The Container Tree 

Nursery Manual

Volume Seven

Chapter 4

Plant Storage

101
102
103

7.4.1 Introduction

Unlike other agricultural commodities that can be stored

for extended periods without a decrease in quality, container nursery crops are living and have a very limited

“shelf life”. Therefore, well-designed storage facilities are

needed at all native plant nurseries. 

Plant storage wasn’t a serious consideration back in the

days when nurseries were established close to the outplanting project. This allowed plants to be dug up in the

nursery one day and outplanted the next. Transportation

was slow and plant handling and packaging was rather

simple (Figure 7.4.1). Reflecting back on those days and

knowing what we now do about plant physiology, it’s

amazing how well those early plantations performed. 

It’s important to realize that plant storage is an operational

necessity, not a physiological requirement (Landis 2000),

because of these four reasons. 

7.4.1.1 Distance between nursery and outplanting site

Today, most native plant nurseries are located at great distances, often hundreds or even thousands of miles, from

the outplanting sites of their customers. This is particularly

true of container nurseries because, as long as the proper

seed source is used, quality plants can be grown in greenhouses in ideal growing environments. The further the distance from nursery to outplanting site, however, the

greater the need for storage.

7.4.1.2. Differences between the lifting window at the

nursery and outplanting windows

As mentioned in the previous section, container nurseries

are often located in different climates than those of their

customers. In mountainous areas, this is especially true

because nurseries are typically located in valleys at low

elevation that have much different climates than outplanting sites at higher elevations. Differences between lifting

and outplanting windows will also depend on the season

of outplanting. If customers desire summer or fall outplanting, then short-term storage is all that is necessary.

Often, however, the best conditions for outplanting occur

the following spring and so it is necessary to protect

plants throughout winter.

Figure 7.4.1—Early forest nurseries did not need storage

facilities because seedlings were shipped and outplanted

within days. Note that the workers are sitting atop the bales

of nursery stock.

7.4.1.3 Facilitating harvesting and shipping

The large numbers of plants being produced at today’s

nurseries means that it is physically impossible to lift,

grade, process, and ship stock all at the same time.

Therefore, one of the primary benefits of storage facilities

is that they help to “spread-out” the scheduling and processing during harvesting and shipping.

7.4.1.4 Refrigerated storage is a cultural tool

Many growers do not appreciate the fact that refrigerated

storage can be used to manipulate plant physiology. Cold

storage temperatures can partially satisfy the chilling

requirement of dormant stock and refrigerated storage has

even been shown to improve plant quality (Ritchie 1989).

Class 2 Douglas-fir (Pseudotsuga menziesii) seedlings

were found to gradually increase in quality to Class 1

while in storage (Figure 7.4.2). A complete discussion of

dormancy and other aspects of plant quality can be found

in Chapter 7.2. 
104

Figure 7.4.2—Refrigerated storage can partially fulfill the

chilling requirement of dormant nursery stock and, in the

case of Douglas-fir, has been shown to actually increase

seedling quality—a move from the yellow to green zone

(modified from Ritchie 1989).

7.4.2 Short-term Storage for Summer or Fall Outplanting

—“Hot-planting”

Container stock that will be outplanted during summer or

fall is not completely dormant or very stress-resistant and

so requires special consideration. The term “hot-planting”

is used to describe this type of operation because no

extended period of refrigerated storage is employed.

Plants are typically held in the hardening structure until

they are shipped, usually a shadehouse or open compound (Figure 7.4.3A). In the Southern United States, hotplanted container stock is stored in coolers or refrigerated

vans at 4 to 21 °C (40 to 70 °F) for no more than a week

(Dumroese and Barnett 2004).

Recent research has shown that nondormant nursery stock

can perform well when hot-planted (Helenius and others

2005). Both actively growing and cold-stored Norway

spruce (Picea abies) container stock were planted and

then subjected to increasing periods of moisture stress.

The nondormant seedlings that were hot-planted had significantly more new roots growing out of the container

plugs (“root egress”) than the cold-stored stock for the first

2 weeks after outplanting (Figure 7.4.3B). 

Hot-planting can be successful in the summer and fall

when conditions are ideal on the outplanting site. This

system offers a lot of flexibility because plants can be

held at the nursery and shipped as they are needed. On

the outplanting site, nursery stock should be stored

upright and kept shaded. Using white boxes helps to

reflect light and keep in-box temperatures lower (Kiiskila

1999). Hot-planting requires close coordination between

the nursery and the customer; therefore, projects are usually close to the nursery and relatively small in size. 
105

Figure 7.4.3—Because they are not dormant or stressresistant, hot-planted stock is held in the hardening area

until shipped (A). In research trials, hot-planted spruce

seedlings had better new root growth in the first 2 weeks

after outplanting compared to stock that had been coldstored (B, modified from Helenius and others 2005).

A

B
106

Figure 7.4.4—Entire crops of

non-hardy plants have been lost

to sudden freezing temperatures

when improperly stored (A).

Sublethal injuries, such as cold

injury to roots (B), are of greater

concern because foliar symptoms are slow to develop under

non-stressful nursery conditions.

The importance of properly overwintering stock is often

overlooked by novice native plant growers because they

are primarily focused on growing the crop. Plants are frequently damaged and some crops have been completely

lost as a result of poorly designed or managed overwinter

storage (Figure 7.4.4A). While dead plants are dramatic,

what is more insidious is sublethal injury where roots are

seriously damaged (Figure 7.4.4B). Unfortunately, sublethally damaged plants often do not develop injury

symptoms under ideal nursery conditions; instead, the

injury is reflected in poor survival and growth on the outplanting site. The risk for overwinter injury is very much

dependent on the physiological condition of the plants at

the time of storage, (discussed in Chapter 7.3) as well as

proper storage techniques and conditions. 

