

contents greater than 25% but below full imbibition continued to 

increase in germinability. Edwards (1981) reported the same 

response with seeds of true firs.

The key concept of the stratification-redry technique is that seeds 

are stratified under cold and moist conditions as usual, then redried 

to a lower moisture content and returned to cold storage (figure 

6.2.23). The benefits are that premature germination during 

stratification is largely prevented, and germination rates are 

improved to the point that total germination can even be higher than 

with traditional presowing treatments (table 6.2.14).

Recent research on hardwood seeds in Europe showed that if 

proper seed moisture content was determined and controlled during 

stratification, full imbibition and subsequent drying was unnecessary 

(Muller and Bonnet-Masimbert 1989). Seeds could be stratified at a 

moisture content that overcame dormancy yet prevented germination, and then be cold stored for up to 5 years without loss of 

viability. Of primary interest to propagators, the seeds could be 

removed from storage and planted at any time without further 

treatment. Seed-use efficiencies are also impressive as the 

stratification-redry technique helped produce 2,500 seedlings/kg of 

beechnuts-an increase of 2.5 times over previous yields (Muller 

1993).

64
65

Challenging species. Lest the novice grower get the impression 

that overcoming dormancy is a routine cultural procedure, some 

species still cause problems including western white pine and 

Rocky Mountain juniper (figure 6.2.26A). Although much research 

has been devoted to achieving prompt and even germination

6.2.5.5 Double or combined dormancy

The seeds of some forest and conservation species are 

particularly difficult to propagate because the dormancy level 

varies in the embryo or is due to a combination of factors (table 

6.2.11).

Warm-moist stratification. There are two cultural objectives of this 

dormancy treatment: to soften a hard seedcoat, and to encourage 

the growth of an underdeveloped embryo. The former is hastened 

by increased microbial decomposition of the seed coat and the 

warmer temperatures simulate the over-summer period that occurs 

in nature (Macdonald 1986). With some species, like white oaks, the 

epicotyl goes through dormancy while the radicle does not, and so a 

warm-moist stratification treatment allows complete germination 

when the seeds are sown. Seeds of some other species germinate 

better when they are given a warm-moist period immediately before 

cold-moist stratification (table 6.2.13).

The requirements and procedures for the warm-moist stratification 

treatment are basically the same as for cold-moist stratification 

except that the temperatures are increased to 18 to 29 °C (65 to 85 

°F). Seeds must be fully imbibed to benefit from warm-moist 

stratification and must be packaged and moved regularly to encourage good aeration. Warm-moist stratification usually takes between 

4 to 12 weeks although this varies considerably between species. 

Some nurseries imbibe seeds and place them in plastic bags, just 

like they do before cold-moist stratification, and then hang them 

inside a warm greenhouse or place them on a heated floor or bench 

in a vegetative propagation structure (figure 6.2.24). If a larger 

volume of seeds must be processed, some nurseries have 

constructed an insulated room for warm .stratification with heat 

lamps to maintain temperatures at 24 to 27 °C (75 to 80 °F) 

(Macdonald 1986).

Combination treatments. Some seeds have a combined 

dormancy that consists of a hard seedcoat plus an embryo 

dormancy (table 6.2.13), and developing seed treatments for these 

species is especially challenging. For example, it is possible to 

overcome the hard seedcoat of eastern redbud with hot water or 

acid soaks, but the seeds will still not germinate due to their internal 

dormancy. A combination of either acid or mechanical scarification 

followed by a relatively short cold-moist stratification period was 

found to be effective (figure 6.2.25).
in the nursery for these problem species, more work still needs to be 

done. Part of the answer may be that the degree of dormancy varies 

from year to year or that some as-yet unknown chemical 

germination inhibitor is present.

In some cases, the answer might be to try and mimic nature. For 

example, Rocky Mountain juniper seed normally requires a 

warm-moist stratification followed by a period of cold temperatures 

(table 6.2.13), and a Colorado nursery has developed a 

"back-to-nature" technique to satisfy these requirements. During 

mid-summer, a mesh bag filled with juniper seeds is buried in soil at 

a depth of 10 cm (4 in.), irrigated, and allowed to naturally stratify 

until late winter. The bag is dug up the following February, when 

seeds are showing signs of germinating (figure 6.2.26B/C). They 

are then sown by hand in containers and grown under normal 

culture (Moench 1995). Seed germination of several tree species 

including maple, hawthorn, and basswood was improved by pretreatment with a compost activator that chemically softens 

seedcoats, and thereby mimics the conditions of natural overwinter 

stratification (Cullum and Gordon 1994).

Nature has devised some intricate tricks to ensure the survival of 

the species by making certain seeds will germinate over a period of 

time. Overcoming complex seed dormancy is one of those factors 

that makes nursery work so interesting because it is as much an art 

as a science.

6.2.5.6 New seed treatments

Chemical soaks. Seed dormancy of some forest and conservation 

species can be overcome with chemical treatments, including 

thiourea, ethylene, hydrogen peroxide, sodium hypochlorite, and 

citric acid (Bonner and others 1994). Stein (1965) reported that a 

48-hour soak in 1 %, hydrogen peroxide hastened germination of 

Douglas-fir in a field test. However, the same treatment had little 

positive effect of sugar pine and ponderosa pine germination. A 

combination of citric acid soaks followed by stratification increased 

nursery germination of baldcypress (Jones 1962). Treating seeds 

with hormones such as gibberellic acid (GA) has shown promise 

with some species. A GA soak for 12 to 24 hours is reported to 

partially replace cold-moist stratification period for northern 

bayberry, and a 100 ppm GA treatment followed by a 7-day cold 

stratification has shown promising results for sweetgum (Macdonald 

1986). Treatment with plant hormones is a logical approach and we 

can only hope that more research will be forthcoming.

