47
Contents

1.3.1 Introduction 49

1.3.1.1 The propagation environment 49

Atmospheric factors 49

Edaphic factors 49

Biotic factors 49

1.3.1.2 Limiting factors and nursery design 49

Crop characteristics 50

Local climate 50

1.3.2 Types of Propagation Environments 52

1.3.2.1 Fully controlled environments 52

Growth chambers 52

Greenhouses 53

1.3.2.2 Semicontrolled environments 54

Shelterhouses 54

Hoop houses and tunnels 55

Shadehouses 56

1.3.2.3 Minimally controlled environments (open

growing compounds) 57

1.3.3 Selecting a Propagation Structure 58

1.3.3.1 Functions and terminology 58

1.3.3.2 Design loads 59

1.3.3.3 Foundation and floors 59

Construction materials 59

Engineering considerations 59

Biological considerations 59

Operational considerations 61

1.3.3.4 Framing 61

Construction materials 62

Engineering considerations 64

Biological considerations 64

Operational considerations 64

1.3.3.5 Coverings 64

Construction materials 64

Engineering considerations 69

Biological considerations 71

Operational considerations 71

48

1.3.4 Designing a Propagation

Environment 73

1.3.4.1 Biological considerations 73

Crop size and type 73

Need for different environments 73

Length of crop rotation 73

Multiple crops per season 73

Pest exclusion 74

1.3.4.2 Economic and political considerations 76

Construction costs 76

Local building codes and taxation policies 76

Water pollution 78

Reliability 78

1.3.4.3 Efficient spatial layout 79

Container dimensions 79

Type of container support and

handling system 79

Open growing compounds 79

Propagation structures 80

Sample space calculations 80

1.3.4.4 The ideal nursery design 81

1.3.5 Service Buildings 82

1.3.5.1 Headhouse 82

1.3.5.2 Pesticide storage 84

1.3.5.3 Office space 84

1.3.5.4 Seedling storage 84

Sheltered storage 84

Refrigerated storage 86

1.3.6 Site Layout and Orientation 91

1.3.6.1 Locating propagation areas and orienting

structures 91

1.3.6.2 Planning for easy access and materials

flow 93

1.3.7 Summary 95

1.3.8 Literature Cited 96
1.3.1 Introduction

Once the nursery site has been selected, the next step in 

developing a container facility is to consider how much 

environmental modification will be needed to produce a crop of 

high-quality seedlings within the given time constraints. To 

accomplish this objective, let's look at the nursery environment and 

discuss how growers can culturally modify it.

1.3.1.1 The propagation environment

The conditions in a container nursery have been radically altered 

from the natural environment, and a term is needed to describe the 

wide range of possible nursery facilities. We like the term 

propagation environment because it is comprehensive without 

being limited to a particular type of structure or production system. 

A propagation environment contains two separate but related parts: 

the atmospheric component and the edaphic component.

Atmospheric factors. The main factors of the atmospheric 

environment are light, temperature, humidity, and carbon dioxide 

(fig. 1.3.1). The atmospheric factors are strongly affected by the 

geographic location and type of nursery facility and so must be 

carefully considered during nursery site selection and construction 

of propagation structures. The climate at the nursery site will 

determine what type of propagation environment will be needed. If 

the ambient environment is mild and time is not a major limitation, 

then container seedlings can be successfully raised in an open 

growing compound or a low-cost propagation structure. On the 

other hand, if the climate is severe and crops must be produced in 

a short time frame, then a fully controlled greenhouse will be 

necessary.

Edaphic factors. The two principal factors of the edaphic 

environment are water and mineral nutrients (fig. 1.3.1). In 

container nurseries, the edaphic factors are independent of 

nursery location and can be completely controlled by the type of 

container, growing medium, and cultural practices. (See volume 

two of this series for more discussion on containers and growing 

media.)

Biotic factors. Both the atmospheric and edaphic components 

contain other organisms that can affect plant growth-either 

positively or negatively (fig. 1.3.1). One of the primary attractions of 

container nursery culture is that growers have more control over 

biological factors and can design propagation environments that 

exclude pests. In fact, in climates where weather conditions are 

otherwise ideal for plant growth, one of the most important design 

considerations is pest exclusion. Nurseries can also encourage 

beneficial microorganisms, for example, by inoculating the growing 

medium with mycorrhizal fungi. (The effect of pests on nursery 

design is discussed in section 1.3.4.1, and a detailed discussion of 

the biological component of the propagation environment is 

provided in volume five of this series.)

1.3.1.2 Limiting factors and nursery design

Container nursery developers can use the concept of limiting 

factors to determine which environmental conditions should be 

modified on their site. The principle of limiting factors states that, 

when a process is governed by several factors, its rate is limited by 

the factor that is closest to the minimum requirement (Odum 1971). 

Conceptually, the idea of limiting factors can be visualized with the 

wooden barrel analogy that Whitcomb (1988) used to explain 

mineral nutrient deficiencies. Plant growth is represented by the 

water in the barrel, which is constructed of wooden staves, each

49
representing a different limiting factor (fig. 1.3.2). The water level 

(plant growth rate) at any one time or location is limited by the 

height of the shortest stave (the limiting factor) in the barrel.

If we expand this concept to nursery design, we can identify those 

environmental factors that are potentially limiting to plant growth. 

The challenge for the nursery developer lies in identifying those 

factors that could limit seedling growth and in designing a 

propagation environment that will keep them as close to optimum 

levels as possible.

Crop characteristics. A good container nursery design will reflect 

both the environmental conditions on the site and the biological 

requirements of the specific crop. A propagation environment that 

is ideal for one group of plants may be biologically or economically 

unsuitable for another. Most nurseries produce a number of 

different species, however, and so often different propagation 

environments must be designed to meet the needs of the various 

crops. On a practical basis, most nursery developers must 

compromise to obtain some sort of average environment in which 

they can grow the entire range of crop species.

Forest and conservation nursery crops can have radically different 

environmental requirements that developers must factor into the 

nursery design process. One characteristic that makes forest and 

conservation seedlings different from most other types of 

horticultural or ornamental nursery crops is that they are typically 

planted in relatively harsh environments without subsequent care. 

Because forest nursery stock must be properly hardened to 

withstand the stresses of outplanting, traditional propagation 

structures and growing regimes have had to be modified. These 

special types of propagation environments are discussed in 

section 1.3.2. Nursery developers can get an idea of the biological 

requirements of typical forest and conservation species by visiting 

other local nurseries or consulting the growing schedules in 

volume six of this series.

Local climate. Next, the nursery developer must determine the 

amount of environmental modification that will be necessary on 

the selected site. Container seedlings have been grown in a wide 

variety of different environments, from open growing compounds 

to sophisticated greenhouses. Of course, the costs of nursery 

development and operation increase with the extent that the 

environment must be modified. Although a sophisti cated 

greenhouse can optimize all the environmental factors that limit 

growth (table 1.3.1), a simpler, less complicated propagation 

structure may be the most economical choice in many 

environments. The nursery that matches the biological 

requirements of the crop to environmental conditions of the site 

will also be the most economical, and so nursery developers 

should devote ample time to site analysis before the type of 

propagation environment is selected. (The specifics of site 

selection are discussed in chapter two of this volume.)
51
1.3.2 Types of Propagation Environments

Container nurseries can be categorized by the relative amount of 

environmental modification: fully controlled environments, 

semicontrolled environments, and minimally controlled 

environments.

1.3.2.1 Fully controlled environments

A fully controlled growing environment requires a propagation 

structure that contains all the environmental control equipment 

necessary to keep all potentially limiting factors at optimum levels 

(table 1.3.1).

Operationally, fully controlled propagation environments have many 

positive biological attributes (table 1.3.2). They are suitable for 

almost any type of climate and, due to the high degree of 

environmental control, the risk of losing a crop to severe weather is 

low. The favorable conditions permit crops to be grown year-round 

with a rotation of 3 to 9 months, making multiple crops a distinct 

possibility. However, fully controlled environments are the most 

expensive to build and operate, primarily due to high energy 

requirements.

Growth chambers. Growth chambers are the only propagation 

environments that completely control all factors that can potentially 

limit growth (table 1.3.1). The main advantage of growth chambers 

over greenhouses is that they artificially supply all aspects of light 

(intensity, duration, and quality). In addition, because they are 

completely insulated from the outside environment, growth 

chambers do not experience the radical temperature fluctuations 

that sometimes result from changes in solar input. Although they 

have been used for seed germination tests and experiments, only a 

few companies have developed growth chambers large enough for 

operational seedling production. Two were constructed 

underground in mines, but these proved to be economically 

unfeasible. Currently, the Iron Range Resources and Rehabilitation 

Board is operating a growth chamber in Chisholm, Minnesota, and 

propagating seedlings for mineland reclamation (fig. 1.3.3). Over 

the past 10 years, they have grown 44 different species of plants in 

two crops per year at competitive prices. Growth chambers are also

used in micropropagation to start the plantlets before they are 

moved out to a standard propagation environment.

52
Greenhouses. Greenhouses are the traditional method of

producing container plants, and they can be equipped to fully 

control the propagation environment (fig.1.3.4A). Greenhouses 

differ from growth chambers in that they use natural sunlight that is 

trapped inside the transparent structure and converted to heat (the 

"greenhouse effect"). The drawback of the transparent covering is 

that greenhouses are inherently poorly insulated and require both 

high-capacity heating and cooling equipment for good temperature 

control. Depending on

whether the climate is arid or humid, the greenhouse environment 

may need humidification or dehumidification. Many forest and 

conservation species are sensitive to changes in daylength, and 

so photoperiodic lighting is often installed to prevent dormancy. 

Carbon dioxide generators can be used to promote faster growth 

rates. Irrigation systems with fertilizer injectors can supply 

optimum amounts of water and all essential mineral nutrients. 

Sophisticated environmental control equipment is used to balance 

all the various factors and keep them at optimal levels.

53
The traditional way of growing forest and conservation nursery 

crops was to start the seedlings in a greenhouse and then move 

them to a shadehouse for hardening. In fact, growers soon learned 

that the hardening phase was the most challenging and began to 

look at ways of modifying the crop schedule. Many began removing 

the greenhouse covering so that they could harden their crops in 

place without the additional labor and expense of moving the 

seedlings (fig. 1.3.413). Others began looking at structural 

modifications to the traditional fully controlled greenhouse.

1.3.2.2 Semicontrolled environments

This category includes a wide variety of growing structures that, as 

their name implies, are designed to control only certain aspects of 

the ambient environment (table 1.3.1). Crops can be propagated in 

semicontrolled structures in all but the most severe climates (table 

1.3.2). Depending on the type of structure, crops can be grown from 

spring to fall with generally one crop produced per year; winter 

crops are not usually considered economical. Some types of 

semicontrolled structures, especially shadehouses and tunnels, are 

also used for hardening and intermittent seedling storage. From an 

economic standpoint, semicontrolled environments are cheaper to 

build and operate, although there is considerable variation between 

the different types of structures.

Shelterhouses. A modification of the traditional greenhouse, the 

original shelterhouse featured a permanent transparent roof with 

movable walls that can be rolled up or otherwise opened (Hahn 

1982). This design permits considerable flexibility in environmental 

control. In the spring or in unusually cold weather at any time 

during the growing season, the sidewalls are kept lowered and 

heat turned on to maintain ideal temperatures. When ambient 

temperatures become favorable, the sides can be raised to permit 

natural ventilation (fig. 1.3.5A), eliminating the need for forced air 

cooling. Other than these structural modifications, shelterhouses 

can be outfitted with any or all of the standard greenhouse 

environmental control equipment to modify most limiting factors.