7.4.3.1 Designing and locating a storage facility

The time to first think about plant storage facilities is during the nursery development phase, but unfortunately this

is often not done. The design and location of a nursery

storage system depends on four factors:

The general climate at the nursery. Most people think

that overwinter storage would be more difficult the further

north or higher in elevation one goes, but that’s not

always the case. Nurseries in the Midwest or southern

Great Plains regions of the United States are often the

most challenging because of extreme weather fluctuations

during winter (Davis 1994). An extreme case is the eastern slope of the Cascade Mountains or Rocky Mountains

where temperatures can vary by as much as 40 °F (22 °C)

or more within a 24-hour period and high, drying winds

are common during winter and early spring. At one nursery in Alberta, a 5-year study on plant quality in outdoor

compounds documented recurring damage and mortality

because of late frosts and unusually warm periods during

late winter (Dymock 1988). It can also be difficult to store

container stock in areas such as the Southwestern United

States, where winter is characterized by many clear,

sunny days. Therefore, each nursery must develop a storage system appropriate for the local climate.

Characteristics of nursery stock. Some plants are easier to

store than others and so storage systems must match the

plant species being grown. Species that tend to overwinter

well are those that achieve deep dormancy and can withstand low or fluctuating temperatures. Deciduous plants

have a definite advantage because their lack of foliage

when dormant reduces the possibility of winter desiccation. On the other hand, evergreen species are prone to

both cold injury and moisture loss and broadleaf evergreen

species are particularly troublesome. Species and ecotypes

from coastal areas are never exposed to freezing and so

tend to be less hardy than those from inland areas. This

makes it particularly challenging for nurseries that grow

seedlots from a wide range of elevations. For example,

coastal sources of Douglas-fir tend to grow late in the season and are much less hardy than seedlots from higher elevations in the mountains. In tropical or semitropical climates, plants never undergo true dormancy and can be

outplanted almost any time of the year. 

All temperate and arctic plants go through an annual

cycle of growth and dormancy—see Chapter 7.2. In nurseries, plants are cultured through an accelerated period of

7.4.3 Overwinter Storage

A B
107

growth that must be terminated before they can be outplanted; this is the hardening period. In Volume 6, we discussed ways in which growers can harden their stock and

prepare them for storage. Plants that are fully dormant

and cold-hardy are in the ideal physiological state for

overwinter storage. Dormant, hardy plants can be thought

of as being in a state of “suspended animation”. They are

still respiring and some cell division occurs in the roots

and stems (see Figure 7.2.35 in Chapter 7.2); evergreen

species can even photosynthesize during favorable periods during winter. The challenge to nursery managers is

to design and manage a storage system to keep their

stored plants dormant while protecting them from stress.

Distance to the outplanting sites. Nurseries located close

to the outplanting sites may be able to hot-plant their

stock with little or no storage. As the distance increases,

however, some type of storage facility is needed.

Nurseries that are in a climate different from their customers need the most sophisticated and expense storage

systems. Because they grow stock from many different

elevations with different outplanting windows,

Weyerhaeuser nurseries in Oregon and Washington use

freezer storage where plants can be held for as long as 6

months (Hee 1986). The USDA Forest Service JH Stone

nursery in southern Oregon has grown commercial

conifers for clients across the Northwestern United States,

but those from high elevations in Idaho require special

handling and therefore incur more costs than those for

local customers. On the other hand, clients in the coastal

forests of Oregon can outplant throughout the winter, and

so receive their plants with minimal storage. 

Number and size of plants to be stored. As already mentioned, larger nurseries face a greater challenge in processing their stock, and storage systems help provide a

buffer. Larger container stock requires special storage

considerations. For example, it is relatively easy to store a

large number of 66 cm3 (4 in3) plants under refrigeration,

but the same number of 328 cm3 (20 in3) plants would

require four times as much space. Even larger stock, such

as 20-L (5-gal), require too much refrigerated storage

space and so must be stored by other means.
Different native plant species have different requirements

for overwinter storage. Because of this, and unique local

climates, four different types of overwinter storage are

commonly used in forest and conservation nurseries.

Most nurseries typically use several types of storage. Three

types of overwinter storage avoid refrigeration and are discussed in this section, whereas refrigeration, the fourth

type of storage, is discussed in section 7.4.5.

7.4.4.1 Open storage

This is the least expensive but most risky overwintering

option in areas with freezing temperatures. This is especially true for small volume container stock that has less

thermal mass of the growing media to protect sensitive

roots from freezing. In addition to more thermal mass,

larger containers also contain more moisture that protects

against overwinter drying. Thus, the smaller the container

size, the higher the risk of injury.

The best locations in a nursery for open storage have some

protection from the wind and are where water and cold

air will drain away. Gravel and/or drainage tile should be

used to promote free drainage of rain or snow melt in the

spring. Packing containers together tightly on the ground

and insulating their perimeter with straw bales or a berm

of sawdust makes use of heat stored in the ground to protect the roots of the stored plants (Figure 7.4.5A). A

research trial in Sweden showed the importance of grouping container plants and placing them directly on the

ground (Lindstrom 1986). Temperatures in the peripheral

containers were consistently lower than those in the interior by as much as 3 °C (5.4 °F) and fluctuated greatly. At

the end of the overwinter period, plants were placed in a

growth chamber to observe their performance and those

stored directly on the ground had much more shoot and

root growth than those stored on pallets that were 10 cm

(4 in) above the ground (Figure 7.4.5B). To keep plant

roots from growing into the ground, underlay all openstored stock with heavy poly sheeting or a copper-treated

fabric that chemically prunes the roots (Figure 7.4.5C).

Open storage is most successful in forested northern climates where adjacent trees create both shade and a

windbreak, and continuous snow cover can be expected.

If tree cover isn’t available, plants can be stored in narrow

east-to-west oriented bays between vertical snowfence

(Figure 7.4.5D). Snow is an ideal natural insulation for

overwintering container plants, but complete and continuous snow cover is not always reliable. Some northern

nurseries have had success with generating snow cover

with snow making equipment (Davis 1994) (Figure

7.4.5E). 