66
67

found that an 11-day pretreatment of Scots pine at -800 kPa (-8 

bars) with a PEG solution improved germination. However, Haridi 

(1985) and Fleming and Lister (1984) in similar experiments, found 

that results varied by species and seed sources. Consequently, 

priming has not become an accepted presowing treatment for 

forest tree species. Seeds provide a convenient delivery system for 

biocontrol agents, such as the beneficial fungi Trichoderma spp. 

and Gliocladium spp. or even mycorrhizal fungi, and so these 

treatments could be a promising new application of seed priming 

(Taylor and Harman 1990).

Priming. Seed priming, or osmoconditioning, describes a presowing 

hydration process in which seeds are incubated at optimum 

temperatures but prevented from germinating until it is time for 

sowing. Primed seeds are commonly used for growing agricultural 

crops, and treated seeds can even be stored for several weeks 

(Kren 1994). Liquid priming uses osmotic solutions, particularly 

polyethylene glycol (PEG), whereas solid matrix priming involves a 

slurry of calcined clay and water. The main benefits of seed priming 

is that seedling germination and emergence is faster and more 

uniform. Simak (1976)
6.2.6 Presowing Treatments To Facilitate

Seed Handling

Many seeds of forest and conservation species are small or 

irregularly shaped, making them difficult to handle during sowing. 

Seeds of some horticultural species are routinely treated with 

coatings or are pelletized to make seed sowing easier and more 

precise. "Coating" denotes the application of a substance to a 

seed that does not appreciably change its size or shape, whereas 

"pelletizing" or "pelleting" refers to the addition of inert fillers to 

increase the uniformity of seed size and weight (Taylor and 

Harman 1990).

6.2.6.1 Coating

Seeds can be coated with a thin layer of colored powder, making 

them easier to handle by increasing the uniformity of the seed coat 

surface. This is particularly important during mechanical sowing to 

assure the proper number of seeds per container cavity. Coatings 

can eliminate the static electricity on the seed coat of small seeds 

that can cause them to clump together and cling to other surfaces. 

Some nurseries use ordinary talc to coat seeds with a white color, 

but other organic dyes are also available. Yellow is popular because 

it is easy to see on the darker-colored growing medium (Kren 1994). 

Growers use coatings of different colors to allow quick identification 

of seeds of difference ecotypes or similar-sized species. Many other 

seed coatings including bioprotectants, fertilizers, and materials that 

help supply oxygen or retard imbibition of water have been used in 

horticulture (Taylor and Harman 1990).

6.2.6.2 Pelletizing

This involves enclosing small seeds within a thicker coating to make 

them larger and more uniform in size and weight. Seeds are 

tumbled in a drum with an inert filler and an adhesive until the 

desired increase in volume is obtained. After treatment, the 

pelletized seeds are dried and can be stored (Taylor and Harman 

1990). Pelletizing makes small or irregularly shaped seeds easier to 

handle, especially during mechanical sowing. Larger and heavier 

pelletized seeds can be picked up better by automatic seeders and 

placed more precisely in the center of the container cavity (Kren 

1994). Many of the commercial forest species, such as pines, have 

large uniform seeds and so pelletizing is not common in most forest 

and conservation nurseries. Seeds of Eucalyptus spp. are very 

small, however, and cannot be separated from the chaff during 

processing. A Swedish forestry firm has been able to clean the 

seeds of several eucalypts and pelletize single seeds into small, 

spherical pellets about 3 mm (0.11

68

in.) in diameter (figure 6.2.27A). In Italy, pelletized seeds of 

eucalypts have been direct-sown into containers for over 10 years, 

thereby overcoming the root deformation associated with 

transplanting (Piotto 1994). Seeds of western redcedar are small 

with wings on both sides and so are routinely pelletized prior to 

sowing in British Columbia container nurseries (figure 6.2.27B). 

Pelletized seed must not be allowed to dry out, and therefore proper irrigation is essential to ensure that the coating breaks down and 

germination occurs normally (Kolotelo 1996). The pelletizing 

process may hold promise for other forest and conservation species 

that are grown in sufficient number to make the process 

economical. As precision sowing becomes more common, 

pelletizing will become a more accepted practice.
6.2.7 Seed Cleansing and Disinfection

Seeds of commercial conifers are normally contaminated with fungi 

and bacteria, but most of the microorganisms are considered 

saprophytic, with no adverse affects on seed germination or 

seedling development (Belcher and Waldrip 1972). With the advent 

of container culture, however, it became apparent that seedborne 

pathogens can be an important cause of nursery disease. A number 

of different pathogenic fungi have been isolated from conifer 

seedcoats (figure 6.2.28A), and pathogens can also be carried 

internally (Littke 1990). For example, 8 to 20% of the seeds from 5 

longleaf pine seedlots were found to be infested with 5 different 

species of the Fusarium fungus, and all were shown to be 

pathogenic (Pawuk 1978). All 12 Douglas-fir seedlots assayed for 

Fusarium in a recent British Columbia study were found to be 

infected, although the level of infection and the virulence of the 

Fusarium species varied considerably (Axelrood and others 1995). 

This variability makes it difficult to predict which species or seedlots 

of seeds will be affected.

The most common symptom of seedborne disease is slow and 

variable seed germination and seedling emergence. Unfortunately, 

these symptoms are usually attributed to poor-quality seed. Even if 

seeds germinate and seedlings emerge normally, seedborne fungi 

can cause post-emergence damping-off or cotyledon blight (figure 

6.2.28B). Sometimes, the symptoms of a seedborne disease 

problem do not become evident until root rot shows up later in the 

season. (A complete discussion of seedborne disease identification 

and control can be found in volume five of this series.)