54
Recently, computer technology and a variety of new shading 

materials have made many different types of semicontrolled 

propagation environments possible. An innovative new 

shelterhouse has been recently developed that features a 

retractable roof that can modify sunlight and crop temperature as 

weather conditions change (Vollebregt 1993). The roof material can 

be constructed of transparent fabric or shadecloth to produce a 

wide variety of light conditions within the propagation environment. 

Just like a shelterhouse, the transparent roof can be left closed 

early in the season and during cool weather and opened later to 

expose the crop to ambient conditions (fig. 1.3.5B&C). This feature 

makes retractable roof structures particularly valuable for forest and 

conservation seedlings because they can be gradually hardened to 

outside conditions yet still be protected from climatic extremes. 

Retractable roof propagation structures can also be designed with 

an inner shadecloth curtain to control light intensity and crop 

temperature. Responding to changes in ambient light intensity, the 

shade curtains can be computer controlled to open or close 

automatically in only 3 to 6 minutes. This allows the crops to receive 

more light early in the morning and late in the day and also 

throughout the day under partly cloudy conditions.

Hoop houses and tunnels. These low-profile metal bow-arch 

structures heat up rapidly in sunny weather, and thus are primarily 

used in cooler climates. Some nurseries have used larger 

structures (high tunnels) for propagation, starting the container 

seedlings in a greenhouse and then transferring them to the 

tunnels for finishing and overwintering (fig. 1.3.6A). High tunnels also can be equipped with portable heaters, so that seedlings can 

be germinated in place. During warm weather, the sides of the 

tunnels are normally rolled up to provide ventilation or are covered 

with shadecloth. In addition to solar or supplemental heat, tunnel 

seedlings receive only irrigation and fertilizer during the growing 

season. This minimal level of culture results in a relatively slow 

growth rate, and so the stock may remain in the structures for a 

second year. Smaller structures (low tunnels) are too short to 

accommodate motorized equipment and are used primarily to store 

stock overwinter (fig. 1.3.613, also see section 1.3.5.4).

55
Shadehouses. Traditionally, shadehouses (or lathhouses) were 

covered with snowfence or wooden slats, but several other types 

of shade coverings are now available in a variety of densities, 

materials, and colors (fig. 1.3.7A). Shadehouses are usually 

equipped with irrigation and fertilization systems but other limiting 

factors remain at ambient levels. Although traditionally used as 

hardening or holding areas, shadehouses are used by forest and 

conservation nurseries to propagate some species, and to finish 

crops in multiple-cropping regimes. In the latter case, seedlings or 

cuttings are started in a greenhouse and then moved to the 

shadehouse after they become established (fig. 1.3.713). In colder 

climates, shadehouses are also used for overwinter storage (see 

section 1.3.5.4).

Shadehouses have found considerable acceptance in the Tropics 

and the Subtropics, where sunlight can be too intense for young 

seedlings and torrential rains and wind can damage the crop. In 

these climates, a shadehouse with a permanent roof and open 

mesh sides (fig. 1.3.7C) will achieve the cultural objectives and 

also exclude insects and oth er pests from the growing area. The 

choice of shade material is important. White shadecloth helps 

keep the growing environment cool by reflecting sunlight, and 

knitted fabrics are superior to woven materials (George 1993).
1.3.2.3 Minimally controlled environments (open

growing compounds)

Open growing compounds were developed to produce an 

inexpensive container seedling that was well acclimated to the 

environment. The compounds are graded for good drainage and 

either covered with weed barrier fabric and gravel or paved with 

asphalt (fig. 1.3.8A). Although they offer little control of the ambient 

climate all are equipped with semipermanent irrigation lines that 

water and injected mineral nutrients can be applied (table 1.3.1). In 

cooler climates, some open growing compounds are equipped with 

photoperiodic lighting (fig. 1.3.813). Containers may be placed on 

benches, pallets, or directly on the ground, although the latter is not 

recommended because of poor air pruning of roots. The containers 

are arranged into elongated beds or bays, the dimensions of which 

are determined by the irrigation or seedling handling system. In 

some nurseries, seedlings are started in special germination 

chambers, but in other locations, germination takes place outside.

Although open growing compounds are the least expensive way to 

produce container stock, seedling growth rates are slow and, 

depending on the climate, it may take 1 to 2 years to produce a 

shippable seedling. Weather damage, such as a killing frost or 

torrential rain, is also a constant concern and so the risk of crop 

loss is the highest of all types of propagation environments (table 

1.3.2). Cold injury to overwintered seedlings is a serious problem 

at nurseries at high latitudes or elevations.
1.3.3 Selecting a Propagation Structure

A variety of different structures are used to modify the environment 

for growing plants, and the purpose of the following discussion is to 

introduce the container nursery developer to the basic terminology 

and concepts. For more detailed information on the engineering 

and operational aspects, the reader should consult standard 

greenhouse texts (Nelson 1991, Aldrich and Bartok 1989, Boodley 

1981, and Hanan and others 1978). Horticultural supply firms also 

can provide information and advice on the best type of growing 

structure for a particular climate and application.

1.3.3.1 Functions and terminology

Greenhouses and other propagation structures come in many sizes 

and shapes (fig. 1.3.9). They can be simply classified by three 

factors: external form, internal support system, and whether they 

are free-standing or gutter-connected.

The external shape of a propagation structure is a reflection of its 

function-to capture the maximum amount of sunlight while protecting 

the crop from adverse climatic conditions. Many structures were 

designed to meet the unique conditions of a particular geographic 

region. In colder climates, for example, gothic structures are 

common because they shed snow more easily. Sawtooth designs 

are designed for good ventilation and so are best suited for warmer 

climates. New innovations also affect the design. The low cost of the 

new types of polyethylene coverings and the superior insulation 

capability have made "double-poly" quonset structures very 

attractive. On the other hand, propagation structures with internal 

columns are rarely used today because truss-frames are now 

available in wide spans.

The type of propagation structure also depends on the financial 

resources of the nursery developer and the availability of local 

material. The wooden rigid-frame structure is relatively 

inexpensive where wood is readily available and can be easily 

constructed by the developer. The design should also reflect 

future expansion plans. For example, the bow frame design is 

only used in gutter-connected applications, but a single structure 

could be constructed until further expansion can be financed 

(Bartok, 1993).

58

Free-standing structures are ideal for new nurseries because they 

offer the most flexibility. Nursery developers can start with one or 

two structures and then add more later as business improves. 

Individual greenhouses are popular in forest and conservation 

nurseries because many different crops can be grown in separate 

environments. They also offer better access and shed snow more 

easily than multispan structures. On the other hand, 

gutter-connected propagation structures offer better space 

utilization, greater labor efficiency, and lower heating costs. For 

instance, one hectare of gutter-connected structures will use about 

25% less heat than an equal area of free-standing structures 

(Bartok 1991 a). They are also most practical where land is limited 

or expensive.

As with all buildings, propagation structures must be 

constructed to support specific design loads.
1.3.3.2 Design loads

There are three types of design loads that are a reflection of the 

engineering characteristics of the structure itself and the local 

climate:

• Dead loads from the weight of the structure

• Live loads caused by building use

• Weather-related loads from wind and snow

In addition to the weight of the frame and covering, dead load 

calculations must include any equipment that is supported by the 

frame. Live loads include people working on the roof and hanging 

plants (Aldrich and Bartok 1989). Wind loads can be significant in 

many environments and snow loading is a serious consideration at 

higher latitudes and elevations. Information on outdoor design 

temperatures, wind speed, and expected average snow loads is 

available (Midwest Plan Service 1983). Container nursery 

developers should consult with an engineer or greenhouse supplier 

to obtain an estimate of design loads before beginning to plan a 

propagation structure.

In the following sections we will discuss the advantages and 

disadvantages of the different types of propagation structures, 

based on the following three functions:

• Engineering-Safely support the design loads

• Biological-Capture maximum sunlight, and protect

the crop from adverse weather and pests

• Operational-Allow easy access and handling of

seedlings and materials

1.3.3.3 Foundation and floors

The most important function of the foundation is to tie the structure 

to the ground because strong winds cause a lifting effect (Aldrich 

and Bartok 1989). A propagation structure must also have a strong 

foundation to counteract all the various loading forces, keep the 

environment clean and pest free, and provide a solid base for the 

container supports and the handling system.

Construction materials. Foundations are made with poured 

concrete, whereas floors can be concrete, asphalt, or gravel 

covering a weed barrier fabric. The best foundation and floor for a 

propagation structure will be a compromise between the 

engineering, biological, and operational considerations and will 

most likely be determined by available funding. Solid concrete 

floors have many advantages, but concrete or asphalt traffic paths 

between the benches are a less expensive option (fig. 1.3.10). 

Bare soil or even soil covered with plastic tarp or weed barrier is 

never recommended in propagation areas.

Engineering considerations. Unlike other buildings, the 

foundations of most propagation structures are not continuous 

around the perimeter of the building. Instead, many structures have 

pier foundations consisting of a series of concrete footings poured 

below the frost line (Boodley 1981). The individual pier footings 

must be engineered to fit design loads and soil conditions, and their 

spacing corresponds to the distance between the primary frame

supports (fig. 1.3.11 A). If primary frame members are spaced more 

than 1.2 m (4 ft) apart, a continuous masonry or poured concrete 

wall is often used (Aldrich and Bartok 1989).

Many designs use a curtain wall of concrete, cement blocks, 

composite panels of aluminum foam and vinyl, or wood around the 

perimeter of the structure to close the area between the piers (fig. 

1.3.11 B). Although it does not provide any structural support, the 

curtain wall should extend below the ground line to exclude pests 

and should be insulated in cold climates (Boodley 1981). The floor 

of the propagation structure does not provide any structural support 

and so operational and biological considerations take priority.

Biological considerations. A solid floor is essential in keeping 

the propagation area clean and free from pests. Moss and algae 

are always a problem in container nurseries, and insects and 

fungal pests breed on weeds growing in or around propagation 

structures. So, a floor surface that can be cleaned regularly is one 

of the basic tenets of a good pest management program. With the 

increasing concern about surface and groundwater pollution, a 

solid floor should be considered whenever possible because 

container nurseries will probably be required to catch and treat all

irrigation runoff in the near future. It is much cheaper and easier to 

build floors during the initial construction than to have to retrofit the 

structure later.

59

Operational considerations. Concrete floors are advantageous 

because they are strong, easy to clean, and light in color. If forklifts 

or other light vehicles will be used in the structure, then the 

concrete must be thick enough to support them safely-at least 10 

cm (4 in). Asphalt is approximately 25% less expensive, but its dark 

color absorbs sunlight. This leads to overheating in hot climates 

and also causes the floor to become soft, which can limit vehicle 

access. Hardtopped traffic aisles that run between the benches are 

sometimes used, with the areas between the aisles covered with 

weed barrier fabric and large gravel.

1.3.3.4 Framing

The function of the frame is to provide support for the covering 

while causing minimal shading and heat loss, yet allowing for 

maximum ease of access and handling within the structure. The 

two basic types of frames are truss frame and quonset (fig. 