7.4.4.2 Structureless storage systems

Next to open storage, these are the simplest and least

expensive ways to overwinter container stock. The term

“structureless” means that plants are enclosed in a protective covering that lacks any mechanical support. Many

different coverings have been used but the basic principle

is the same—to provide a protective, insulating layer over

the stored plants. Clear plastic should never be used

because it transmits sunlight so that temperatures within

the storage area can reach damaging levels. All plastic

coverings will be eventually photodegraded by direct sunlight so they should be stored in a dry, dark location

when not in use (Green and Fuchigami 1985). Any structureless storage system is only effective if applied after

plants have developed sufficient hardiness and, most

importantly, removed before plants lose dormancy in the

spring.

White plastic sheeting. Single layer films, such as a 4-mil

white copolymer plastic sheeting, are the most common

coverings in structureless systems. White is preferred

because it reflects sunlight and keeps temperatures from

building-up under the covering. Some growers group

blocks of containers together with the roots to the inside

and then cover them with white plastic (Figure 7.4.6). This

is less effective, however, than grouping the containers

together on the ground to take advantage of the heat

stored in the soil (Figure 7.4.5A). 

White StyrofoamTM sheets and panels. Microfoam® is a

breathable Styrofoam-like material that is lightweight,

reusable, and easily removed and stored. It is available in

rolls or sheets of various widths, lengths, and thicknesses

(Figure 7.4.7A). Sheets can be placed directly over plants

(Figure 7.4.7B), or supported by wood stakes or wiring.

Because Microfoam® is so lightweight, it needs to be

secured well so that it does not rip or blow away during

windstorms. Typically, the foam sheets are secured around

108

7.4.4 Non-Refrigerated Storage Systems
109

Figure 7.4.5—Open storage can be effective when plants are blocked on the ground and surrounded by insulation (A).

Both shoot growth and root growth capacity (RGC) of seedlings overwintered on the ground was much greater than those

stored on pallets (B). Copper-treated fabrics, like Tex-R® (C), are ideal for ground storage because they chemically prevent

plant roots from growing into the ground. Open-stored plants should be protected from direct sun and wind by natural or

artificial snowfences (D). Snow is an excellent insulator and northern nurseries have augmented natural snowfall with

snow-making equipment (E). (B, modified from Lindstrom 1986; C, courtesy of Stuewe & Sons; E, courtesy of Maurice

Dionne).

A

B C

D E
110

Figure 7.4.6—White plastic reflects the warming rays of the

sun but, by itself, has no insulation value and so it is better

to leave the containers on the ground.

the edges with concrete blocks, wooden planks, or even a

berm of sand. In a comprehensive trial in Ontario,

Styrofoam SMTM blankets protected conifer seedlings from

temperatures below -30 °C (-22 °F) with a significant cost

savings compared to refrigerated storage (Figure 7.4.7C).

The authors recommended removing insulating covers during warmer weather to allow condensation to escape and

prevent overheating of stock nearest the ground. Subsequent

outplanting trials produced almost identical results in survival and subsequent growth (Figure 7.4.7D) (Whaley and

Buse 1994). In another test, however, one layer of

Microfoam®, in the absence of reliable snow, did not provide enough protection in the harsh climates of northern

Minnesota and North Dakota (Mathers 2004). As with all

new techniques, nurseries considering using insulating covers should install small trials before attempting operational

use.

Plastic bubble-wrap sheeting. This material has better

insulation than regular plastic sheeting and is reported to

be cheaper and more durable than Microfoam®sheets

(Barnes 1990). Because it is clear, however, heat build-up

would still be a problem on sunny days.

Frost fabrics. Woven and nonwoven landscape fabrics have

been used for structureless storage. White frost fabrics retard

solar heating while permitting infiltration of rain or snow

melt, and allow stored plants to “breathe”. Horticultural

suppliers offer frost fabrics in a range of weights and thicknesses giving 2 to 4.5 °C (4 to 8 °F) of thermal insulation.

Arbor Pro® is a feltlike material that has been used successfully for conifer storage in eastern Canada (White 2004).

Plastic film with layer of insulating material. In harsh,

northern climates without reliable snow cover, some nurseries cover their container stock with a “sandwich” of

straw or other insulating material between two layers of

clear plastic sheeting. Because the clear plastic and straw

absorb solar heat on clear, frigid days, and the straw provides insulation during the night, this layering provides

more overwinter protection than other structureless systems (Mathers 2003). Although layered coverings provide

good insulation, they cannot be removed or vented during

periods of sunny warm winter weather (Iles and others

1993).

For nurseries considering overwintering with coverings or

in poly tunnels, Green and Fuchigami (1985) provide

operational costs for various systems.

7.4.4.3 Storage structures

The next level of sophistication and cost are storage structures, which range from traditional cold frames to fullycontrolled units.

Cold frames. The term “cold frame” is a traditional name

for a propagation structure that receives its heat only

through absorbed sunlight. When sheltered from direct

sunlight and insulated, however, cold frames can be a

low-cost alternative for overwinter storage. In northern

Alberta and Alaska, cold frames constructed of wooden

sideboards lined and topped with rigid StyrofoamTM panels have proven effective for overwintering conifer

seedlings (Figure 7.4.8A). Use of insulated cold frames

has resulted in a significant increase in plant survival at

the Weldwood Nursery in Alberta (Matwie 1991). Cold

frames constructed of wooden pallets supported by

cement blocks and covered by white plastic poly sheeting

are considered the most cost effective overwintering system for conifer seedlings at a nursery in eastern Canada

(White 2004). 
111

Figure 7.4.7—Microfoam® plastic foam sheeting makes an excellent overwinter cover (A). Many ornamental nurseries group

their containers together on the ground and cover them with Microfoam® (B). When properly designed and applied,

StyrofoamTM blankets protected conifer stock as well as refrigerated storage (C&D) (B, courtesy of Richard Regan; C&D,

modifed from Whaley and Buse 1994).