Historically, nursery managers treated all their seeds with 

fungicides before sowing. However, pesticide use has come under 

increasing scrutiny because of phytotoxicity to germinating seeds 

as well as newer concerns about worker safety and environmental 

pollution. The following steps are recommended for managing seed 

diseases (Campbell and Landis 1990):

1. Determine if a seedborne disease problem exists by

carefully observing germination and monitoring seed

use efficiency

2. If a problem is suspected, conduct seed assays to

identify the specific pathogens involved

3. Treat seedlots that are shown to be contaminated

It is a good idea to go ahead and assume that seeds do carry 

pathogens and institute cultural practices that prevent potential 

pathogens from entering the normally sanitary container nursery environment. However, if growers want to 

confirm that they have a seedborne disease problem, then a 

sample of seeds can be assayed for pathogens. Some seed testing 

laboratories offer a phyto sanitary test (figure 6.2.12), or a sample 

can be sent to a nursery pathology lab. An enzyme-linked 

immunosorbent assay (ELISA) test has been developed to confirm 

the fungal pathogens Sirococcus strobilinus and Caloscypha 

fulgens on spruce seed (Littke 1990). Once a specific fungi is 

confirmed, then one of the following treatments, which are listed in 

order of increasing toxicity, can be institu ted.

6.2.7.1 Aerated water rinse

As already discussed in section 6.2.5, soaking seeds in an aerated 

rinse is a standard procedure for preparing nondormant seeds for 

sowing or as a treatment prior to stratification. It has also been 

shown to be an effective way to control nursery diseases and 

reduce the use of toxic pesticides (Dumroese and others 1990). 

Cleansing conifer seeds under running water for 48 hours was 

effective in reducing Fusarium levels and this simple treatment also 

will wash away other pathogens on the seedcoat. Seeds should be 

poured into a nylon mesh bag with enough room to allow them to 

move, and the bag placed in a tank of water. Inserting a irrigation 

hose will generate enough water pressure to gently agitate the 

seeds and wash contaminants out with the water running over the 

top of the tank. Some nurseries have used modified hot tubs to treat 

large volumes of seed before stratifi cation.

6.2.7.2 Chemical sterilants

Hydrogen peroxide (H2O2) has been used to sterilize seeds for 

many years, although the concentration and treatment time are 

critical (figure 6.2.28C/D). A 40minute soak in 30% H2O2 (laboratory 

grade) virtually eliminated all seedborne organisms on Douglas-fir 

seeds, and also was effective on southern pines with treatment 

times ranging from 15 minutes to 1 hour (Barnett 1976). The 

common antiseptic grade of H2O2 is only 3% and much less caustic 

than the laboratory-grade chemical. Recent operational trials found 

that a 4-hour treatment with 3% H2O2 was very effective in reducing 

levels of seedborne Fusarium on coastal Douglas-fir and western 

larch seeds. Subalpine fir was more sensitive, however, so a 1-hour 

treatment gave the best control with the least chance of 

phytotoxicity (Neumann and others 1997).

69
70

In addition to effectively sterilizing seed coats, H2O2 increases 

germination of some pine seeds. Germination of some longleaf pine 

seedlots, especially those with low viability, can be increased by a 

30- to 60-minute soak in H2O2 (Campbell 1982). The North Carolina 

Claridge State Nursery operationally soaks their longleaf pine

seeds in 30% H2O2 to reduce fungal contamination and improve 

germination and found that this treatment increases seedling 

stocking by about 10% after 90 days. Lots of 9 to 11 kg (20 to 25 

Ibs) of seed are placed in porous nylon mesh bags and soaked for 

55 minutes in the H2O2 solution at 24 °C (75 °F). Then, the bags 

are
drained and thoroughly rinsed in 3 separate containers of clean 

water and allowed to surface-dry (Barnett and McGilvray 1997).

Chlorine bleach (sodium hypochlorite) also has been used on seeds 

as a surface sterilant. Operational nursery use of solution of 2 parts 

common household bleach (5.25% sodium hypochlorite) to 3 parts 

water indicate that it is a less-reliable seed treatment than hydrogen 

peroxide, but it is safer to use (Wenny and Dumroese 1987). The 

other advantage to household bleach is that it is inexpensive and 

widely available.

6.2.7.3 Hot water

Hot water soaks are a traditional seed sterilization technique for 

horticultural seeds and also would be effective on those of forest 

and conservation species. Seeds are placed in nylon mesh bags, 

immersed in a tank of warm water--49 to 57 °C (120 to 135 °F)-for 

15 to 30 minutes, and then cooled in running tap water (Handreck 

and Black 1994). Aerated steam is an even safer way to apply heat 

because it does not leach the seeds. Seeds are spread on a mesh 

screen in an insulated chamber connected to an aerated steamer so 

that the steam is dry when it reaches the chamber. Temperatures 

reach the same range as the hot-water method and the treatment 

lasts 30 minutes, but treatments as short as 10 to 15 minutes also 

may be effective. At the end of the treatment, temperatures must be 

lowered rapidly by a cold-water rinse and the seeds allowed to dry 

(Hartmann and others 1997).

Although the best temperature/time treatment depends on the 

pathogen to be controlled and would have to be

71

determined for different species, hot water or steam is a safe, 

simple and environmenta lly friendly way to cleanse seeds.

6.2.7.4 Pesticides

Historically, fungicides were routinely applied to seeds to control 

diseases, especially damping-off. In 1981, captan, thiram, and 

benomyl were listed as seed fungicides for the control of the 

Fusarium fungus (Bloomberg 1981); however, the number of 

pesticides registered for nursery use continues to be reduced every 

year. In container nurseries, the possibility of phytotoxicity from 

seed pesticides is more serious because the biological and chemical buffering capacity of artificial growing media is very low 

(Sutherland and Van Eerden 1980). Therefore, use of fungicidal 

seed treatments must be justified by the identification of a specific 

pathogen and operational nursery trials to show that these 

treatments are truly effective.

The use of protective repellent coatings to limit bird depredation is 

not needed in fully enclosed propagation structures. However, 

more container seedlings are being grown in open compounds and 

so it is necessary to protect seeds from birds. The use of repellent 

coatings including thiram or anthraquinone is common in bareroot 

nurseries in the South. Anthraquinone with a latex sticker is safer 

to handle and has less effect on germination, but its powder 

formulation makes it troublesome to apply. Only label rates of 

anthraquinone or thiram should be used, because heavier rates 

may slow germi nation (May 1985). Again, nurseries should confirm 

that bird-repellent coatings are safe and effective before initi ating 

full-scale use.
6.2.8 Seed Sowing

The actual process of sowing seeds into containers varies with the 

type of seed and seed quality as determined by a viability test. 