1.3.12), and these can be further categorized by whether they 

have internal supports or a free span (table 1.3.3). In structures 

with internal supports, vertical columns carry the weight of the 

design loads (fig 1.3.13A), whereas free-span structures are 

engineered so that columns are unnecessary (fig. 1.3.13B&C). 

Internal columns are primarily used only for shadehouses and 

other low-cost structures because modern trusses can support 

spans of over 12 m (40 ft).

61
Construction materials. The three materials used for framing 

propagation structures are galvanized steel, aluminum alloy, and 

wooden timbers. Each material has its advantages and 

disadvantages.

Steel. Steel is popular because it has high strength-toweight 

properties, is relatively inexpensive, and can be easily fabricated. 

Because steel will corrode in the highhumidity propagation 

environment, it must be pretreated with hot-dip galvanizing. On-site 

fabrication must be done carefully to make sure that the galvanizing 

is not scratched or the framing will corrode at that point (Garzoli 

1988).

Aluminum. Because they are not as strong, aluminum structural 

members are relatively larger than those of steel. Most of the 

aluminum used in propagation structures is extruded and must be 

manufactured to

62

detailed specifications, which increases the cost. Galvanized steel 

bolts or screws used to secure aluminum framing will produce 

electrolysis and eventual corrosion (Langhans 1980), but this is not 

a problem with commercially available structures.

Wood. Wood frames are inexpensive and can be hand fabricated. 

Information on the engineering properties of various types of wood 

and their uses in structures is available (e.g., Midwest Plan 

Service 1983). Because high humidity and temperature favor 

decay and insect damage, redwood or cedar have traditionally 

been used in propagation structures, but the high cost of these 

materials has made them less attractive. Other types of wood must 

be treated with preservatives. Chemical treatments must not 

release vapors that could be phytotoxic to the crop or leach 

excessively, which could pollute runoff water. Preservatives such 

as chromated
63
copper arsenate (CCA), ammoniacal copper arsenate (ACA), 

copper chromate acid, and chromated zinc chloride are very 

resistant to leaching and will not vaporize. Wood can be treated in 

three ways: pressure treatment, dipping, and painting. Pressure 

treatment is most effective and can increase the life of wood by 10 

times (Langhans 1980). Glue-laminated arches are also 

sometimes used but they are relatively expensive.

Engineering considerations. Hoop or quonset frames have a 

lateral outward force induced at the ground level and therefore 

must be restrained by solid footings. The frame consists of hollow 

pipe bows and lateral purlins (fig. 1.3.12). Both galvanized steel 

and aluminum extrusion bows can be used, but the latter are 

considerably more expensive. Although a single layer of film 

plastic is sometimes used, double-layer plastic that is inflated with 

a small blower is much stronger and adds considerable strength to 

the structure. Wooden structures must be well constructed, not 

only to support their own weight but to withstand heavy wet snows 

in colder climate s.

Biological considerations. Growing structures with trusses or 

other internal supports intercept sunlight and create internal 

shadows. Because they are relatively larger, wood trusses 

intercept more sunlight than metal ones (fig. 1.3.13C), although 

this can be lessened by painting them white to increase their 

reflectivity. Shading, which is much more of a problem during the 

late fall and winter when the angle of the sun is low, can be 

reduced by proper orientation (see section 1.3.6).

Operational considerations. Column-supported structures can 

cover wide spans, but the vertical columns create shadows and 

limit the use of large materials-handling equipment (fig. 1.3.13A). 

Some growing space is lost along the sides of quonset structures 

without high curtain walls because the angle of the roof restricts 

good air movement and access for workers.

1.3.3.5 Coverings

The function of the propagation structure covering is to capture 

sunlight while maintaining the desired temperature, humidity, and 

carbon dioxide levels for the species of plant being produced. The 

importance of the covering cannot be overemphasized because, 

during cold weather, it serves as the only barrier between an ideal 

propagation environment and disaster (Gray 1992). A wide variety 

of transparent materials have been used for covering propagation 

structures.

64

Construction materials. Glass has been used to cover 

conservatories since Greek times and was the only covering 

available until around 1950. In fact, the term "glazing," which is still 

sometimes used to refer to greenhouse coverings, is derived from 

the use of glass panels. During the past 40 years, a wide variety of 

different transparent plastics have been developed for covering 

propagation structures, including hard plastic panels and flexible 

plastic sheeting (Boodley 1981). Since 1979, plastic films have 

dominated the market and currently 80% of new greenhouses are 

constructed with double-polyethylene film (Reilly 1992). The type of 

structural framing must be considered when choosing a covering 

because some materials are more appropriate for some frames 

than others (table 1.3.3). Some structures, including greenhouses, 

typically have the same covering on the roof and sides. 

Shelterhouses, on the other hand, have one material on the roof 

and another flexible material on the sides, which can be rolled up 

(fig. 1.3.5A).

Plastic films or sheeting. Flexible, transparent films of 

polyethylene, polyester, polyvinyl fluoride, and polyvinyl chloride 

have been used to cover propagation structures. They are roughly 

the same in light transmittance, but vary in heat transmittance, 

durability, and cost (table 1.3.4). Although early trials used single 

sheets, growers quickly converted to double-layered coverings after 

the energy crisis of the early 1970's. Not only does the double layer 

provide better insulation, but the air inflation makes the covering 

much more resistant to wind damage. Today, double-layered 

plastic film is the standard of the industry.

Low-density polyethylene ("poly"). This polyethylene is the best 

known and most widely used plastic used to cover propagation 

structures in the United States. It is chemically inert, stays flexible 

at low temperatures, and is permeable to both oxygen and carbon 

dioxide. Sheets of 4- or 6-mil-thick (0.004- to 0.006-in) poly are 

typically, used, and a number of different brands are available in 

dimensions up to 80 by 150 ft. Agricultural or construction grade 

poly is not recommended because it has a useable life of only 9 

months. Greenhouse grades of poly can last from 3 to 5 years, 

depending on weather conditions and application (table 1.3.4).
65
Most propagation structures are covered with a double-layer of 

polyethylene sheeting inflated with a small fan, which increases its 

durability and can result in heat savings of 30 to 40% (fig. 1.3.14A). 

The outer layer should be 6 mil thick and the inner layer 4 mil thick 

(Boodley 1981). Single layers of poly sheeting have also been 

installed over fiberglass and other coverings to increase their 

insulation value (fig. 1.3.14B). Polyethylene film for nursery use 

comes in rolls and contains antioxidants and ultraviolet inhibitors to 

counteract the deleterious effect of sunlight and to extend its usable 

life. New products are continually being developed. One contains an 

infrared-blocking agent to stop heat loss, end another features a 

wetting agent to prevent condensation on the inner surface (Aldrich 

and Bartok 1989).

Other plastics. Although other types of plastic sheeting have been 

used as greenhouse coverings in research studies, none are as 

inexpensive or practical as polyethylene. Therefore, the following 

information is included for completeness only and the following 

coverings are not recommended for operational use.

Polyvinylfluoride (PVF) has th e highest light transmission of all 

plastic films. It also has a long usable life because it is resistant to 

abrasion, tolerates a wide range of temperature, and is transparent 

to ultraviolet light (Boodley 1981). Rolls of PVF are narrow, 

however, and the film must be spliced together to cover most 

propagation structures (Nelson 1991). Polyvinylchloride (PVC) film 

is durable and retains heat better than polyethylene, but attracts 

dirt and dust, which lower its life as a covering. It also becomes 

brittle at low temperatures and soft on very warm days. Another 

drawback of PVC film is that it comes in very narrow rolls. Although 

more common in Japan and Scandinavia, ethylene vinyl acetate 

copolymers (EVA) films are not used in the United States because 

of their expense (Boodley 1981).

Rigid sheets. In addition to traditional glass, several types of 

rigid plastic sheets are used to cover propagation structures.

66
Glass. Because of its high light transmittance and durability, glass 

has been a popular greenhouse covering for decades (fig. 1.3.15). 

Although several grades and weights of glass panels have been 

used for covering propagation structures, tempered glass such as 

Solatex® is recommended (table 1.3.4). Recent technological 

innovations have produced panels that are 0.9 to 1.8 m (3 to 6 ft) 

wide, which greatly reduces the number of supporting sash bars 

and caulking (Bartok 1993). Smaller panels may still be best where 

breakage due to hail or other causes is a problem (Boodley 1981). 

Glass is actually too transparent for some crops and so stippled 

panels that produce a more diffuse light are available. Double-layer 

Thermopane® glass is more expensive but reduces heat loss. 

Although sash bars were traditionally made of wood, extruded 

aluminum supports are becoming more popular because they 

require less maintenance and reflect more light.

Structured plastic panels. Fiberglass-reinforced polyester was the 

first type of rigid plastic panel used to cover propagation structures 

(table 1.3.4). Corrugated fiberglass panels contain strands of 

fiberglass, which scatter the light entering the structure, producing 

a diffuse light that is ideal for plant growth. Although inexpensive, 

the surface of fiberglass panels weathers relatively easily, and the 

ultraviolet rays in sunlight cause it to yellow with age (fig. 1.3.16A). 

Treating the surface of fiberglass panels with polyvinyl fluoride 

(Tedlar®) or sprayed-on coatings retards yellowing. One of the 

biggest drawbacks to fiberglass panels is that they are highly 

flammable and, although fire retardant brands are available, they 

are not suitable for nursery use (Nelson 1991). Although they still 

can be seen on many older structures, fiberglass panels are 

decreasing in popularity and are being replaced by newer types of 

corrugated polycarbonate sheets (fig. 1.3.16B).

67
Structured panels of acrylic and polycarbonate have only been 

available for the last 10 to 15 years, and although relatively 

expensive, they are rapidly increasing in popularity (Nelson 1991). 

Acrylic and polycarbonate panels are lightweight and durable, and 

they have excellent light transmission properties (table 1.3.4). 

Structured plastic panels are available in both single layer 

corrugated sheets (fig. 1.3.1613) and chambered flat

68

panels, which can be double or triple layered (fig. 1.3.16C). 

Recently, double-layer acrylic panels with increased insulation 

ratings have entered the market. Although initially used for covering 

the end walls on film plastic greenhouses, structured plastic panels 

are flexible enough to fully cover quonset propagation structures 

(fig. 1.3.16D). Like fiberglass, acrylic panels are flammable but 

polycarbonate panels are not (Boodley 1981).
Rigid polyvinyl chloride panels were introduced as an inexpensive 

substitute for fiberglass, but operational use showed that their life 

expectancy was only 2 to 5 years (table 1.3.4). They are not 

commonly used today, because like their film plastic counterparts, 

they discolored under ultraviolet light and became brittle (Nelson 

1991).

Shade coverings. Many semicontrolled propagation structures 

feature coverings producing varying amounts of shade, such as 

wooden lath or shadecloth. Shadehouses can be constructed of 

narrow slats nailed individually on the frame but snowfence, a type 

of fencing made of lath connected with wire, is commonly used 

(fig. 1.3.17A). Commercial snowfence provides 55% shade and is 

constructed of wood that has been pressure-treated to resist 

decay. It is available in 1.2- or 1.8-m (4- or 6-ft) widths and should 

be installed so that the individual pickets are oriented north to 

south to adjust for movement of the sun (Hummert 1993). 

Shadehouses with lath on the sides are also effective in providing 

wind protection and excluding large animals.