A C

B D
112

Figure 7.4.8—Cold frames of wood and rigid StyrofoamTM

sheet insulation have been used to overwinter container

plants in northern climates (A). When weather conditions

permit, the top layer of insulation is removed so that plants

can be irrigated (B). Cold frames can be extensive (C) and

automated to protect plants during freezing temperatures

(D) or retracted during heavy snowfall (E) (photos A&B

courtesy of Larry Matwie; photos C-E courtesy of JD Irving

Limited).

C D

E

A B
113

Figure 7.4.9—Cloches and polyhouses are simple overwintering structures covered with white plastic (A) or

MicrofoamTM sheeting (B). The ends or sides are opened

during warm and sunny winter weather for ventilation (C).

Cold frames take advantage of the heat stored in the

ground and the insulating covering retards heat loss and,

more importantly, prevents winter drying. To be most effective, plants should be placed in the cold frames as soon as

they are hardy and before the ground freezes. Heat buildup can still be a problem during warm or sunny periods in

the winter and, on these occasions, the top insulation

panel can be removed for ventilation and to allow irrigation (Figure 7.4.8B). As soon as weather conditions permit

in the spring, the tops of the cold frames should be

removed to prevent heat build-up and subsequent loss of

bud dormancy. 

The Juniper tree nursery in New Brunswick uses large

sophisticated cold frames to overwinter their stock (Figure

7.4.8C). The accordion-like covers can be extended to

protect plants from freezing temperatures or drying winds

(Figure 7.4.8D), or opened during heavy snowfall (Figure

7.4.8E). Although expensive to construct, they are much

less expensive than refrigerated storage (Brown 2007).

Cloches and polyhouses. These two storage structures are

similar except for their length; cloches are shorter and do

not offer worker access, whereas polyhouses typically have

doors in the end walls. Both feature wooden or pipe frames

covered by white plastic sheeting (Figure 7.4.9A), or a

panel of MicrofoamTM placed between two layers of plastic

(Figure 7.4.9B). The ends of these structures are opened for

cooling during sunny, warm periods during winter (Figure

7.4.9C). Although a single layer of white poly sheeting is

adequate protection in milder climates, a double layer of

white plastic that is inflated by a small fan provides better

insulation in colder locations. In locations with frigid temperatures below -18 °C (0 °F), plants overwintered in polyhouses need the additional protection of a white poly film

or MicrofoamTM blanket (Perry 1990). In milder climates,

growers supply just enough heat in their polyhouses to

keep the ambient temperatures just above freezing; this

approach has proven effective for overwintering a wide

variety of native plants in Colorado (Mandel 2004). 

If possible, cloches and polyhouses should be oriented

south-to-north to minimize and equalize solar heating. In

structures oriented east-to-west, plants on the south side

receive more light and heat than those on the north and

may require irrigation during the winter. Any closed storage structure needs to be monitored carefully throughout

A

C
114

the winter to determine if ventilation is needed on sunny

days during late winter and early spring (Figure 7.4.9C).

Ventilation can also be provided by opening end doors,

or a thermostatically-controlled fan can be installed on

one end with intake louvers at the other end. To prevent

desiccation, mount fans and louvers in the top of the

structures where heat build-up will be greatest. 

Shadehouses. Shadehouses are traditional hardening

structures that have also been used for overwintering all

sizes of container stock (Figure 7.4.10A), and are particularly useful for larger container stock that requires too

much space in refrigerated storage. Tall containers like

TreepotsTM need to be supported, so nurseries have developed heavy wire rack systems. Some nurseries use

cement blocks to support prefabricated “stock panels”

that can be purchased from any ranch or farm supply

(Figure 7.4.10B). 

Shadehouse design varies with nursery climate and location. Where prolonged cold temperatures aren’t typical,

plants can be overwintered under shadecloth or shadeframe. In wet climates, a waterproof roof is desirable for

overwinter storage to prevent excessive leaching of nutrients from 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

winter to allow the snow to fall through and insulate the

crop. Light, dry snow will not damage plants and actually

serves as an excellent insulator over the crop. 

The typical shadehouse for overwinter storage has shading on both the roof and sides that protect plants from

adverse weather, such as 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 percent. This shade

and the reduced wind speeds significantly lower transpirational water losses; this prevents the scorching known

as winter desiccation. To protect sensitive roots, plants are

blocked together on the ground and surrounded by an

insulating material like sawdust or StyrofoamTM panels

(Figure 7.4.10C).

Greenhouses. Very sensitive plants, such as newly rooted

cuttings, can be overwintered in a greenhouse with miniFigure 7.4.10—Shadehouses are traditional structures that

can be used for hardening and then overwintering container stock (A). They are particularly useful for large stocktypes that must be supported by heavy wire racks (B).

Before freezing temperatures are expected, the containers

should be grouped together on the ground and surrounded by perimeter insulation to protect the roots (C).

A

B

C
115

Figure 7.4.11—Fully-enclosed greenhouses are not good for overwintering, especially in climates with sunny winters (A-B).

Snow removal is necessary in cold climates (B). Retractable roof greenhouses (C) are better for overwinter storage because

they can be opened to allow heat to escape and snow to cover seedlings.

A B

C D

mal heating to keep air temperatures above freezing. It

must be emphasized, however, that greenhouses should

not be considered for routine overwinter storage especially

in areas with clear sunny winters (Figure 7.4.11A).