Large or irregularly shaped seeds must be sown by hand, whereas 

small seeds of uniform shape can be mechanically sown. One seed 

can be sown per container for high-quality seedlots such as those 

from seed orchards, but 2 to 6 seeds per container are used for 

seedlots with lower germination percentages. Only the highest 

quality seeds should be used in container nurseries, however, 

because there are cultural and economic drawbacks to 

multiple seed sowing. Some nurseries have seed quality 

standards and will not accept seedlots with germination tests below 

a certain minimum. When several seedlings emerge in the same 

container they compete for light, water, and nutrients, resulting in 

initial growth rates that are lower until the seedlings are thinned to 1 

per container. Thinning costs are higher with multiple sowing and 

there is always the possibility of damaging the crop seedling when 

the oth ers are removed.

Three different sowing techniques have been used in forest and 

conservation nurseries: direct seeding, planting germinants, and 

transplanting emergents (table 6.2.15).

6.2.8.1 Direct seeding

Direct seeding is defined as the placement of seeds directly into the 

growth container when they are ready to germinate and grow (table 

6.2.15). If seeds are dormant, then they must be treated before the 

planned sowing date. By far the majority of seeds of commercial 

tree species such as pines are direct-sown, but the situation is 

different for other forest and conservation species because most 

have seeds with irregular shapes and diffi cult dormancy 

requirements. In a native plant nursery in the Intermountain West, 

only 10% of the species were direct--seeded (Landis and Simonich 

1984).

The seedling operation begins with the calculation of the sowing 

rate based on the results of germination tests-as tempered by the 

grower's past experience.

Determining sowing rate per container. For a container nursery 

to reach maximum production efficiency, empty cavities must be 

avoided. Unfortunately, seeds of most forest and conservation 

species will rarely have germination percentages higher than 90%. 

Orchard seed of commercial conifers is the exception, with the 

germination percentage of some seed lots approaching 100% and 

therefore these highly valuable seeds are sown 1 seed

72

per container. For most species, however, growers sow several 

seeds per cavity to make sure that they have no empty 

containers. Growers use several strategies to achieve maximum 

container occupancy when direct seeding:

• Sow 1 seed per cavity but sow some extra containers

(oversowing)

• Single- sow and transplant emergents from additional

seed trays

• Multiple-sow and thin to 1 emergent per container

The decision will depend on seed availability and cost, germination 

test results, container type, labor costs, and available growing 

space. If the grower has extra space and uses containers with 

removable cells that can be consolidated, then the oversowing 

option works well if the sowing of another species can be delayed 

for a month or so. The second option of single-sowing with 

subsequent transplanting of emergents that are grown in separate 

seed trays is viable if labor costs are not prohibitive. (See section 

6.2.8.3 for more information on transplanting emergents.)

Most nurseries sow multiple seeds per container and then thin 

down to 1 seedling after germination is complete. The decision 

of how many seeds to sow can be calculated directly or 

determined from tables that use sowing rates and expected 

germination to predict the number of vacant and stocked cavities 

(Balmer and Space 1976). A complete set of sowing probability 

tables can be found in Tinus and McDonald (1979).

The direct calculations are relatively simple, however, and most 

growers should be able to compute the proper sowing density with 

little effort. The technique is based on the concept of binomial 

probability -a seed either grows or it does not. If "X" equals the 

probability of a seed germinating and "Y" equals the probability of 

its failing to germinate, a binomial expansion can be constructed 

that includes all possible occurrences. The following example 

shows the possibilities when 2 seeds are sown per container 

(Schwartz 1993):

(X+ Y)2 = X2 + 2XY +Y2

where: X2 = the probability of both seeds germinating

2XY = the probability of only one seed

germinating

Y2 = the probability of neither seed gernIinating
73
6.2.291). (See section 1.4.5.3 in volume one of this series for a 

complete discussion on seeding equipment.)

The success of direct seeding depends on the accuracy of seed 

test information, the sowing technique, and conditions in the 

propagation environment. Novice growers must realize that actual 

seedling emergence may be different from the results of laboratory 

germination tests which are conducted under ideal environmental 

conditions. Therefore, nursery managers must adjust for this 

discrepancy based on their own operational experience.

As long as germination test data are known, the proper number of 

seeds to sow per container can be easily determined by entering 

the "germination failure" on a hand-held calculator with a universal 

power key (YX). The procedure consists of keying-in the decimal 

equivalent of the germination failure, pushing the universal power 

key, entering the number of seeds you might sow, and finally 

pushing the "equals" key. If your calculator does not have a 

universal power key, then just use repeated multiplication. For 

example, a seedlot with 78% germination has a 22% failure score:

For 1 seed per cell: (0.22)1 = 0.2200 = 22% empty cells

For 2 seeds per cell: (0.22)2 = 0.0484 = 4.8% empty cells

For 3 seeds per cell: (0.22)3 = 0.0106 = 1 .1 % empty cells

For 4 seeds per cell: (0.22)4 = 0.0023 = 0.2% empty cells

Thus, the calculation becomes a "law of diminishing returns" and 

the best number of seeds to sow will depend on seed availability, 

seed cost, cost of thinning, and the reliability of the germination 

test. In this example, most nurseries would be satisfied with 

sowing 3 seeds per cell.

Many nurseries have "Seed Use Forms" that show the necessary 

information for each seedlot. Not only are these invaluable for 

internal record keeping but are a good way to show customers 

how the seed requirement is calculated for each order (table 

6.2.16).

Hand-sowing. Small seedlots are always sown by hand because it 

takes a certain minimum amount of seeds to operate a seeder. At 

the present time, there are no seeders available that can handle 

large seeds (figure 6.2.29A), and so they also must be hand-sown. 