Several types of shadecloth are commercially available and provide 

various percentages of shade (30 to 95%) depending on the 

closeness of the weave, the thickness, and the color of the fabric 

(fig. 1.3.17B&C). For forest and conservation species, 55% 

shadecloth is normally used, but 30% sheeting is popular in more 

cloudy climates. Installation and useable life of shadecloths depend 

on the type of material. Shadecloth only comes in standard lengths 

and widths and so has to be custom-fabricated to fit many 

structures. Shadecloth is available in several fabrics, each having 

different properties. Polypropylene is strong and durable and will 

shrink only 1 %. Saran shrinks about 3%, however, which means 

that it should be installed with a slight sag. Polyester is fire and 

mildew resistant. Lock-stitched polyethylene will not run or fray 

when cut or punctured and, being resistant to ultraviolet light, has a 

long operational life (Bartok 1990b).

Engineering considerations. A growing structure must be 

designed so that the covering is suitable for the frame. Although 

most frame types can be covered with rigid plastic sheets, plastic 

films are not recommended for structures with internal support 

columns (table 1.3.3). The covering must also be able to support 

the structural loads, especially in areas with significant winter snow 

accumulation (fig. 1.3.17D).

Rigid sheets require relatively more fasteners than plastic films and 

so the panels should be as large as possible. Plastic films must be 

tightly secured to the frame to avoid wind damage and, when 

double layers are inflated, to minimize air leakage. Wooden furring 

strips are an inexpensive option and can even be used to fasten 

double layers of plastic (fig. 1.3.18A). Several different fastening 

systems are commercially available. One features a quick 

snap-locking feature (fig. 1.3.1813) making replacement quick and 

easy, which is important when damaged films need to be replaced 

during cold or windy weather. Because the dimensions of plastic 

film change with temperature, sheets can be pulled taut when 

installed in cold weather, but 5 to 8 cm (2 to 3 in) of slack must be 

left when temperatures are near 27 °C (80 °F) to allow room for 

contraction.

Double-layer plastic (double-poly) sheets that are air inflated have 

many positive features compared to single-sheet applications. Poly 

tubes are now available so that the double covering can be applied 

in one step. In addition to their superior heat insulation value, 

double-poly coverings are considerably stronger than single layers 

of the same material. They also extend the life of the plastic 

because the tighter fit reduces wind abrasion against the frame. The 

thickness of the outer layer should be 0.152 mm (6 mils) to offer 

more resistance to mechanical damage, while the inside sheet can 

be only 0.102 mm (4 mils) thick. The ideal inflation height should be 

1.25 to 10 cm (0.5 to 4 in) because internal air currents reduce the 

insulation value at greater heights (Nelson 1991). The layers are 

inflated with a simple squirrel-cage blower (fig. 1.3.18C) that can 

maintain 5.1 to 7.6 mm (0.2 to 0.3 in) of water static pressure; 

higher pressures of 13 mm (0.5 in) are necessary under high wind 

conditions. For a greenhouse measuring 8 x 29 m (26 x 96 ft), a 

blower (1 amp, 155 W) that can deliver 5.7 to 11.3 m3/min (200 to 

400 ft3/min) of air at 1 mm (0.5 in) of static water pressure is 

sufficient. The inflation pressure is controlled by adjusting the size of 

the intake on the blower. Manometers are used to monitor inflation 

pressure and can be homemade or purchased commercially (Bartok 

1990a).

69
70
Operational considerations. At higher latitudes and elevations, 

heat conservation becomes a major characteristic when selecting a 

covering. The heat transmittance values vary considerably for the 

major types of coverings (table 1.3.4), and nurseries in cold 

climates generally choose multiple layered coverings that provide 

the best insulation. Air-inflated double-poly structures can save 

about one-third of the heating cost (Bartok 1990a). Maintenance 

and replacement of coverings are also important factors. Rigid 

panels can last as long as 25 years or more, compared to only a 

few years for plastic films (table 1.3.4). Replacing plastic films can 

be impossible in windy conditions, but with good weather and 

experienced workers, most nurseries have mastered the process 

so that it is routine. Some nurseries use specially constructed 

scaffolds to make changing the covering both easier and safer (fig. 

1.3.19A). Disposing of used poly coverings used to be a drawback, 

but recycling is becoming more of an option (Bartok 1992a). After 

removal, the used plastic sheets can be mechanically gathered into 

either rolls or bales (fig. 1.3.19B&C) that can be processed by 

plastic recyclers.

71

Biological considerations. The main biological consideration is 

light transmission, and all coverings allow a high percentage of 

photosynthetically active solar radiation to penetrate (table 1.3.4). 

These maximum light values may need to be moderated for some 

crops or at some stages during the growing season. Light quality is 

also affected by the type of covering, and some new colored 

plastics can affect the ratio of various wavelengths reaching the 

crop. Different colors of shadecloth are also available (fig. 1.3.17C).
All coverings collect dust and dirt over time and loose their light 

transmission capabilities, and so periodic: cleaning is necessary. 

For example, without cleaning, the light transmission of poly films 

was found to decrease 4 to 7% over a period of 18 months 

(Giacomelli and Roberts 1993). One of the drawbacks of doublepoly houses is that they are so airtight that they restrict air 

exchange and are more difficult to cool and dehumidify. 

Condensation on the inside of the cover causes problems because 

it reduces light transmission and drips onto the crop. New plastic 

films with "anti-fog" properties can reduce this problem, however.

72

The life of shadecloth is several years if it is properly installed; it 

must be stretched tight and secured to prevent whipping and 

tearing in high winds (fig. 1.3.17B). Shadecloth should be furled or 

taken down in winter snow areas and is also more prone to wind 

damage when the weather is cold. Shadehouses made of lath can 

also be damaged by heavy snowfall (fig. 1.3.17D). In areas with 

heavy snowfalls-30 cm or 12 in of wet snow-lath on the roof should 

be in rolls (such as snowfencing) so that it can be rolled up and 

removed in winter. Even if lath on the roof of a shadehouse is 

removed, the shadehouse walls should be left up to protect the 

seedlings from high winds.
1.3.4 Designing a Propagation Environment

The next step in container nursery design is to determine how 

many different propagation environments are needed and how 

large each should be. If the seedlings are to be grown in an open 

compound, then the calculations are relatively easy. However, 

when structures must be built, there are many biological, economic, 

and political considerations that affect both external and internal 

design.

1.3.4.1 Biological considerations

Crop size and type. Based on the identified demand for seedlings 

or market survey, the nursery developer should have a good idea 

of which species and how many seedlings to grow per crop. The 

number of seedlings will define the total size of growing area that 

is required, and the biological requirements of the different species 

will determine what types of propagation environments will be 

needed. However, nursery developers should recognize that it 

may become necessary to increase production in the future or 

grow additional species that were not initially considered. 

Therefore, it is important to retain enough flexibility during nursery 

development to be able to respond to future business 

opportunities. It is possible to design a container nursery without 

specific production information, but persons entering the market on 

a speculative basis must decide on some seedling production 

estimates and crop species for planning purposes.

Need for different environments. If the desired crop can be 

grown on the same schedule and in a relatively similar 

environment, then large propagation structures are warranted (fig. 

1.3.20A). Large structures are inherently less expensive to build 

and more cost effective to operate per unit area. Their irrigation and 

fertilization systems are simpler to design, and they cost less to 

heat because they have less perimeter area for heat loss. These 

design features have cultural as well as economic implications, 

because seedling growth is often poorer around the edges of the 

growing area, the "edge effect." Large propagation structures can 

be divided into separate environments with movable curtains or 

sliding doors (fig. 1.3.20B), although the environmental control 

systems must be designed accordingly.

A number of smaller propagation environments can provide the 

grower with some distinct advantages (fig. 1.3.20C). Smaller 

structures can be used to generate a range of environments to 

accommodate the requirements of a variety of species and also 

allow different growing schedules during the year. If the crop 

consists

of species with radically different requirements, then the developer 

must divide them into biologically compatible groups. The number of 

these groups will determine the minimum number of propagation 

environments that will be needed, unless they can be grown at 

different times of the year in a multiple-cropping system. Having a 

number of smaller environments also gives the grower mare 

flexibility in propagating, handling, and delivering. From a nursery 

development standpoint, small environments are advantageous 

because they can be added sequentially over time without 

disrupting ongoing production. The risk of total crop failure is less 

because fewer seedlings will be lost if one structure fails, and the 

duplication of equipment provides backup in case of emergency.

Length of crop rotation. The crop rotation is the amount of time 

that it takes to propagate a crop within a given environment and 

grow it to the size and quality when seedlings can be either shipped 

or transplanted to another location (fig. 1.3.20D). Although there 

are variations with crop species, stock types, and local climate, 

nursery developers must calculate an average rotation for planning 

purposes. The average duration of each crop rotation and the 

number of rotations per year will determine the number and size of 

propagation environments that should be developed. Many forest 

and conservation species can be grown to shippable size in as little 

as 3 to 4 months under ideal conditions. The duration of typical crop 

rotations can be obtained from other nurseries in the local area, but 

often estimates must be made based on similar species. Typical 

crop rotations are discussed in volume six of this series and can be 

used for general planning purposes, although growth rates can vary 

significantly from nursery to nursery.

Multiple crops per season. Some nurseries are able to grow 

more than one crop per year in fully controlled greenhouses (fig. 

1.3.20D). Other growers start their crops in a greenhouse or 

germination chamber and then move them to other growing 

structures, such as shadehouses or tunnels, or out into open 

compounds. Although most efficient from a spatial standpoint, 

multiple cropping requires considerable experience and skill. In 

some areas, the trend has been away from multiple greenhouse 

crops and towards nursery systems employing simpler propagation 

structures and cultural regimes. Whether multiple cropping is 

feasible depends on the inherent growth rate of the species and 

their desired size. The experience of other local growers is 

probably the best guide, although this can vary considerably with 

cultural and economic factors.

73
Pest exclusion. One of the most attractive features of container 

nurseries is that they offer the grower the ability to effectively 

eliminate most pests from the propagation environment. The 

ability to exclude pests depends on the type of propagation 

environment. Even in open growing compounds, soil-related pests 

can be eliminated because the seedlings are grown in containers 

filled with artificial growing media. Reusable containers are 

sterilized between crops and the components used in artificial 

media are also sterile. A couple of new container design features 

are also helping reduce
pest problems. Styrofoam® block containers called Copperblocks® 

have cells that have been coated with copper carbonate. Although 

primarily used to chemically root prune seedlings, the copper 

compound is also an effective fungicide and keeps root pathogens 

from developing in the cells. The Ventblock® is another type of 

Styrofoam container that is designed with ventilation holes between 

the cells to encourage air movement and reduce humidity within the 

seedling canopy (fig. 1.3.21). Gray mold, caused by the fungus 

Botrytis cinerea (Pers.:Fr.), thrives in high-humidity environments, 

and operational trials with Ventblocks have shown them to 

significantly reduce gray mold infection of Douglas-fir seedlings 

(Peterson and Sutherland 1990).

