NURSERY CROPS PRODUCTION MANUAL Preparing Nursery Plants for Winter AG-454

Part 2: PROTECTION TECHNIQUES

Field Nurseries

Select a site that reduces plant damage during winter months by avoiding excessive wind, frost pockets, rodent populations, and abnormally early warming in the spring or during winter thaws.

Barriers can help prevent wind bum if placed properly. Windbreaks raise the wind and reduce air movement (Figure 6). A windbreak will protect plants on the leeward side (away from the wind) for a distance of five times its height. Poorly placed windbreaks can create swirls, causing more damage than protection. Plants that are dug and overwintered need to be placed in shade with the rootballs heavily mulched to protect the root system from freezing temperatures (Figure 7).

Seedbeds and Liner Nurseries

Small or recently set plants usually have a reduced root system. Because of their size and small root system these plants are more likely to be "heaved" out of die ground, dry out, and die during the many freeze and thaw cycles of a normal western North Carolina winter. Fewer frost-heaving problems occur in piedmont and coastal plantings.

To safeguard against frost-heave damage, these plants are usually mulched heavily during the first winter after germinating or transplanting. Soils with a higher clay or organic-matter content seem to be more prone than sandy soils to frost heaves. Best results are obtained by mulching with 6 to 8 inches of hardwood leaves, pine needles, or clean straw after the plants are fully dormant. Covering plants with a single layer of spun-bonded polyester or polypropylene fabric has provided good winter protection as well (Figure 8). However, heat may build up under these fabrics, causing plants to break dormancy early.

Container Nurseries

Growing plants in containers presents many special winter survival problems. The two major problems involve drying out and a lack of root hardiness. Winter desiccation, or drying out, is the most common winter injury of container grown evergreen nursery crops. If broadleaved evergreens are not watered adequately, they often turn bronze and their shoots later die. When temperatures remain below freezing for an extended period of time, the root ball can freeze completely in containers making water unavailable to the roots. To remedy the problem, irrigate adequately during the winter. When cold, sunny, or windy conditions are forecast, irrigate before the arrival of the weather front. Irrigation increases plant turgor and helps the plant move water through cell membranes. Also, if containers are irrigated to their capacity, an additional thermal resistance to freezing is provided. Plant roots are not as hardy as shoots. Roots will die at much higher temperatures than the above-ground portions of the plant (See Table 1). Plants that endure freezing temperatures in the landscape may have roots that would normally be killed at temperatures of 20^OF to 25 ^OF. In North Carolina, soil rarely drops below 20⁰F, except in the very upper-most soil portion. The soil provides insulation during low ambient air temperatures. Roots in unprotected containers are much more vulnerable to freezing temperatures.

Avoid laying plants over on their side for long periods of time during the winter. In light conditions, buds and shoots will turn upward and the result will be asymmetrical growth. If dormant trees are laid over and exposed to full sunlight, sunscalding on the main branches and trunk may also occur. This type of winter damage is often mistaken for mechanical injury.

A variety of winter protection techniques have been used successfully for container-grown plants. For example, in western North Carolina, nursery operators that grow only very hardy container plants may cluster them together in a sheltered, shaded location, mulching over the tops of the containers and placing bales of straw around the perimeter of the clustered pots (Figure 9). The bales and mulch trap the air during the day, and when the temperatures drop at night, the air trapped around the containers remains warmer than the air around the tops of the plants. Shade also protects the leaves of evergreens from sun and wind, reducing water loss on bright, cold days. Spring frost burn, which occurs when the sun shines on frozen or frost covered leaves, is also prevented by shade.

In the piedmont and eastern North Carolina, container grown plants are pushed tightly together in blocks. Some nursery operators wrap the container blocks with plastic or paper to reduce air movement between containers (Figure 10).

In recent years, many nursery operators have experimented with structureless winter protection methods. In hardiness zones 8 and 9, they have successfully protected plants by preparing them as if they were going to be placed into a structure, then laying a cover of white copolymer film or thermal blankets over the top of the plants. The sides are securely fastened and the cover checked to make sure it is unpunctured (Figure 11). Problems with this technique have been the abrasion of plants from the plastic flapping in the wind, breakage if a heavy ice or snow storm occurs, and heat and moisture buildup under the cover. Other growers have used the structureless system, covering plants with shade cloth or fabric (Figure 12).