Greenhouses heat-up rapidly during periods of sunny

weather, causing plants to rapidly lose dormancy (Figure

7.4.11B). In snowy climates, heat must be used to keep

heavy wet snow from building-up and damaging the structure (Figure 7.4.11C). On the other hand, retractable roof

greenhouses (Figure 7.4.11D) are excellent for overwinter

storage because the roof can be opened during sunny

weather to allow heat to escape and keep nursery stock

dormant. During snowfall, the roof can be left open to

allow plants to be covered with a protective layer of snow. 
116

7.4.5 Refrigerated Storage 

The basic concepts of refrigeration and design of refrigerated storage are covered in section 1.3.5.4 of Volume I of

this series so this section will focus on its operational use

in forest and native plant nurseries. Refrigerated storage

has become the standard in many modern forestry nurseries, especially in the Pacific Northwest, and has been

the focus of most storage research.

The two different types of refrigerated storage used in

native plant nurseries are cooler storage and freezer storage; they are differentiated by their temperatures (Figure

7.4.12A) and the recommended duration of storage (Table

7.4.1). A review of nursery research and operational

experience shows that cooler storage is best for periods of

2 months or less whereas, freezer storage is recommended for longer storage periods. Cooler storage is preferred

when nursery stock is outplanted throughout the winter.

For example, in the southern states, cooler storage periods

vary from a week or less in the late summer or fall to as

long as 3 months (Dumroese and Barnett 2004). While no

research has been published on the subject, operational

experience has shown that many broadleaved trees and

shrubs store better in coolers (Davis 1994) (Figure

7.4.12B), and many other native plant species can be

stored this way as well (Table 7.4.2). Some species, such

as black walnut (Juglans nigra) and dogwood (Cornus

spp.), have serious problems with root rots in cooler storage. Considerable variation exists between species, however, so there is no substitute for practical experience. 

Freezer storage has become the standard operating procedure for many commercial conifer nurseries (Hee 1987;

Kooistra 2004), but less is known about how other native

plants tolerate it. Because plant carbohydrate reserves

decrease during refrigerated storage, freezer storage is

recommended for storage durations longer than 2 months;

Table 7.4.1—Comparison of types of refrigerated storage.

Type of Storage In-Container Temperature Recommended Length of Storage Best Type of Packaging

Cooler Storage 1 to 2 °C (33 to 36 °F) 2 weeks to 2 months Kraft-poly bags, 

or cardboard boxes 

with plastic bag liners

Freezer Storage -2 to -4 °C (30 to 25 °F) 2 to 8 months Cardboard boxes with

plastic bag liners

even so, 6 to 8 months appears to be the practical limit

for freezer storage (Ritchie 2004). Although carbohydrate

reserves are conserved better with freezer storage, the primary reason for choosing freezer storage is the reduced

incidence of storage molds. Because freezing converts all

the free water in the storage container to ice, the development of pathogenic fungi such as gray mold (Botrytis

cinerea) is retarded (Trotter and others 1992). After packing, plants should be frozen as quickly as possible to minimize carbohydrate loss and reduce the possibility of

mold development (Kooistra 2004). 

To ensure good air circulation in the storage unit, boxes

of nursery stock are loaded onto pallets and then stacked

onto shelving (Figure 7.4.13A) to improve air flow and

prevent heat buildup. Refrigerated vans (“reefers”) are

sometimes used for temporary storage (Figure 7.4.13A)

but are prone to breakdown and therefore no substitute

for well-designed refrigeration units. Information on

designing and operating refrigerated storage can be found

in Chapter One of this series. 

7.4.5.1 Physiology of Plants in Refrigerated Storage.

Although refrigeration is the most expensive way to store

nursery stock, it offers significant physiological advantages

over other methods. As shown earlier (Figure 7.4.2),

refrigerated storage can even increase the quality of

stored plants. Camm and others (1994) present an excellent over-view of the subject although the authors do not

always distinguish between bareroot and container stock.

Another review of the subject is also informative (Ritchie

1987). More information on all aspects of seedling quality

can be found in Chapter 7.2. 
117

Figure 7.4.12—The actual temperature difference between

cooler and freezer storage is minimal (A), and studies of 

photosynthetic recovery after outplanting show little differences (B) Species differences exist, however, and operational experience has shown that some broadleaves trees

and shrubs do better in cooler storage (C). (A, modifed

from Paterson and others 2001; B, modified from Mattson

and Troeng 1986; C by Thomas D. Landis)

A B

C
118

Dormancy. Most research has been done on bud 

dormancy, and its intensity is measured by days to bud

break (DBB). Refrigerated storage temperatures can partially satisfy the chilling requirement of dormant stock

(Burr and Tinus 1988), and then prolong the release of

dormancy until later in the spring (Dunsworth 1988).

Several studies have proven that freezer storage is as

effective as cooler storage in releasing dormancy (Figure

7.4.14A) provided plants have reached a certain level of

cold hardiness prior to storage. In one study with white

spruce (Picea glauca) (Harper and others 1989), dormancy release in freezer storage continued for as long as 6

months. Cooler storage and freezer storage maintain dormancy equally and plant performance after outplanting

appears to be similar. For example, when the photosynthetic recovery of Scots pine (Pinus sylvestris) seedlings

stored in cooler or freezer storage was measured during

the first season after outplanting, little difference was

noted in outplanting performance (Mattsson and Troeng

1986).