Obviously, the opportunities for pest exclusion are much greater in 

enclosed propagation environments. In

climates where ambient conditions are ideal for plant growth, one 

of the main purposes for building a propagation structure is to 

exclude nursery pests. Research has shown that 90% of the 

insects that infest container nursery crops enter propagation 

structures through vents or open doors. Thus, growers are 

beginning to install insect screens as one part of an integrated pest 

management (IPM) program. Screens of both stainless steel and 

woven synthetic fabric have been used to exclude insect pests, 

including thrips and whiteflies. Covering vents with fine-mesh 

screens significantly reduces airflow, however, which can lead to 

high temperature problems in naturally ventilated structures. 

Therefore, growers applying exclusion screens to existing 

structures will have to make adjustments to reduce solar heating, 

such as converting to higher static pressure fans or using 

shadecloth (Neat 1992).

75
Special plastic film coverings can also help control fungal 

diseases. For example, researchers in Israel have developed a 

polyethylene film that screens out the ultraviolet wavelength of 

sunlight that is necessary for the germination of Botrytis spores 

(Lieberth 1991). In another trial, vinyl plastic film with infrared 

inhibitors reduced the relative humidity inside the greenhouse and 

resulted in better plant growth and lower incidence of gray mold 

and other fungal diseases (Vakalounakis 1992).

These new pest exclusion practices, as part of an IPM program, 

will reduce pesticide use and the potential of polluting surface 

water and groundwater.

1.3.4.2 Economic and political considerations

Construction costs. Construction costs vary considerably 

between different types of structures and are typically quoted per 

area of covered production space (table 1.3.5). Some growers 

prefer to erect their own structures whereas other, 

less-experienced nursery developers use the expertise of 

commercial contractors. Be sure to supply contractors with the 

same detailed set of specifications and conditions so that the bids 

will be comprehensive and comparable.

76

Make certain that you have included everything in your cost 

estimate. Many novice developers consider only the basic structure 

and forget about all the other associated costs of developing an 

operational nursery. For example, the price of a standard 

double-poly greenhouse may be quoted as $16.14/mz ($1.50/ftZ). 

However, this price does not include labor, environmental control 

equipment, electrical wiring, and plumbing which increase the cost 

to $58 to 84/ml ($5.40 to 7.86/ ft'). Finally, add the cost of 

purchasing and grading the land, constructing service buildings, 

drives and parking areas, and the cost can increase another 30 to 

45% (Nelson 1901).

An economic analysis should also consider other associated 

operating expenses when comparing types of propagation 

structures. For example, difficulty in obtaining materials, a shortage 

of reliable labor, or just the inconvenience of maintaining and 

recovering a structure may make a rigid plastic covering more 

practical than plastic sheeting in the long run. Fuel savings are 

another major consideration in cold climates; heating costs can be 

as much as 30 to 40% less in a double-poly structure than in a 

structure with single glass panels (Nelson 1991). These types of 

specific costs can be difficult to obtain, however, and so new 

nursery developers will want to contact other local nurseries for 

estimates.

Local building codes and taxation policies. The selection of the 

best nursery design can also be affected by local building codes, 

taxation policies, and land use laws. Greenhouses and other 

propagation structures are subject to local building codes, which 

cover every aspect of design and construction to protect public 

safety and the environment. The building codes of most states 

have been adapted from model codes published by three different 

organizations (fig. 1.3.22). The Basic Building Code (BOCA) is 

used in the Northeast, the Uniform Building Code (ICBO) in the 

West, and the Standard Building Code (SBCCI) in the South. Local 

municipalities may adopt any of these codes or even modify them 

to suit local needs, and so nursery developers are urged to consult 

local authorities during the nursery design phase to make certain 

that they are aware of local requirements (Aldrich 1993).
The method of appraising land and propagation structures can 

differ significantly between States and even within counties or 

cities. Some appraisal systems consider propagation structures 

covered with glass or structural plastic panels to be permanent 

structures, whereas those covered with polyethylene sheeting are 

considered to be temporary. In other locations, some types of 

temporary propagation structures are classed as "Agricultural 

Personal Property" and so are tax-exempt. So nursery developers 

should consult other growers in the area and tax consultants to 

determine local codes for appraising land and structures. Counties 

or other government taxing agencies maintain a current listing of 

property assessments and tax rates (Bartok 1991 b). The difference 

in annual taxes could be significant enough to favor one type of 

propagation structure over another.

77
Water pollution. Water quality is one of the most important 

ecopolitical issues at the present time, and new legislation is being 

enacted to regulate the chemical discharge from all agricultural 

activities. Chemical fertilizers and pesticides have contributed 

greatly to the increased seedling growth that can be realized in 

container nurseries, but they can also pollute surface and 

groundwater if they are allowed to leave the propagation 

environment. The principal agricultural pollutants of concern are 

pesticides and their degradates, as well as nitrates and phosphates. 

Pesticides and nitrates that have leached into the groundwater can 

pose a risk to human health in relatively low concentrations. Runoff 

of nitrates and phosphates leads to eutrophication of surface water 

bodies.

Container nurseries can contribute to point-source pollution when 

fertilizers or pesticides are injected into the irrigation system and 

then enter the water discharge leaving the nursery. Nurseries can 

also be accused of non-point-source pollution if water quality 

tests show groundwater to be contaminated with nitrates or 

pesticides (Landis and others 1992).

Water pollution restrictions will only become more stringent with 

time. Container nursery developers should incorporate water control 

features in their nursery design and layout so that they will have a 

no-discharge facility (fig. 1.3.23). The first design feature should be 

impermeable flooring in all propagation environments. In previous 

years, many container nurseries were designed with gravel floors so 

that irrigation runoff could just soak into the ground. Even those 

propagation structures with solid floors were graded so that runoff 

drained either into an adjacent ditch or pond. The next feature 

should be a nursery layout designed so that all irrigation runoff is 

collected into a pond where it can be either recycled or treated. 

Several ornamental nurseries in California and Oregon have 

pioneered irrigation recycling for container nurseries (Skimina 

1992). Operational trials at the University of Idaho Forest Research 

Nursery have shown that water discharge can be effectively 

collected and treated on the nursery site with a constructed wetland 

(Dumroese and others 1992). Several closed-loop irrigation systems 

are also being developed that completely eliminate discharge from 

the propagation environment.

78

Many of these water pollution control features are relatively 

inexpensive when they are incorporated into the initial nursery 

design and layout, but can be significantly more costly if an 

existing facility must be retrofitted.

Reliability. A nursery that has been poorly designed can be 

disastrous, and some container nurseries have failed because the 

wrong type of facility was constructed (McDonald 1982). Even in a 

fully controlled propagation structure, equipment failure or severe 

weather can cause rapid, disastrous environmental changes. If the 

facility has not been properly matched to the environment, the 

grower will have to struggle to maintain suitable growing 

conditions. These design flaws will be reflected in poor-quality 

seedlings and higher-than-normal nursery operating costs.

Developers should visit other nurseries in their area to learn what 

types of facilities have been successful, and ask them what they 

would do differently if they could built their nurseries over again. 

A decision matrix approach can also be used to evaluate the 

many different factors that must be considered (see section 

1.2.4).
1.3.4.3 Efficient spatial layout

Once the decision on the types and sizes of propagation 

environments has been made, then the internal space can be 

designed. Production space is defined as that area within the 

propagation environment that is covered with seedlings. Other 

areas such as aisles do not directly contribute to seedling 

production. Seedling costs are a direct reflection of production 

space efficiency, and so developers should carefully design the 

layout of their nursery facilities. Any space within the propagation 

environment that is not producing seedlings becomes part of the 

spatial overhead and increases unit costs.

Container dimensions. The most critical factor affecting 

production space efficiency is container size. A large variety of 

types and sizes of containers have been used to produce forest 

and conservation species and each has its advantages and 

disadvantages. (The criteria for selecting a container are discussed 

in detail in volume two of this series.)

The most important characteristic for layout design is the amount 

of production space that each container will occupy. Most 

containers used in forest and conservation nurseries are 

aggregates called blocks or trays (fig. 1.3.21), and their lengths 

and widths can be used to calculate surface area per unit. When 

growing different species or stock types, several different container 

sizes may be needed. If this is the case, some types of container 

such as Styrofoam blocks have the same outside dimensions for a 

range of different cell capacities (table 1.3.6). This allows growers 

to interchange container sizes without changing the layout of the 

production space and will also produce the best overall space 

efficiencies.

Type of container support and handling system. The decision to 

grow seedlings on benches, pallets, or other supports also affects 

the space efficiency within the propagation environment. One 

traditional container support system consists of permanent benches 

or tables with regularly spaced aisles to allow access to all parts of 

the production area. There are two standard configurations for 

permanent benches: longitudinal and peninsular (fig. 1.3.24). 

Although they are more space efficient, peninsular benches may be 

more restrictive for conveyors or other motorized handling systems. 

Movable or rolling benches are a relatively recent innovation that 

can greatly increase production space efficiency by 25% over fixed 

bench systems and can approach 90% overall space utilization 

(Langhans 1980).

The type of container handling system will affect growing space 

estimates because it determines the number and dimensions of 

aisles and corridors within the propagation structure. For example, if 

a forklift and pallet system will be used, aisles and doors must be 

wide enough to accommodate them and provide adequate turning 

radius. (Specific information on container support and handling 

systems is provided in section 1.4.2 of this volume.)

Open growing compounds. Because they have no external 

structural limits, open growing compounds are the simplest to 

design. Open compounds are typically divided into sections or 

bays, the dimensions of which are determined by the coverage of 

the irrigation system and the need for worker's access. If fork lifts 

or other equipment will be used within the compounds, then 

additional aisle space must also be allowed (fig. 1.3.25).

79
Propagation structures. Some special considerations are 

necessary in fully controlled greenhouses with heating and cooling 

systems. The main objective of any layout is to achieve maximum 

space utilization while allowing for reasonable access for workers 

and keeping seedlings a safe distance from heating and cooling 

equipment. Usually 30 to 60 cm (1 to 2 ft) of space between the 

greenhouse walls and the benches is provided to keep the plants 

away from hot or cold air flowing down the wall and to encourage 

good internal circulation. Areas near cooling pads and exhaust fans 

are often more subject to excessive drying, and so containers 

should be located about 1 m (3.3 ft) away.

Sample space calculations. Consider a greenhouse that is 9.3 m 

(30 feet) wide and 30 m (98 ft) long with heaters, fans, and cooling 

pads located on the end walls. The seedlings will be grown in 

Styrofoam block containers measuring 36 x 60 cm (14 x 24 in) and 

will be placed on wooden pallets, which can be handled with a 

pallet jack or forklift. A standard pallet measures 1.2 x 3.7 m (4 x 

12 ft), and the containers could be oriented on the pallets either 

sideways or endways (fig. 1.3.26A). Placing them sideways will 

allow 3 rows of 6 blocks or 18 in all, whereas the endways 

orientation produces 2 rows of 10 blocks for a total of 20 per 

pallet-the most space-efficient option. Each pallet contains 4.44 

m2, of which the blocks occupy 4.32 m2, for a space efficiency of 

97%, which is a very good fit.

Because the greenhouse must be loaded from the ends, the 

pallets would have to be oriented sideways across the 

greenhouse (fig. 1.3.26B). This orientation would allow for a 

center aisle of 1 m (3.3 ft) and side aisles of
approximately 0.5 m (1 .2 ft). Leaving 1.0 m (3.2 ft) on one end of 

the greenhouse for a buffer from the fans and 2.0 m (6.4 ft) on the 

other for the heaters results in 2 bays of 22 pallets each. 