Very small seeds are also difficult to handle, not only because of 

their size but also because static electricity makes them clump 

together. However, they can be sown with a wet tooth pick or metal 

pin (figure 6.2.29B-G).

Seeding equipment. Seeds of uniform shape and with a smooth 

seedcoat, such as pines, can be mechanically sown. Sowing 

equipment ranges from relatively inexpensive shutterboxes and 

homemade plate vacuum seeders (figure 6.2.29H) to commercial 

needle-point vacuum seeders that can deposit 1 seed per container 

with a high rate of accuracy. In mechanized nurseries, the seeder is 

included in a sowing line where containers are filled with growing 

medium, sown, and covered in one operation. However, the 

precision of many seeders is such that workers still have to fill 

empty containers by hand (figure

74
75
Again, the importance of accurate and detailed record-keeping 

cannot be overemphasized.

6.2.8.2 Planting germinants from stratification

Germinant sowing consists of placing pregerminated seed into the 

growth containers, and is particularly well- suited for large seeds 

(table 6.2.15). This procedure is sometimes referred to as planting 

"sprouts" (Finnerty and Hutton 1993) but we feel that "germinants" is 

a more accurate and descriptive term. Planting germinants is most 

commonly used for seeds requiring cold-moist stratification but can 

also be used for seeds requiring warm-moist stratification. It can 

even be used for seeds of maples or junipers that require both 

treatments. Germinant sowing is particularly useful for seedlots of 

variable quality or for those for which no germination test data are 

available. It is commonly used for commercial conifers like true firs 

that typically have low germination tests and for large hardwood 

seeds like those of the oaks (figure 6.2.24). In an Intermountain 

West nursery specializing in native plants, the germinant technique 

was used for about 15% of the species (Landis and Simonich 1984). 

It is also popular in developing countries that do not routinely test 

their seed and where labor costs are low. Germinant sowing not 

only ensures that a live seed is placed in every container but that 

the resultant seedlings are larger because they can begin to grow 

immediately (table 6.2.17).

The germinant sowing process is relatively simple. Fresh seeds or 

seeds from storage are soaked in an aerated water tank for 24 to 

48 hours to achieve full imbibition and cleanse their seedcoats. 

They are then either placed in either cold-moist or warm-moist 

stratification until they begin to germinate. Some propagators coat 

their

76

seeds with a protective fungicide to reduce mold development 

(Finnerty and Hutton 1993), but this should only be done when 

absolutely necessary because of possible phytotoxicity and health 

concerns. Larger seeds are sometimes placed in plastic bags and 

mixed with a moist medium such as Sphagnum moss or wet burlap 

to maintain high humidity (figure 6.2.30 A). Smaller seeds are 

stratified naked in a tray so that germinating seeds are easier to 

locate. A variety of common trays have been used including 

stryrofoam meat trays or cake pans with clear plastic lids. One 

innovation involves spreading the seed in the fold of a mesh cloth 

between layers of moist peat moss-vermiculite growing medium 

(figure 6.2.30B). For cold-moist stratification, the bags or trays are 

placed under normal refrigeration at 1 to 2 °C (34 to 36 °F) or left 

under a moist cover in a greenhouse for warm-moist stratification.

The seeds are checked every few days or weeks to monitor 

germination, and as soon as the radicle begins to emerge (figure 

6.2.30C), that seed is sown into a growth container. Larger seeds 

can be planted with the fingers, but smaller ones require tweezers 

(figure 6.2.30D). Some growers like to prune the radicle of dominant 

tap-rooted species, such as oaks, prior to planting to ensure a 

more-fibrous root system. No more than 1 .5 to 3.0 mm (0.06 to 

0.12 in.) is trimmed with scissors or clipped with the thumbnail 

(Emery 1988). The placement of the germinating seeds is very 

important. The seeds must either be sown on their sides or with the 

radicle extending downward. Poorly placed seeds will develop a 

crook in their stems (figure 6.2.30E) that will become brittle and 

break when they become larger (figure 6.2.30F). The major disadvantage of the germinant technique is that sowing will be 

extended over several weeks or even months, depending on the 

degree of seed dormancy, and the resultant crop development will 

be uneven, producing seedlings with a wide range of sizes and 

ages (table 6.2.15).

6.2.8.3 Transplanting emergents from seed trays or

"pricking out"

Because many seeds of forest and conservation species are too 

small or fragile to be direct-seeded or even planted as 

germinants, another technique consists of sowing seeds into 

shallow trays. After germination, the emerging seedlings 

(emergents) are transplanted into growth containers (table 

6.2.15). This procedure is also commonly-known as "pricking out" 

or "spotting off" (Emery 1988). Transplanting emergents is 

particularly
77
only reduce growth in the nursery but also causes mechanical 

weakness or mortality after outplanting (figure 6.2.31 E). Reducing 

the length of the primary root of spruce seedlings by 50% was found 

to greatly improve transplanting success, but this clipping treatment 

was less effective as the emergents grew larger (McCure 1995; 

Singh and others 1984). In another innovative way to overcome root 

kinking, growers have developed an transplanting tool consisting of 

a sharpened probe with a V-notch in the tip (figure 6.2.31 F). The 

top of the emergent is held with one hand and the bottom of the root 

is hooked with th e notched tip of the transplanting tool (figure 6.2.31 

G). The root is pushed down into the soil or growing medium until 

the seedling is at the proper depth. Then, while still stabilizing the 

seedling, the hooked bottom of the root is cut off and the tool

removed (figure 6.2.31 H). This simple technique leaves the 

emergent transplanted without the possibility of a "J-root" or other 

deformation so that roots can develop normally (figure 6.2.31 I).

Transplanting emergents requires some degree of skill but can be 

easily mastered with some simple training (table 6.2.15). The 

procedure is also labor intensive, compared to direct seeding or 

sowing germinants, but an experienced worker can transplant up to 

2,000 emergents in an 8-hour day (Landis and Simonich 1984).