Table 7.4.2—Non-timber native plant storage at Coeur d’ Alene Nursery varies with species and 

outplanting window (Burr 2004)

Scientific name Common Name Packaging Type of Storage Outplanting Window

Alnus rubra Red alder Pull, bag, and box Freezer Spring outplant

Alnus sinuata Sitka alder Pull, bag, and box Freezer Spring outplant

Amelanchier alnifolia Serviceberry Pull, bag, and box Freezer Spring outplant

Arctostaphylos uva-ursi Kinnikinnick Pull, bag, and box Cooler Spring outplant

Arctostaphylos uva-ursi Kinnikinnick Overwinter in Cooler Spring regrowth,

greenhouse Summer/fall outplant

Ceanothus velutinus Snowbrush Overwinter in Cooler Spring regrowth,

greenhouse summer/fall outplant

Menziesia ferruginea Fool’s huckleberry Overwinter in Cooler Spring regrowth,

greenhouse summer/fall outplant

Rosa woodsii Woods’ rose Pull, bag, and box Freezer Spring outplant

Rosa woodsii Woods’ rose Pull, bag, and box Cooler Spring outplant

Rosa woodsii Woods’ rose Overwinter in Cooler Spring regrowth,

greenhouse summer/fall outplant

Salix spp. Willows Pull, bag, and box Freezer Spring outplant

Spirea betufolia White spirea Pull, bag, and box Cooler Spring outplant

Spirea douglasii Rose spirea Pull, bag, and box Cooler Spring outplant

Symphoricarpus albus Snowberry Pull, bag, and box Freezer Spring outplant

Xerophyllum tenax Bear-grass Overwinter in Cooler Spring regrowth, 

greenhouse summer/fall outplant
119

Cold hardiness. Obviously, nursery stock must be cold

hardy to tolerate overwinter storage, but the operational

importance of cold hardiness is its relationship to overall

stress resistance. Cold hardiness tests are routinely used

as a storability index (see Chapter 7.2). Exactly how

refrigerated storage affects the development or maintenance of cold hardiness is an important question, but

unfortunately, no research has been published on container plants. With bareroot stock, however, cold hardiness of

Douglas-fir and Sitka spruce (Picea sitchensis) seedlings in

cooler storage did not harden as deeply as those overwintered in the nursery but still achieved adequate hardiness

(Figure 7.4.14B). Compared to overwintered outside,

these seedlings only dehardened slightly during refrigerated storage and still tolerated freezing of around -15 °C (5

°F) at the time of outplanting (Cannell and others 1990).

Stress resistance. This quality attribute reflects a plant’s

overall tolerance to the many physical and physiological

stresses during harvesting, storage, shipping, and outplanting. Again, little research has been done with container stock, but Douglas-fir seedlings stored under refrigeration showed improved tolerance to low temperatures,

root desiccation, and handling stresses (Ritchie 1986). 

Root growth capacity (RGC). Most studies of new root

growth of plants under refrigerated storage show variable

results with no discernable trends. For example, when white

spruce seedlings were removed from freezer storage at intervals through the winter and potted to observe new root

growth, RGC was found to increase through 3 to 4 months

and then decrease (Figure 7.4.14C: Harper and others

1989). This agrees with Mattson and Lasheikki (1998) who

found that RGC of Siberian larch (Larix sibirica) container

stock decreased after about 4 months of refrigerated storage.

Stored carbohydrates. Once plants are harvested and

placed in dark, refrigerated storage they begin to use

stored carbohydrates, even in freezer storage (Figure

7.4.14D). Carbohydrate reserves are measured as total

nonstructural carbohydrates (TNC) as opposed to structural

carbohydrates which cannot be used for energy. Ritchie

(2004) estimated that conifer seedlings contain from 15 to

20 percent dry weight of TNC when they are harvested,

and they gradually decrease during refrigerated storage.

Obviously, the longer plants are stored under refrigeration,

Figure 7.4.13—To ensure uniform temperatures throughout the refrigerated storage units, boxes must be spaced

on shelving to allow good air flow (A). Portable reefer

vans (B) should only be used for short-term refrigerated

storage.

A

B
120

Figure 7.4.14—Cooler and freezer storage are effective in fulfilling the chilling requirement once plants have attained a certain level of cold hardiness (A) and also have other effects on plant physiology. Compared to outside storage, conifer

seedlings maintained their cold hardiness better under refrigeration (B). Root growth capacity increased for about 4 months,

and then declined (C). Freezer storage slows the decline in stored carbohydrates more than cooler storage, and so is preferable for long-term storage (D) (A from Ritchie (2004); B, modified from Cannell and others 1990; C, modified from Harper

and others 1989; D, modified from Ritchie 1982).

the less energy reserves they have for survival and growth

after outplanting. Because of species differences and the

wide variation of outplanting site conditions, the lower

limit for TNC varies significantly. Coastal sources of

Douglas-fir seedlings are in a critical area when they reach

10 to 12 percent of total dry weight (Ritchie 2004).

7.4.5.2 Handling, thawing, and outplanting frozen stock

Freezer storage is a relatively new practice and nursery

clients have expressed concern about whether frozen

stock can be safely transported. Experience with commercial conifers has shown that frozen seedlings can be

shipped without serious injury (Kiiskila 1999) but, like all

stock, frozen stock should always be handled with care. 

The speed at which frozen plants are thawed has also

caused concern with many nursery customers. Operational

practice had been to thaw slowly but rapid thawing is now

gaining support. In the most comprehensive experiments to

date, Camm and others (1995) studied the physiological

effects of thawing regimes on the physiology of container

A B

C D
121

spruce seedlings. They found no significant differences

between rapid (1 to 2 days at 15 °C [60 °F]) and slow

thawing (17 days at 5 °C [41 °F]). For instance, seedling

moisture stress was found to recover in only 4 to 5 hours

during rapid thawing (Figure 7.4.15A). These positive

results were confirmed by operational trials in British

Columbia (Silim and Guy 1998) that showed rapid thawing

of frozen stock (15 °C [60 °F] for 1 to 2 days) resulted in

less carbohydrate loss and produced better outplanting performance (Figure 7.4 15B). The Nursery Technology

Cooperative at Oregon State University did a similar study

and found no significant differences between slow or rapid

thawing periods or for stock that was rapid thawed and

then held in cold storage (Rose and Haase 1997).

Obviously, common sense must be applied and thawing

should be done out of direct sunlight, but it seems that the

quicker, the better. The problem of increased susceptibility

to storage molds is a rather specious concern because fungi

will develop in cold storage.