Therefore, this layout would permit 44 pallets of 20 containers 

each or 880 containers in all, for an average space efficiency of 

70%. Multiplying this by the pallet space efficiency (0.70 X 0.97) 

yields an overall greenhouse production space efficiency of 68%.

This same general procedure can be used for other container 

types and bench systems. The best spatial arrangement will be a 

compromise between access for workers and equipment and 

maximum production space. Note that more seedlings can be 

produced in the same greenhouse by substituting a smaller 

capacity container. Using the Styrofoam block with the largest cell 

capacity will produce 39,600 seedlings per crop, whereas using the 

smallest capacity container will yield 211,200 seedlings (table 

1.3.6). Seedling size and quality would be significantly different 

between the two different types of containers, however, and some 

species will not tolerate higher growing densities.

1.3.4.4 The ideal nursery design

It should be obvious by now that there is not one "ideal" 

propagation structure for every application. The best choice will 

depend on many factors, and nursery developers should attempt to 

get as much information as possible before the final decision is 

made. Read the latest nursery trade journals to get the very latest 

technical information because that in books is usually several 

years old. Contact universities with horticulture or agriculture 

schools, horticultural supply firms, and other local growers to gain 

practical perspectives. Be wary of product claims or replacement 

guarantees, as most cover only the purchase price of the 

materials, not the labor for installation or the cost of a lost crop 

(Jozwik 1992).

In the final analysis, the container nursery developer must consider 

all aspects of a growing structure: engineering, biological, 

operational, and economic. These factors can be assigned 

weights based on relative importance and the decision-making 

procedure from section 1.2.4 can be used to make a final 

selection. Be certain to include political considerations, especially 

for government or company nurseries. One type of growing 

structure may be the most economically and biologically sound 

option, but it may not be the best alternative if the company or 

agency will not accept its appearance or the risk of crop loss 

associated with it.

81
1.3.5 Service Buildings

A nursery is more that just the propagation structure or growing 

compound. A successful nursery facility will also include a 

headhouse, storage buildings, and offices to support the work force 

and provide a handy source of tools, equipment, and supplies.

1.3.5.1 Headhouse

The main service building in support of a propagation house is 

referred to as a headhouse, which is an analogy to the critical 

function that it serves to the rest of the nursery facility. Depending 

on the sophistication and size of the nursery facility, the headhouse 

provides many services; it

• Shelters the environmental control equipment

• Provides materials and equipment storage

• Serves as a workroom during sowing or packing

• Provides a location for the office, restrooms, and

lunch room

• Functions as a repair shop

Headhouses are an excellent place to locate the main electrical 

panel, environmental controls and computers, irrigation valves and 

switches, and the emergency alarm system. Because fertilizers 

and pesticides can be stored nearby, fertilizer injectors are often 

located in the headhouse. If pesticides must be stored in the 

headhouse, they should be stored in a separate room that is 

properly designed to minimize possible spills and contamination of 

the rest of the headhouse (see section 1.3.5.2)

Regardless of its functions, the headhouse should be located so 

that workers, supplies, and the crop can be moved quickly and 

efficiently. To maximize access and minimize shading, the 

headhouse is usually attached to the north side of a single unit or 

row of structures, or at the center of a range of structures (fig. 

1.3.27). The size and internal layout of the headhouse depends on 

the types of activities, size, and storage requirements of each 

nursery. The proper size for a headhouse in a container nursery 

will vary considerably depending on many factors. Actually, 

headhouse size is not a fixed propor82

Some nurseries use inactive space or aisles in the propagation 

structures to perform all their labor-intensive activities, such as 

sowing and packing (fig. 1.3.28). This can decrease the amount of 

headhouse space that will be required but requires careful advance 

planning and an emphasis on cleanliness.

The headhouse should be laid out so that materials and people can 

flow efficiently between all the various operations with a minimum 

of extra handling and cross traffic. Work-area space requirements 

should allow adequate room for workers and equipment to work 

safely and efficiently during all the various nursery work periods: 

shipping, receiving, sowing, grading and packing, and 

maintenance. All equipment should be portable so that it can be 

stored when not in use. Headhouse floors should be concrete and, 

if heavy materials-handling equipment will be used in the 

headhouse, at least 10.2 to 15.2 cm (4 to 6 in) thick. Ceilings 

should be a minimum of 7.3 m (24 ft) high. Artificial lights should be 

fluorescent or metal halide and produce 215 lux (20 foot candles) at 

the work surface and 108 lux (10 foot candles) in office areas 

(Bartok 1992b). Doors between the headhouse and the 

propagation structures should be large enough to accommodate 

materials transport systems and equipment; Nelson (1991) 

recommends service doors that are 3 m wide by 2.7 m high (10 by 

9 ft). A loading dock is also a good idea to facilitate unloading 

supplies and shipping seedlings (see section 1.3.6.2).

Several headhouse plans should be developed, and flow 

diagrams made for each phase of work. Then, the design that 

best meets all the various requirements can be selected.

tion of total production space, but the relative area decreases 

as the size of the nursery increases (Bartok 1992b):

1.3.5.2 Pesticide storage

Although there are currently no Federal laws mandating where and 

how pesticide storage facilities must be built, others prohibit the use 

of land that may have previously been contaminated (Dwinell 1992). 

Several states in the Midwest have already established some 

minimum regulations and other states are following suit. Nursery 

developers would be well advised to incorporate certain features, 

such as a containment pad, into their site plan. Indoor pesticide 

storage and mixing areas should have a concrete pad with an 

approved coating. Larger nurseries using motorized spray 

equipment will require an enclosed outdoor pad. Containment pads 

should be sloped to drain into a sump, which will enable spilled 

pesticides to be diluted with water and then pumped back into the 

sprayer or into a disposal tank (Bartok 1992c). Other design 

specifics are contained in two excellent publications: Designing 

Facilities for Pesticide and Fertilizer Containment (Kammel and 

others 1991) and the Proceedings from the National Symposium on 

Pesticide and Fertilizer Containment: Design and Management 

(Midwest Plan Service 1992). Both can be purchased from the 

Natural Resources Management and Engineering Department at 

the University of Connecticut in Storrs.

1.3.5.3 Office space

Office space may seem to be a luxury when designing a container 

nursery, but it becomes very important once business begins. 

Accordingly, an office should be included in the nursery plans if at 

all possible. Larger nurseries design separate office buildings for 

clerical and managerial staff whereas, in smaller facilities, offices 

are usually located in the headhouse near the main entrance. Allow 

approximately 9.3 m2 (100 ft2) per person in the office area (Bartok 

1992b). Because the amount of paperwork and recordkeeping 

increases exponentially with the size and sophistication of a 

nursery, office areas should be designed so that they can expand 

as the operation grows.

1.3.5.4 Seedling storage

Forest and conservation seedlings are a perishable commodity. 

Unlike many other products that can be held without a decrease in 

quality, nursery crops are living and therefore have a limited shelf 

life. Developers must give special consideration to the type and 

amount of storage space that will be required. There are two basic 

types of seedling storage: sheltered storage and refrigerated 

storage

84

Sheltered storage. In mild climates, container seedlings are stored 

in the propagation area until they are shipped to the outplanting site 

(fig. 1.3.29A). The seedlings continue to receive irrigation and are 

protected from drying winds by shelterbelts. At higher latitudes and 

elevations where freezing weather is likely, the container seedlings 

are placed directly on the ground for the winter to lessen the 

possibility of cold injury to the roots. This operation can be labor 

intensive because all the seedlings must be taken off the 

propagation benches or pallets and placed on the ground. Growers 

have developed some innovative ways to make this procedure 

easier. At one Canadian nursery, the seedlings are supported on 

specially-designed supports made out of fencing and kept off the 

ground by collapsible wooden spacers (fig. 1.3.29B). This allows air 

pruning during the growing season, but when winter comes, the 

tension on the end cables is released and the entire bay of 

seedlings drops to the ground. In very cold and windy locations, the 

bays of seedlings should be insulated around the perimeter.

Shadehouses have been used as a combination hardening and 

storage structure for many years. The typical shadehouse for 

overwinter storage had shading on both the roof and sides (fig. 

1.3.17A) that protected seedlings from adverse weather, including 

high winds, intense rains, hail, and heavy snow. Shadehouse 

storage reduces seedling temperature below what it would be in 

direct sunlight by reducing sunlight by about 30 to 50%. This shade 

and the reduced wind speeds can significantly lower transpirational 

water losses during the growing season and also protect against the 

scorching caused by desiccation of foliage in cold weather when 

roots are cold or frozen. Fully enclosed shadehouses also protect 

seedlings from large animal pests such as deer and rabbits. This 

protection may actually increase the potential for small animal 

damage, however, because mice and other rodent populations can 

quickly increase when protected from their predators.

Shadehouse design will vary with nursery climate. In mild climates, 

a waterproof roof is desirable for overwinter storage of trees to 

prevent overwatering from rain and the leaching of nutrients from 

the containers. In areas that receive heavy wet snowfall, 

shadehouses for overwinter storage must be significantly stronger 

than temporary storage structures. Another option is to remove the 

shade covering during the winter to allow the snow to fall through 

and insulate the crop. Light dry snow will not damage the seedlings 

and actually serves as an excellent insulator.
Hoop houses and tunnels are low-cost quonset structures that have 

been used to store forest and conservation seedlings in some 

high-latitude nurseries. After the growing season, the coverings are 

removed from the quonsets and the seedlings placed on the ground 

for overwinter storage. In other systems, the seedlings are grown in 

greenhouses and then moved to the hoop houses that are covered 

with white poly sheeting to relect sunlight, while providing protection 

from wind

and precipitation (fig. 1.3.29C). Often, hoop houses are designed 

with side vents to allow air exchange during warm periods in the 

winter when inside temperature could become high enough to 

cause seedlings to break dormancy (fig. 1.3.29D).

The size of sheltered storage area that will be needed depends 

primarily on the type of propagation system, the number of crops 

produced per season, and the length of

85
86

time that the seedlings will have to be stored. Nurseries that 

produce more than one crop per year will need to carefully analyze 

the necessary amount of storage space; experience has shown that 

a storage area of 2 to 3 times the propagation space is often 

required.

Refrigerated storage. Bareroot seedlings have been stored under 

refrigeration for many years, but this practice is relatively new for 

container nursery stock. When forest and conservation seedlings 

were first produced in containers, it was assumed that container 

seedlings could be planted all year and many nurseries shipped 

their stock to the outplanting site in the growth container. Then, 

several things happened that gradually changed how container 

stock was stored and shipped. First, shipping the seedlings in the 

container was found to be costly and cumbersome and many of the 

reusable containers came back to the nursery dirty and damaged 

(fig. 1.3.30A). Second, growers observed that container stock 

stored in sheltered storage broke dormancy very early, especially in 

their root systems (fig. 1.3.30B). This often occurred before the 

planting window on many sites, and operational trials revealed that 

nondormant container seedlings did not tolerate the stresses of 

handling very well. So, to minimize storage volume and maintain 

the seedlings in a fully dormant condition until the outplanting sites 

were ready, nurseries began extracting (pulling) the seedlings from 

their containers, grading them, and packing them for refrigerated 

storage.