6.2.8.4 New sowing techniques

Nursery managers are constantly searching for better and 

more-cost-effective ways of sowing seeds.

Miniplug transplants. A relatively new seed propagation option is 

to direct sow the seeds in small volume containers ("miniplugs") or 

peat pellets. Once the seedlings have become established in these 

smaller cells, they can be removed and transplanted into the growth 

container. Transplanting is usually done by hand, and growers have 

developed innovative tools, such as dibbles and spatulas to make 

the procedures easier and faster (figure 6.2.32A). The stage of 

seedling development at the time of transplanting is very important 

because they must have a firm enough root plug to with stand 

handling but not so many roots that they will become deformed 

after transplanting (figure 6.2.32B).

Miniplugs have been successfully used for bareroot transplants for 

several years (Gelinas 1990), but this procedure is still very new in 

containers. This technology has particular application for nurseries 

growing large conpopular for native species because of their complex dormancy 

requirements and small or irregularly shaped seeds (Finnerty and 

Hutton 1993). In one native plant nursery, about 65% of the 

species were propagated by transplanting emergents (Landis and 

Simonich 1984).

Seeds requiring scarification must be treated before sowing, and 

those that require cold-moist stratification can be sown in trays in 

the fall and then placed in a refrigerator or even outside in a 

sheltered location to undergo natural stratification. In the latter case, 

the seed trays must be irrigated periodically to prevent desiccation 

and protected against rodent predation. When such trays are 

brought into the greenhouse in the spring, the seeds germinate 

almost immediately. Some species may germinate better at lower 

temperatures. For example, seeds of wintergreen are sown in seed 

flats in cold frames where the temperature is around 13 °C (55 °F) 

(Date 1994).

Transplanting emergents begins with sowing seeds in the 

germination trays. These shallow trays are filled with about 5 cm (2 

in.) of standard peat moss-vermiculite growing medium that is lightly 

tamped until it is firm, but not compacted. Larger seeds are 

scattered over the surface of the medium by hand, or smaller seeds 

can be sown with a salt shaker with the holes in the top enlarged. 

The sown seeds are covered with a light application of a 

fine-textured mulch such as sand-blasting grit, irrigated, placed into 

a greenhouse and misted lightly (figure 6.2.31A). When the 

germinating seedlings reach the cotyledon stage and begin to grow 

their first set of primary leaves, they are ready for transplanting to 

the growth containers (figure 6.2.31 B). The best size or age for 

transplanting varies by species, however. Broadleaf species with 

large cotyledons, such as sumacs, should be transplanted at the 

2-leaf stage whereas those with smaller cotyledons, such as 

monkeyflower, should not be transplanted until the 6- or 8-leaf stage 

(Emery 1988). With black spruce and jack pine, transplanting was 

impracticable after primary needles have developed, as the 

incidence and severity of root deformity increased with the amount 

of time past the erect hypocotyl stage (Scarratt 1991).

Transplanting consists of working the emergents loose from the 

seed tray (figure 6.2.31 C), making a dibble hole in the growing 

medium in the growth container, placing a plant in the hole, and 

firming the medium around the stem (figure 6.2.31 D). 

Unfortunately, this procedure sometimes produces a "J-root" or kink 

in the seedling stem (Gordon and Hayes 1994). This deformity can 

not

78
79
tainer stock because valuable growing space can be conserved by 

growing the miniplugs in a greenhouse. Once they are transplanted, 

they can be moved to shadehouses or open growing compounds 

which are much less expensive to operate. The biggest drawback to 

this technique is the cost of handling and transplanting (table 

6.2.15). The savings of greenhouse space and heating must 

compensate for the cost of the additional containers and 

transplanting. Plug transplanting equipment has been used for 

flower and vegetable plugs for many years and miniplug 

transplanters are now available for forest and conservation 

seedlings. (See section 1 .4.5.6 in volume one of this series for 

more information.)

Fluid drilling. The technique of fluid drilling, which is

80

also known as gel seeding, involves germinating seeds in aerated 

water at the optimum temperatures for germination until radicles 

emerge. Non-viable and slow-to-germinate seeds are removed from 

germinating seeds with density separation. The germinating seeds 

are mixed with a viscous gel and sown with specialized seeders 

(Taylor and Harman 1990). Fluid drilling maintains good moisture 

around the seeds and also protects them from rough candling. 

Another advantage is that gels can be used to deliver biocontrol 

agents such as fungicides to control seedling diseases such as 

damping-off.

In South Africa, seeds of eucalypts and pines with less Khan 90% 

germination have been operationally sown with gel seeding since 

1986 (South and Young 1994). reeds of the same size and mass 

are germinated in water and then separated using a sugar solution. 

After separation, the germinants are sown with special precision 

vacuum seeders. Gel seeding is still experimental in the United 

States but has promise for high-value seeds. Barnett (1985b) 

showed that if high-quality southern pine seedlots (90% or greater 

germination) are prechilled for "I[--)out 60 days and then soaked in 

aerated water at 24 °C 75 °F), about 85% of the seeds will have 

their radicles emerge within 4 to 5 days.

Single-seed sowing. The number of seeds to sow per ~ ell is 

typically computed based on germination percentage as discussed 

in section 6.2.8.1. However, with seeds that are valuable or of 

limited availability, the economics of single-seed sowing are 

becoming more attractive. Increased labor costs for thinning also 

are a consideration: South and Young (1994) calculated that 

thinning costs were 34% greater for sowing 2 seeds/cell compared 

to the costs of single-seed sowing.

Single- seed sowing also has genetic benefits because of the 

unequal selection pressures posed by certain container nursery 

practices. It has been estimated that germination, thinning, and 

culling contribute 66, 20, and 14% respectively to the total variation 

in the final crop (EIKassaby and Thomson 1996). The authors 

recommend single-seed sowing as one way to maintain natural 

biodiversity in wild seedlots or to assume reasonable gain with 

genetically improved seed.
many emerging seedlings were damaged during an 

unusually sunny spring.