Changing weather can shut down outplanting projects

very quickly, which raises the question about what to do

with the thawed plants. No research has been published

on this problem but Ritchie (2004) recommends cooler

storage if the delay will only be a few days but freezing

the stock if several weeks will elapse. The very latest

research involves the direct outplanting of still frozen

seedlings. Comparison outplantings showed that when

frozen container stock was planted it thawed rapidly

without any significant effects on plant growth (Kooistra

and Bakker 2002, Jacobs and others 2008). This can pose

an operational challenge, however, because freezer storage usually results in plants freezing together to form one

large mass. Therefore, direct outplanting of frozen stock

cannot be accomplished unless plants can be easily singulated.

Figure 17.4.15—Although slow thawing of frozen nursery

stock was initially favored, rapid thawing has been proven

to have no adverse affects in terms of plant moisture stress

(A), or other physiological variables. Outplanting trials have

confirmed that rapid thawing is actually beneficial to seedling growth (B) (A, modified from Camm and others 1995;

B, modifed from Silim and Guy 1998).

A

B
122

During overwinter storage, plants can be considered as

being in a state of “suspended animation”—the plants are

alive but their physiological functions have slowed to a

minimum. The critical limiting factor that maintains dormancy during storage is temperature. Therefore, temperature should be rigorously monitored throughout the overwinter storage period (Kooistra 2004). Electronic thermometers with long probes are very useful for monitoring

temperature in storage containers (Figure 7.4.16A). Small

and inexpensive data loggers are self-contained recording

devices that monitor temperature, humidity, and other

weather variables that contribute to plant stress (Figure

7.4.16B). New models like the Hobo are small enough to

place in storage packages where they detect both incidence and duration of exposure (McCraw 1999).

Thermochron iButtons® are even smaller and almost

indestructable (Gasvoda and others 2003). Both can monitor temperature over time and the data can be downloaded to a computer (Figure 7.4.16C). Any thermometer

or temperature recording device must be calibrated annually to make sure that they are accurate; an easy way is to

place the temperature probe in a mixture of ice and water

and the temperature should read exactly 0 °C (32 °F)

(Figure 7.4.16D). 

It is important to measure temperature within the containers as well as in the storage building because the two

locations tell different things. Because stored plants are

still respiring, they generate a small amount of heat,

which means that the in-bag or in-box temperature will

always be a couple of degrees warmer than the ambient

environment. For this reason, the setpoint temperature for

the storage environment should always be 1 to 2 degrees

cooler than the desired temperature in the container

(Kooistra 2004). For example, you may have to operate

with a setpoint of -2 °C (28 °F) to obtain a temperature of

-1 °C (30 °F) in-box temperature. The temperature in the

storage facility should be monitored as well because it

tells whether the compressors are working properly and

good distribution of cold air is occurring (Landis 2000).

After temperature, the next most critical factor to monitor

is moisture. Even hardy, dormant plants can dry out during overwinter storage. This is more of a concern with

evergreen species because they will begin to transpire

whenever exposed to heat and light. As mentioned earlier,

desiccation is a continual threat in refrigerated storage,

especially freezer storage. Even deciduous species can be

damaged so it’s important to check plants occasionally

during the storage period and irrigate, if necessary.

Because water is so heavy, weighing storage boxes is the

most accurate way to monitor moisture loss during storage. Many nurseries measure plant moisture stress with a

pressure chamber prior to and during harvesting (see

Chapter 7.2), and this equipment also affords a quick and

accurate way of monitoring the degree of plant desiccation during storage (Landis 2000).

7.4.6 Monitoring plant quality in storage
123

Figure 7.4.16—Temperature can be monitored with long-stemmed electronic thermometers (A). Small hygrothermographs

like the Datapod® can monitor both temperature and relative humidity (B). Even smaller, the iButton® can monitor temperature for weeks or months and the data can be downloaded to a computer (C). Calibrate any thermometer in a water-ice mixture to make sure they are accurate (D).

A B

C D
124

7.4.7 Causes of Overwinter Damage

Overwinter storage has many potential hazards for stored

plants (Table 7.4.3) and growers should periodically monitor stock for the following:

7.4.7.1 Cold injury

Cold injury can develop from a single frost or during an

extended period of cold weather. Damage is most common in the late fall or early spring, when plants are entering or coming out of dormancy. Cold injury is directly

related to plant dormancy or cold hardiness. The shoots of

native plants that have been properly hardened can tolerate freezing temperature extremes expected in geographic

area from which they originate but cold hardiness and

dormancy are lost as winter progresses. The lateral meristem at the root collar, can be injured by frost (Figure

7.4.17A) as can the buds. This type of injury is very difficult to diagnose without destructive sampling because

symptoms may not become evident until later in the

spring.

Root systems need special protection because they are

injured at much higher temperatures than shoots.

Furthermore, young roots are less hardy than mature roots

and will be injured at higher temperatures than mature

roots. Rooted cuttings are particularly vulnerable to injury

because their roots have not developed protective layers yet.

Young roots are typically on the outside of the container

and are the first to be injured by cold temperatures (Figure

7.4.17B). Where freezing temperatures occur, cold injury to

roots is the most common type of overwinter damage.

Because shoots do not show symptoms immediately, root

injury often goes unnoticed and the damage becomes evident after outplanting. Therefore, growers should design

their overwintering systems to protect all roots from damaging temperatures during overwinter storage. 

7.4.7.2 Desiccation

Winter drying is actually desiccation injury and occurs

whenever plants are exposed to extreme moisture stress,

especially wind and/or direct sunlight (Figure 7.4.17C).

Damage is most severe when the growing medium and

roots remain frozen for extended periods. Plants can even

become desiccated when they are stored under frost-free

refrigeration without proper packaging. Winter drying is

not directly related to plant dormancy or cold hardiness—

even the most dormant and hardy stock can be damaged.