There are two different types of refrigerated storage used in 

forest and conservation nurseries-cooler storage and freezer 

storage. They are distinguished by the temperatures at which 

they hold seedlings:
Cooler storage is recommended when seedlings are going to be 

stored for less than 3 months and when seedling shipments occur 

throughout the storage period. When the storage period is going to 

be more than 3 months, many nurseries use freezer storage. This 

extends the allowable storage period because the lower 

temperatures suspend seedling metabolic activity and conserve 

stored carbohydrates. Freezer storage also significantly reduces 

the incidence of storage molds. Because freezing the seedlings 

turns all the free water in the storage container to ice, the 

development of pathogenic fungi is retarded. Storage pathogens, 

such as gray mold (Botrytis cinerea), can spread rapidly at 

temperatures slightly above freezing and ruin entire boxes of 

stored seedlings. (Refer to volume five of this series for more 

information on storage molds and other storage-elated problems.)

To retard desiccation and protect them during handling and storage, 

container seedlings are typically wrapped in plastic film or put into 

plastic bags. These bundles are then placed into waxed cardboard 

boxes that are often lined with another plastic bag (fig. 1.3.31A). 

The boxes are stored on pallet racks (fig. 1.3.31 B), which can be 

moved by forklift into the refrigerated storage units where they are

stacked into bays (fig. 1.3.31C&D).

Basic concepts. Refrigeration is the process of removing heat 

from a substance until a desired storage temperature is reached 

and maintained (Hardenburg and others 1968). Nursery stock is 

typically cooled by the room cooling method, in which the boxes of 

seedlings are exposed to cold air moving at a velocity of 60 to 120 

m/min (200 to 400 ft/min). The basic components of a refrigeration 

system are the vapor compression refrigerant, the compressor, the 

condenser, and the evaporator (fig. 1.3.32). The refrigerant is 

pumped through a pipe to the thermostatic expansion valve, where 

it is released evenly into the evaporator (A in fig. 1 .3.32), where it 

"boils" under the reduced pressure. Because evaporation is a 

cooling process, heat is absorbed by the evaporating refrigerant, 

cooling the air around the finned evaporator coils. Circulation fans 

blow air through the evaporator where it is cooled and

then distributed throughout the storage area. The refrigerant, which 

is now a gas at low pressure and temperature, is returned to the 

compressor (B in fig. 1.3.32), where the pressure is increased. The 

hot compressed gas is then pumped to the condenser (C in fig. 

1.3.32). Condensation is a heat-releasing process and so when air 

is blown across the condenser fins, heat is removed from the gas 

and it changes back into a high-pressure liquid (Bartsch and 

Blanpied 1990). The liquid is then stored in the receiver, ready to be 

sent back to the evaporator when cooling is required.

All vapor compression refrigerants are referred to by an "R" 

number. The refrigerants used in small refrigeration systems are 

halocarbons, also know as chlorofluorocarbons. Fully chlorinated 

refrigerants, such as R-11 and R12, have been implicated in the 

destruction of the earth's ozone layer and so are being phased out. 

Alternative refrigerants are being developed but will be much more 

expensive and will not be compatible with existing refrigeration 

systems. Recent legislation mandates that all halocarbon 

refrigerants be recovered and recycled. Therefore, new nursery 

refrigeration systems will need to be periodically serviced by 

portable recovery and recycling units that remove the moisture and 

any contaminants from the refrigerant so that it can be reused.

The amount of cooling needed to keep seedlings at the desired 

temperature is called the refrigeration requirement or box load 

and is expressed as tons of refrigeration or British thermal units 

(BTU). A ton of refrigeration absorbs 12,660 kJ\h (12,000 BTU/h). 

The refrigeration requirement should be based on peak refrigeration 

load, which depends on many factors such as the specific heat of 

the seedlings, the target temperature, heat leakage, and heat of 

seedling respiration (Hardenburg and others 1986). Obviously, this 

is a complicated process, and so nursery developers should contact 

a refrigeration specialist during this phase of the nursery design 

process.

87

Designing refrigerated storage. The volume of storage required 

will depend on the total number of shippable seedlings, their size, 

the type of storage container, and the arrangement within the 

storage area. The first step is to determine how many seedlings 

will fit in a storage container. Nurseries growing several stock 

types may require different container sizes. Next, calculate how 

many seedlings will fit in a given volume of refrigerated storage 

(table 1.3.7). Dividing the total seedling production by the 

seedlings per cubic volume of storage will provide an estimate of 

the total volume of storage needed. The internal arrangement of 

the storage area varies greatly between nurseries, however. 

Nurseries that palletize their storage containers can stack them 

higher but need wider aisles for forklift access. Some nurseries 

store all their seedlings in bulk boxes and then assemble individual 

orders throughout the storage period, so that space is needed for 

temporary storage until the seedling orders are shipped and 

picked up.

89
The final decision is whether to rent refrigerated storage space or 

to build your own. Nurseries in agricultural areas often are able to 

rent refrigerated storage when the facilities are not in their normal 

use. For example, apple processors often have storage space 

available in the spring when it could be used to store tree 

seedlings. Other nurseries rent refrigerated vans from trucking 

companies for the seedling storage season.

If you decide to build, there are two options: purchasing a 

prefabricated unit or constructing one from scratch. Several 

companies will help design and construct refrigerated storage 

buildings that can be expanded at a later date (fig. 1.3.33). 

Because desiccation is always a

problem, cooler storage is often designed with humidity control to 

keep relative humidities near 100%. This is very important where 

seedlings will be stored in open bins but is not of critical importance 

when all stock is properly packaged. If freezer storage will be 

required, then the building must be designed with no internal water 

lines that could freeze and break. Considering the high value of 

stored seedlings, designing the refrigeration building with an alarm 

system and back-up compressor is recommended. The cost of a 

prefabricated freezer storage building can run around $176/ml 

($5.00/ft3) without storage racks (Wenny 1993). (More detailed 

information on seedling storage structures and their operation can 

be found in volume seven of this series.)
1.3.5 Site Layout and Orientation

A container tree nursery should be laid out to maximize seedling 

growth while promoting maximum operational efficiency. Nursery 

developers should begin the layout process by sketching out the 

rough locations of the various nursery components (fig. 1.3.34). 

These sketches should be roughly to scale and done in pencil so 

that changes can easily be made. Several different computer 

programs are also available that make preliminary design work even 

simpler.

1.3.6.1 Locating propagation areas and orienting

structures

The propagation environment is the heart of the nursery and so the 

entire facility should be arranged with respect to it. Container 

nurseries should always be laid out to maximize sunlight in the 

propagation areas. Open growing compounds should be situated to 

receive maximum sunlight while receiving protection from wind. 

They should not be located near large trees, buildings, or other 

obstructions that can cause shade during a significant portion of the 

day (see figure 1.2.1). As a general rule, a growing compound 

should be located at a distance at least 2.5 times the height of the 

nearest object to the south, east, and west (Walker and Duncan 

1974).

Properly located shelterbelts can reduce adverse effects of wind. A 

well-designed wind barrier or a tree shelterbelt can significantly 

reduce heat losses of propagation structures, decrease the irrigation 

wind drift in shelter houses or open growing compounds, and 

provide protection from damaging storms (fig. 1.3.29A). The zone of 

protection varies with the height and location of the wind barrier. 

Tree shelterbelts should be located in the direction of the prevailing 

winds, and the growing area should be no closer than 4 to 6 times 

the height of a mature tree in the downwind direction. A mixture of 

evergreen and deciduous trees is best and the windbreak should be 

wider than the growing area to reduce wind turbulence around the 

ends. Tree species should be carefully selected to make certain that 

they will not harbor damaging insects or diseases. A fence 

windbreak approximately 3 to 3.6 m (10 to 12 ft) high can be 

constructed of lath snowfence or shadecloth that is 50 to 60% 

porous (not the same as percent shade) and supported by posts at 

3-m (10-ft) spacing (Roberts and others 1989).

Because of the shading of framing and internal environmental 

control equipment, the orientation of propagation structures 

requires special consideration. Orientation refers to the compass 

direction along the main ridgeline of the structure. Because of the 

low solar angle and short days, solar orientation is most important 

in nurseries growing winter crops at high latitudes. During 

midwinter, container nurseries in northern climates receive only 

about one-third of the sunlight that they would in the summer 

(Roberts and others 1989). The proper orientation is different for 

free-standing and gutter-connected propagation structures.

Because more internal shadows will be created when the winter sun 

has to penetrate the end walls, the ridgeline of free-standing 

propagation structures should be oriented east to west at latitudes 

greater than 38° (fig. 1.3.35). At lower latitudes the orientation is 

less critical although a north-to-south solar orientation is preferred. 

Gutter-connected structures at all latitudes should be oriented north 

to south in order to minimize the shading that is caused by the 

adjacent structures (Nelson 1991). Solar orientation should also 

take local climate and topography into account. At locations where 

the mornings tend to be more cloudy than the afternoons, the 

ridgeline should be canted slightly more to the northwest. If, for 

example, a hill blocked solar access in the mornings, the ridgeline 

of the propagation structure should be canted more in the 

northwest-to-southeast direction (Husby 1973). Access corridors 

that connect a series of growing structures should also be on the 

north side.

The direction of the prevailing wind is also an important factor to 

consider when laying out a container nursery because wind 

increases heat loss in the winter and cooling efficiency in the 

summer. If winter crops will be regularly grown, then the structures 

should be oriented so that the end walls, which have the least 

surface area and can be insulated easily, are facing the prevailing 

wind direction. In like manner, the structure should be oriented with 

the vents windward and the exhaust fans leeward for maximum 

ventilation (Boodley 1981).

91
92
At high latitudes or elevations where snow accumulation can 

become a problem, the distance between propagation structures 

must be wide enough to allow for snow removal. Snow slides off 

the roofs of structures and can accumulate to considerable depths 

between the buildings, where it may remain for weeks or months. 

Allow enough room between structures so that bucket tractors or 

other equipment can remove the accumulated snow and take it to 

another location.

1.3.6.2 Planning for easy access and materials flow

The entire site layout should be integrated so that the flow of 

workers, supplies, and seedlings is efficient both within and into 

and out of the nursery. The best design will depend on the type of 

container handling system. Nurseries that handle everything by 

hand or conveyors will want the distances between buildings to be 

as short as possible. If forklifts and other motorized equipment will 

be used, however, more space will be required for turning and 

maneuvering. Use the nursery layout sketch (fig. 1.3.34) to 

construct flow diagrams for each phase of work (Appleton 1986). 

Sketching the material flow throughout the nursery will help identify 

the most efficient layout. Inexperienced developers should contact 

existing nurseries and observe both the positive and negative 

aspects.

93

The headhouse needs to be readily accessible to the growing 

structures to permit easy flow of workers, materials, and seedlings 

(fig. 1.3.36). In larger facilities, a covered access corridor is 

recommended for safe all-weather access to the propagation areas 

and storage facilities. Loading docks should be considered if 

economically feasible because they will greatly aid in receiving 

materials and shipping seedlings. Generally, a loading dock in the 

headhouse is most convenient although some nurseries ship their 

seedlings directly out of storage. Most loading docks are 3.6 m (12 

ft) wide and 1.2 m (4 ft) high to accommodate large trailers, 

although the actual size depends on what types of trucks will be 

used. All-weather roads should be at least 7.9 m (26 ft) wide and 

should be able to support in excess of 18,144 kg (40,000 lbs). 