Sterility. Because germinating seeds are very susceptible to 

fungal or bacterial attack, any material in direct contact with them 

obviously should be sterile. Some seed coverings are inherently 

sterile, including vermi culite and perlite, because they are heated 

to high temperatures during processing. Sphagnum peat moss is 

often considered to be sterile, but this has recently been shown to 

be false. Seed coverings need to be tested or sterilized with heat 

prior to use.

pH. Growers should try to maintain slightly acid (pH 5.0 to 6.0) 

conditions around germinating seeds to discourage damping-off 

fungi. Some seed coverings are made of crushed seashells, which 

are 90% calcium carbonate, and some coarse sands can also be 

calcareous. Such materials can cause the pH around the 

germinating seed to be too high, so growers should always test any 

potential new material. Sphagnum peat moss has the ideal pH and 

the large fibers can be milled to the proper size by forcing them 

through a 6.4-mm (0.25-in.) mesh screen (Emery 1988).

Weight. Seed coverings, such as grit or sand, are relatively heavy 

and can be costly to ship, so growers usually try to find a local 

supplier. Perlite is very lightweight, which can be a disadvantage 

because it tends to blow off the containers until they are irrigated, 

and can even be displaced by heavy water drops (Tinus and 

McDonald 1979).

6.2.9.2 Proper and uniform depth

The ideal depth of seed covering depends on the size of the seeds 

and, to some extent, on the type of irrigation system. A good 

general rule is to cover seeds to a depth that is approximately twice 

their smallest diameter (figure 6.2.33C) but this effect varies with 

seed size. In a study with Douglas-fir and western hemlock, the 

larger Douglas-fir seeds germinated and emerged at all the sowing 

depth treatments, whereas the small hemlock seeds had trouble at 

all but the shallowest depths (Minore 1985). Very small seeds or 

those requiring high light levels should be left uncovered and misted 

frequently or covered with a fine-textured material such as milled 

Sphagnum peat moss (Emery 1988). For example, Atlantic 

white-cedar seeds germinated best when pressed into the soil, left 

uncovered and then watered by a mist81

6.2.9 Seed Coverings

After seeds are sown into the container or cavity, they must be 

covered or "mulched" with a material that will keep them moist and 

protected until they can germinate. Seed coverings help hold the 

seed in contact with the growing medium and also reduce 

development of cryptogams such as moss, algae, and liverworts 

(figure 6.2.33A). (See section 5.1.5.4 in volume five of this series 

for more information on managing these pests.)

6.2.9.1 Types of coverings

Many materials have been used to cover seeds including standard 

growing medium components-shredded peat moss, vermiculite, and 

perlite. Organic materials work reasonably well although they 

encourage the growth of cryptogams in the moist environment 

necessary to germi nate seeds (Tinus and McDonald 1979). 

Therefore, other inorganic materials such as coarse sand or granite 

grit have become more popular in recent years. Some mate rials are 

unpleasant or even hazardous to use, however. Because ordinary 

horticulture grades of perlite contain dust that can irritate the lungs 

of workers, masks must be worn (figure 6.2.33B). Some brands 

contain a warning label noting that prolonged inhalation of 

crystalline silica can cause the delayed lung injury called silicosis 

and also that it can cause cancer in laboratory animals.

When shopping for a seed covering, the following characteristics 

need to be considered.

Texture. One of the primary functions of a seed covering is to 

maintain a "moist, but not wet" environment around the germinating 

seed. To the novice grower, it might seem logical to choose a 

material that holds water but this is not the case. A good seed 

mulch functions by creating a break in texture at the top of the 

growing medium (figure 6.2.33C). Because the growing medium is 

finer textured, water will not move into the coarser seed covering 

and so the surface of the medium remains moist, creating ideal 

conditions for seed germination.

Heat buildup. Germinating seeds are easily injured by heat. 

Temperatures at the surface of a dark growing medium or seed 

covering can significantly exceed that of the ambient growing 

environment. The intense sunlight in a greenhouse can quickly 

cause temperatures to reach damaging levels when dark seed 

mulches are used, so light-colored materials should always be 

used. Nurseries in coastal British Columbia used a dark grit for 

many years but switched to a very light gray material when
82

ing system so that the growing medium surface remained damp 

(Boyle and Kuser 1994).

It is particularly difficult to maintain a uniform depth of seed mulch if 

containers are unevenly filled with growing medium. Any seed 

covering must be of uniform depth or seed germination and 

emergence will vary widely. Most seeds are phototropic and will 

germinate and emerge more quickly when they are not covered too 

deeply. The depth of the seed covering not only affects the rate of 

germination but also the size of the developing seedlings. The time 

for Siberian larch seedlings to emerge almost doubled as the seed 

covering increased from 5 to 10 mm (0.2 to 0.4 in.) and was 

different for vermiculite and sand (figure 6.2.33D). The size of the 

larch seedlings also decreased significantly as the depth of seed covering increased for both materials (figure 6.2.33E). In addition, 

seeds of low vigor may not have the energy to push through deep 

seed coverings.
83

Seeds of most forest and conservation species are normally 

contaminated with fungi and bacteria and should be cleansed so 

that pathogens are not introduced into the nursery environment. The 

actual process of sowing seeds into containers varies with the type 

of seed and seed quality as determined by a viability test. Growers 

use three seed propagation methods (direct sowing, planting 

germinants, and transplanting emergents) and the choice depends 

on seed quality and dimensions. Direct sowing is most common 

and, although large or irregularly shaped seed must be sown by 

hand, the majority of commercial conifer seeds are mechanically 

sown. The number of seeds sown per container depends on seed 

quality and ranges from 1 to as many as 6. Seeds are covered with 

a mulch to protect the germinant and keep it moist. With multiple 

sowing, the extra seedlings must be physically thinned. Seed 

propagation will continue to be the most popular technique in forest 

and conservation nurseries and new innovations, such as miniplug 

transplants and precision sowing, will become more common.