Plants stored near the perimeter of open compounds or

sheltered storage are most susceptible (Figure 7.4.17D)

but even plants covered by snow can be damaged if their

tops become exposed. This type of desiccation can be

prevented if open or shelter-stored stock can be irrigated

during the winter storage period, and effective perimeter

insulation is used. 

7.4.7.3 Loss of dormancy 

This type of injury is most common when container stock

is overwintered in greenhouses. Often during winter periods of clear, sunny weather, greenhouses heat up and

cause plants to lose dormancy. Loss of dormancy

becomes progressively more serious during late winter

and early spring when plants have fulfilled their chilling

requirements and cold temperatures are the only thing

keeping them from growing (Figure 7.4.17E). Although

refrigerated storage is the best prevention, using white or

reflective coverings in structureless storage reflects sunlight and prevents heat buildup. Monitor temperature frequently in sheltered storage and ventilate if necessary. 

7.4.7.4 Storage molds

The type of storage conditions will determine the types of

disease problems that will be encountered. Although fungal diseases can be a problem in open storage or shadehouses, they are most serious when plants are overwintered under refrigeration (Table 7.4.3). Some fungi, such

as Botrytis cinerea, actually prefer the cold, dark conditions in storage bags and boxes and will continue to grow

and damage plants whenever free moisture is available

(Figure 7.4.17F). Some nurseries apply fungicides before

overwinter storage but careful grading to remove injured

or infected plants is the best prevention. Freezer storage

has become popular because it prevents the further development of storage molds. Much more information is provided in section 5.1.6 in Volume Five of this series. 
125

Figure 7.4.17—Overwinter storage is a time of considerable

risk for nursery stock: Cold temperatures can damage nonhardy tissue, even buds (A). Roots are particularly susceptible

because they will grow whenever temperatures permit (B).

Winterburn (C) is actually desiccation and is particularly

severe around the perimeter of storage areas (D).

Overwintered plants gradually lose dormancy and can break

bud during late winter or early spring (E). Storage molds (F)

are most serious in cooler storage whereas animal damage can

be a real problem in sheltered storage (G).

A B C

D

E F

G
126

Table 7.4.3—Plants can be injured by several types of stresses during overwinter storage.

Type of Damage Cause Preventative Measures for Types of Storage

Open Sheltered Refrigerated

Cold injury Temperatures below plants’ Properly harden plants to tolerate maximum

(Fig. 7.4.16A-B) cold hardiness level expected cold temperatures

Roots are much more 

susceptible than shoots.

Drought injury Exposure to intense sunlight and Fully saturate media before storing

(Winter desiccation) especially drying wind

(Fig. 7.4.16C-D) Shade plants Cover stock Use moistureand protect with moisture- retentive

from wind retaining film packaging

Loss of dormancy Temperatures above 5 °C (40 °F) Not possible Monitor and Maintain cold

(Fig. 7.4.16E) ventilate as in-box

needed temperatures.

Storage Molds Warm temperatures; Prevent injury to plant tissue,

(Fig. 7.4.16F) latent infections of Botrytis cull damaged plants

Keep foliage Keep foliage Use freezer

cool and dry cool and dry storage if more

than 2 months

Animal Damage Small rodents and even rabbits Exclude larger animals with Not a 

(Fig. 7.4.16G) can girdle stored nursery stock fencing; use poison baits problem

for rodents

7.4.7.5 Animal damage

The only type of overwinter storage where animals will

not pose a threat is refrigerated storage. Small rodents,

such as mice and voles, can be pests in shadehouses and

structureless systems because the pests are protected from

natural predators and from harsh weather conditions.

Baiting or trapping to keep populations low can be effective if started early in the season. Large animals, such as

deer and rabbits, can be pests in open, structureless, and

shadehouse storage, but fencing is an effective way to

prevent damage. See section 5.1.6 in Volume Five of this

series for more specific information.
127

7.4.8 Summary and Conclusions

Non-dormant plants destined for nearby outplanting may

go from the nursery to the field with little or no storage

(hot-planted). More commonly, dormant plants are stored

during winter awaiting optimum ouplanting conditions.

Storage becomes more important as the distance from the

nursery to the outplanting sites increases, differences

between climates at the nursery and field sites are great,

and nurseries produce large quantities of plants requiring

months to process. Therefore, storage is an operational

necessity rather than a physiological requirement. 

Overwinter storage should be developed to meet local

climate, plant type, and production factors. In general,

three types of overwinter storage are used: open, structureless, and structured. In open storage, plants are left

outdoors, on the ground, and protected from sun and

wind by larger trees and snowfall. Plants stored in a structureless system are also outdoors and on the ground, but

are protected from the vagaries of winter weather by various applications of plastic and/or Styrofoam sheets.

Structured storage can be very simple, such as a cold

frame, progressing through modest structures such as

polyhouses and shadehouses that provide some climate

control, to the most complex systems: refrigerated units.

Refrigerated storage includes coolers (temperatures just

above freezing), which are best for short-term storage of

plants (2 weeks to 2 months), and freezer storage (temperatures just below freezing), which is best for long-term

storage (2 to 8 months).

Regardless of the type of storage used, plants should be

regularly monitored to ensure that pests (animals and storage molds) are not becoming a problem, temperatures are

in the proper range to keep plants dormant, and medium

moisture is appropriate to avoid dessication.

After storage, plants should be shipped carefully to the

field. Stock kept in freezer storage can be safely shipped

frozen, but if thawed at the nursery, the thawing process

should be rapid to reduce carbohydrate losses and development of storage molds.

Successful storage of container plants is one of the most

challenging and important aspects of nursery management. Many types of over-wintering systems can be

employed depending on location, climate, and species

grown; more than one system may be used at a nursery.

Determining when it is safe to harvest plants so that they

will maintain a high level of quality throughout the storage period and to the outplanting site is one of the most

challenging aspects of nursery management.
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