Docks must be designed for good drainage and, in climates where 

ice accumulates during the winter, should be either located on the 

sunny side of the buildings or covered (fig. 1.3.37). A well-designed 

loading dock will also have good lighting, weather seals, and a 

system to remove exhaust fumes (Aldrich and Bartok 1989).
A good nursery layout will make the most of the selected site and 

will take both current needs and the possibilities for future 

expansion into consideration. Many nursery developers make the 

mistake of designing a layout that is only adequate for current 

needs. Make certain that the headhouse and other critical facilities 

are centrally located so that they will be accessible to future 

expansion. The proposed direction of expansion can also be 

included in the site layout sketches (fig. 1.3.34). (See section 

1.2.2.4 for more discussion of this subject.)

94
95

1.3.6 Summary

The challenge to the nursery manager is to design a container 

facility that will modify the environmental conditions on the selected 

site so that a crop of high-quality seedlings can be grown within the 

given time constraints. To do this, it is necessary to evaluate which 

potentially growth-limiting environmental factors must be controlled 

and estimate the required costs. A well-designed nursery facility 

will produce the best propagation environment for the particular 

crop at the least cost.

A variety of different propagation structures has been used for 

growing forest and conservation crops, but the primary functions are 

to capture the maximum amount of sunlight, protect the crop from 

adverse climatic conditions, and allow easy access and handling of 

seedlings and materials. The type of structure will vary with the 

financial resources of the nursery developer and the availability of 

local material. The decision on how many different propagation 

environments are needed and how large each should be will 

depend on factors that are unique to each nursery. Seedling costs 

are a direct reflection of production space efficiency and so 

developers should carefully design their propagation environments. 

Any space that is not producing seedlings becomes part of the 

spatial overhead and increases unit costs.

A successful nursery facility will also include a headhouse, storage 

buildings, and offices to support the work force and provide a 

handy source of tools, equipment, and supplies. A container tree 

nursery will be laid out to maximize seedling production while 

offering optimum operational efficiency. A good nursery layout will 

make the most of the selected site and will take both current needs 

and the possibilities for future expansion into consideration. There 

is no one "ideal" container nursery. The best design will depend on 

many factors: engineering, biological, operational, economic, and 

(in the case of government nurseries) political considerations. 

These site factors can be assigned relative importance "weights," 

giving the developer an objective selection method that produces a 

numerical result.
1.3.7 Literature Cited

Aldrich, R.A. 1993. You are entering the building code zone. 

Greenhouse Grower]] (5):71-72.

Aldrich, R.A.; Bartok, J.W., Jr. 1989. Greenhouse engineering. 

Bulletin NRAES-33. Ithaca, NY: Cornell University, Northeast 

Regional Agricultural Engineering Service. 203 p.

Appleton, B.L. 1986. Container nursery design. Chicago: American 

Nurseryman Publishing Company. 122 p.

Bartok, J.W., Jr. 1993. Personal communication. Storrs, CT: 

University of Connecticut, Natural Resources Management 

and Engineering Department.

Bartok, J.W., Jr. 1992a. Recycling film plastic. In: Proceedings, 

1992 International Summer Meeting of the American Society of 

Agricultural Engineers; 1992 June 21-24; Charlotte, NC. Pap. 

92-4031. St. Joseph, MI: American Society of Agricultural 

Engineers. 6 p.

Bartok, J.W., Jr. 1992b. A headhouse can give your business the 

space to operate more efficiently. Greenhouse Manager 

10(9):143.

Bartok, J.W., Jr. 1992c. Most states are preparing proposals for 

pesticide, fertilizer containment. Greenhouse Manager 

10(12):90-91.

Bartok, J.W., Jr. 1991 a. Choosing the right structure. 

Greenhouse Grower 9(6):48-50.

Bartok, J.W., Jr. 1991 b. Figure structure value, depreciation 

when considering tax assessments. Greenhouse Manager 

10(3):130-131.

Bartok, J.W., Jr. 1991 c. Several techniques can counter 

decreasing winter greenhouse light. Greenhouse Manager 

10(7):92-93.

Bartok, J.W., Jr. 1990a. The basics of air inflation. 

Greenhouse Manager 9(6):133-134.

Bartok, J.W., Jr. 1990b. Shading: it's simple. Greenhouse 

Manager 9(2):121-122.

Bartsch, J.A.; Blanpied, G.D. 1990. Refrigeration and controlled 

atmosphere storage for horticultural crops. Publ. NRAES-22. 

Ithaca, NY: Cornell University, Northeast Regional Agricultural 

Engineering Service. 44 p.

Boodley, J.W. 1981. The commercial greenhouse. Albany, 

NY: Delmar Publishers. 568 p.

Cyro Canada, Inc. 1991. Technical Bulletin DSS 7.1. 

Mississauga, ON.

Dumroese, R.K.; Page-Dumroese, D.S.; Wenny, D.L. 1992. 

Managing pesticide and fertilizer leaching and runoff in a 

container nursery. In: Landis, T.D., tech. coord. Proceedings, 

Intermountain Forest Nursery Association; 1991 August 

12-16; Park City, UT. Gen. Tech. Rep. RM-211. Ft. Collins, 

CO: USDA Forest Service, Rocky Mountain Forest and Range 

Experiment Station: 1-15.

96

Dwinell, S.E. 1992. Federal laws affecting the design and 

management of pesticide mixing and loading facilities. In: 

Proceedings, National Symposium on Pesticide and Fertilizer 

Containment: Design and Management. Publ. MSWPS-C1. 

Ames, IA: Iowa State University, Midwest Plan Service. 160 p.

Garzoli, K. 1988. Greenhouses. Canberra, Australia: 

Australian Government Publishing Service. 185 p.

George, D.L., Sr. 1993. Personal communication. Renton, 

WA: Sharp and Sons, Inc.

Giacomelli, G.A.; Roberts, W.J. 1993. Greenhouse 

covering systems. HortTechnology 3(1):50-58.

Gray, H.E. 1992. Greenhouse glazing uncovered. 

Greenhouse Grower 10(8):54, 56-57.

Hahn, P.F. 1992. Personal communication. Eugene, OR: 

Georgia-Pacific Research Center.

Hahn, P.F. 1982. Practical guidelines for developing 

containerized nursery programs. In: Guldin, R.W.; Barnett, 

J.P., eds. Proceedings of the Southern Containerized 

Forest Tree Seedling Conference; 1981 August 25-27; 

Savannah, GA. Gen. Tech. Rep. SO37. New Orleans: 

USDA Forest Service, Southern Forest Experiment Station: 

97-100.

Hanan, J.J.; Holley, W.D.; Goldsberry, K.L. 1978. Greenhouse 

management. New York: Springer Verlag. 530 p.

Hardenburg, R.E.; Watada, A.E.; Wang, C.Y. 1986. The 

commercial storage of fruits, vegetables, and florist and 

nursery stocks. Agric. Handbk. 66. Washington, DC: USDA 

Agricultural Research Service. 130 p.

Hummert, P. 1993. Personal communication and sales catalog. St. 

Louis, MO: Hummert International. 448 p.

Husby, K. 1973. A tree seedling greenhouse: design and costs. 

Rep. ED&T 2340. Missoula, MT: USDA Forest Service, 

Missoula Technology Development Center. 77 p.

Jozwik, F.X. 1992. The greenhouse and nursery handbook. 

Mills, WY: Andmar Press. 511 p.

Kammel, D.W.; Noyes, R.T.; Riskowski, G.L.; Hofman, V.L. 

1991. Designing facilities for pesticide and fertilizer 

containment. Publ. MWPS-37. Ames, IA: Iowa State 

University, Midwest Plan Service. 116 p.

Landis, T.D.; Campbell, S.J.; Zensen, F. 1992. Agricultural 

pollution of surface and groundwater in forest nurseries. In: 

Landis, T.D., tech. coord. Proceedings, Intermountain Forest 

Nursery Association; 1991 August 12-16; Park City, UT. Gen. 

Tech. Rep. RM211. Ft. Collins, CO: USDA Forest Service, 

Rocky Mountain Forest and Range Experiment Station: 1-15.
Langhans, R.W. 1980. Greenhouse management. Ithaca, NY: 

Halcyon Press. 270 p.

Lieberth, J.A. 1991. A film that inhibits Botrytis. Greenhouse 

Grower 9(4): 74, 76.

McDonald, S.E. 1982. Fully-controlled or semi-controlled 

environment greenhouses-which is best? In: Guldin, R.W.; 

Barnett, J.P., eds. Proceedings, Southern Containerized 

Forest Tree Seedling Conference. 1981 August 25-27; 

Savannah, GA. Gen. Tech. Rep. SO-37. New Orleans: USDA 

Forest Service, Southern Forest Experiment Station: 81-85.

Midwest Plan Service. 1992. Proceedings, National Symposium 

on Pesticide and Fertilizer Containment: Design and 

Management. Publ. MWPS-C1. Ames, IA: Iowa State 

University. 116 p.

Midwest Plan Service. 1983. Structures and environment 

handbook, 11 th ed. Publ. MWPS-1. Ames, IA: Iowa State 

University.

Neal, K. 1992. Screen pests out, reduce chemical use. 

Greenhouse Manager 10(12):54-55.

Nelson, P.V. 1991 . Greenhouse operation and management. 

Englewood Cliffs, NJ: Prentice-Hall. 612 p.

Odum, E.P. 1971. Fundamentals of ecology, 3rd ed. 

Philadelphia: W.B. Saunders Company. 574 p.

Paterson, J.M. 1991. Field performance of black spruce container 

stock: 1. Comparison of two growing environments and four 

container types-second-year field results from 1989 

outplanting. Forest Research Report 127. Sault Ste. Marie, 

ON: Ontario Forest Research Institute. 10 p.

Peterson, M.J.; Sutherland, J.R. 1990. Controlling gray mold on 

container-grown Douglas-fir by modified Styroblocks and 

under-bench, forced air ventilation. Western Journal of 

Applied Forestry 5(3):75-79.

Began, R. 1993. Personal communication. Aurora, OR: Oregon 

State University Extension Service.

Reilly, A. 1992. New developments in greenhouse structures and 

equipment: 1. Structures and coverings. Foliage Digest 

15(1):1-3. [As cited in: Giacomelli, G.A.; Roberts, W.J. 1993. 

Greenhouse covering systems. HortTechnology 3(1):50-58.]

Roberts, W.J.; Bartok, J.W., Jr.; Fabian, E.E.; Simpkins, ]. 1989. 

Energy conservation for commercial greenhouses. Publ. 

NRAES-3. Ithaca, NY: Cornell University, Northeast Regional 

Agricultural Engineering Service. 42 p.

Skimina, C.A. 1992. Recycling water, nutrients, and waste in the 

nursery industry. HortScience 27(9):968971.

Vakalounakis, D.J. 1992. Control of fungal diseases of 

greenhouse tomatounder long-wave infrared-absorbing plastic 

film. Plant Disease 76(1): 43-46.

Walker, J.N.; Duncan, G.A. 1974. Greenhouse location and 

orientation. PubI.AFN-32. Lexington, KY: University of 

Kentucky, Department of Agricultural Engineering.

Vollebregt, R. 1993. Personal communication. Brantford, ON: 

Cravo Equipment Ltd.

Wenny, D.L. 1993. Personal Communication. Moscow, ID: 

University of Idaho, Forest Research Nursery.

Whitcomb, C.E. 1988. Plant production in containers. Stillwater, 

OK: Lacebark Publications. 633 p.

97