6.2.10 Summary

The majority of forest and conservation species are propagated 

from seed to preserve the wide genetic adaptation that is critical to 

successful seedling establishment and growth in the natural 

environment. Growers must become familiar with the seed anatomy 

of the plants they wish to propagate and so a basic understanding 

of seed biology is mandatory. The seed source is important 

because it affects seedling growth in the nursery but, more 

importantly, seedlings must be adapted to the outplanting site. If the 

seed is not supplied by the customer, growers should insist on the 

proper seed source when purchasing seeds. High seed quality is 

essential to consistently produce crops of superior seedlings, and 

the only way to determine seed quality is by testing. Seed testing 

takes several weeks, so growers must allow enough time for testing 

before seeds are prepared for sowing. Unlike seeds of horticultural 

crops, which have been bred to germinate immediately after 

sowing, those of many forest and conservation species become 

dormant after they mature. Therefore, most need some sort of 

presowing treatment such as cold-moist stratification.
6.2.11 References

6.2.11.1 Literature cited

AOSA [Association of Official Seed Analysts].] 995. Tetrazolium 

testing handbook (No. 29). Lincoln, NE: AOSA. 22 p.

Alexander NL, Flint HL, Hammer PA. 1984. Variation in 

cold-hardiness of Fraxinus americana stem tissue according to 

geographic origin. Ecology 65(4): 1087-1092.

Axelrood PE, Neumann M, Trotter D, Radley R, Shrimpton G, 

Dennis J. 1995. Seedborne Fusarium on Douglas-fir: 

pathogenicity and seed contamination. New Forests 9 (1): 

35-51.

Balmer WE, Space JC. 1976. Probability tables for containerized 

seedlings. Atlanta: USDA Forest Service, Southern Area, 

State and Private Forestry. 27 p.

Banerjee M. 1994. A presowing treatment for spruce and lodgepole 

pine. Seed and Seedling Extension Topics 7(2): 7-8.

Barnett JP. 1971. Aerated water soaks stimulate germination of 

southern pine seeds. Res. Pap. SO-67. New Orleans: USDA 

Forest Service, Southern Forest Experiment Station. 9 p.

Barnett JP. 1972. Drying and storing stratified loblolly pine seeds 

reinduces dormancy. Tree Planters' Notes 23(3): 10-11.

Barnett JP. 1976. Sterilizing southern pine seeds with hydrogen 

peroxide. Tree Planters' Notes 27(3): 17-19.

Barnett JP. 1981. Imbibitional temperatures affect seed moisture 

uptake, germination, and seedling development. In: 

Proceedings,1 st Biennial Southern Silvicultural Research 

Conference. Gen. Tech. Rep. SO-34. New Orleans: USDA 

Forest Service, Southern Forest Experiment Station: 41-45.

Barnett JP. 1985a. Estimating seed vigor by sugar exudates 

and radicle elongation. Tree Planters' Notes 36(31): 16-19.

Barnett, J.P. 1985b. Fluid drilling: a technique for improving 

nursery seedling establishment. In: Proceedings, 3rd Biennial 

Southern Silvicultural Research Conference. Gen. Tech. 

Rep. SO-54. New Orleans: USDA Forest Service, Southern 

Forest Experiment Station: 38-41 .

Barnett JP. 1985c. Techniques for improving the performance of 

southern pine seeds in nurseries. In: Practices for the 

southern pines. Proceedings, International Symposium of 

Nursery Management, Montgomery, AL. Auburn University, 

AL: Alabama Agricultural Experiment Station: 102-112.

84

Barnett JP. 1986. Principles of sowing southern pine seeds. In: 

Schroeder, R.A., comp. Proceedings, Southern Forest Nursery 

Association Annual Meeting; 1986 July 22-24; Pensacola, FL. 

Tallahassee: Florida Division of Forestry: 22-31.

Barnett JP. 1997. Practical guidelines for producing longleaf pine 

seedlings in containers. Gen. Tech. Rep. SRS-14. Asheville, 

NC: USDA Forest Service, Southern Research Station. 28 p.

Barnett JP.; McGilvray JM. 1997. Practical guidelines for 

producing longleaf pine seedlings in containers. Gen. Tech. 

Rep. SRS-14. Asheville, NC: USDA Forest Service, Southern 

Research Station. 28 p.

Barnett JP, McLemore BF. 1967. Germination of loblolly pine seed 

hastened by soakings in aerated cold water. Tree Planters' 

Notes 18(2): 24-25.

Barnett JP, McLemore BF. 1984. Germination speed as a 

predictor of nursery seedling performance. Southern Journal of 

Applied Forestry 8: 157-162.

Barnett JP, Pesacreta TC. 1993. Handling longleaf pine seeds 

for optimal nursery performance. Southern Journal of 

Applied Forestry 17: 180-187.

Belcher EW, Leach GN, Gresham HH. 1984. Sizing slash pine 

seeds as a nursery practice. Tree Planters' Notes 35(2): 5-10.

Belcher EW, Waldrip BJ Jr. 1972. Effect of thiram on seed mold 

and germination of slash pine. In: Proceedings, Association of 

Official Seed Analysts No. 62. Corvallis: Oregon State 

University: 91-93.

Bergsten U. 1993. Removal of dead-filled seeds and invigoration of 

viable seeds: a review of a seed conditioning concept used on 

conifers in Sweden. In: Edwards DGW, ed. Dormancy and 

barriers to germination. Proceedings, IUFRO Project Group 

P2.04-00. Victoria, BC: Forestry Canada, Pacific Forestry 

Centre: 7-15.

Bir RE. 1992. Growing and propagating showy native woody 

plants. Chapel Hill: University of North Carolina Press. 192 

p.

Bloomberg WJ. 1981. Disease caused by Fusarium in forest 

nurseries. In: Nelson PE, Toussan TA, Cook RJ. Fusarium: 

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