Research with porous row cover fabrics indicates that they protect some nursery crops as well as if the crops were placed in winter protection structures. Shade cloth or row cover fabrics reduce sunlight and wind movement around evergreen and broadleaved evergreen plants. This reduces desiccation and discoloration of foliage, leaving greener plants with greater sales appeal for early spring marketing. During periods of bright, sunny, warm days, remove the fabrics but keep them accessible. Removal helps reduce early shoot development.

Many nursery operators build temporary "ag454-wintering" structures (Figure 13). Orientation of wintering structures covered with white copolymer plastic is not as critical as with clear plastic. However, houses oriented north to south will be somewhat cooler than those facing east to west. Plants that are fully dormant or have hardened are placed in these structures, which are then covered with a plastic film. To ensure the greatest degree of hardiness, do not cover houses until the onset of extended cold winter temperatures is imminent.

Figure 13. An example of a typical polyhouse.
Click on a section of Figure 13 to view a detailed enlargement.

Approximately six weeks to a month before covering, apply preemergence weed controls. If slow-release fertilizers were used during the growing season, conduct a test to determine the salt levels in the containers. You may use a procedure called the Virginia Tech Extraction Method (VTEM) to test the conductivity (salt level) in containers. Irrigate the containers before collecting the leachate. Approximately 5 fluid ounces (350 milliliters) of distilled water is required to obtain a leachate from a 1-gallon container; approximately 12 fluid ounces (350 milliliters) is needed for a 3-gallon container. If a VTEM pour-through leachate collection is made, salt levels of the leachate should be below 0.5 millimhos (50 Mhos). If the medium is taken from the container to do a salt test, measure 50 cc of medium and add 100 ml of distilled water. The conductivity (salt) reading should be less than 0.2 millimhos (20 Mhos). If conductivity levels are higher, leach the containers by applying approximately 1 inch of irrigation. Check the containers during the winter and do not let them become excessively dry. Random salt testing of containers in winter-protection houses may indicate the need for further irrigation.

Shortly before they are covered, plants must be thoroughly watered and sprayed with a fungicide to prevent infection by diseases that are active at the low temperatures and high humidities found in wintering structures. Once the plant foliage has dried, the structures may be covered. The most popular covering material in North Carolina is 4- or 6- mil white copolymer plastic film. The white film provides shade, while preventing rapid temperature changes within the house. Houses covered with clear plastic film are hotter during the day and colder at night than those covered with white copolymer film. These large temperature changes are responsible for damage to plants in houses covered with clear plastic film.

Make arrangements to ventilate overwintering structures. A ventilation fan activated by a thermostat and mounted on the leeward end of the house, with louvers on the windward end, will provide the most consistent ventilation. If ventilation is provided by opening end doors, block the air movement at plant height and direct ventilation to the upper portions of the house (Figure 14). This reduces the air movement around plants. Some growers ventilate houses by cutting progressively larger holes in the film on the sides of the houses.

Operators of nurseries located below 2,000 feet in elevation who ventilate their clear-plastic-covered greenhouses on hot days and irrigate regularly during the winter can successfully overwinter plants. Less hardy plants or colder locations may require the use of supplemental heat or greater insulation as provided by devices such as thermal blankets or inflated double-poly houses.

All nursery operators who protect plants in wintering houses must make provisions for snow and ice. Unless overwintering structures have sufficient structural strength, they may collapse during a snowstorm. (Figure 15). Plants that have been crushed and broken by tons of snow are an expensive loss that can be prevented by adequate planning.

Figure 15. A winter protection house collapsed by snow.

In spring, remove covers as early as possible so that heat buildup under the cover does not result in excessive bud swelling. However, remove covers late enough to avoid subfreezing conditions in which root and shoot damage occurs. If active shoot growth begins in enclosed structures, uncovering will cause frost damage to new shoots unless plants are left covered until after the last expected frost date. Leaf and shoot expansion under the low light conditions of white copolymer film will be wide and thin. When the film is removed, the new growth must be shaded for several weeks to prevent sunscalding. Dates for covering and uncovering vary from one location to another and from one year to the next. Growers must develop an intuitive feeling for these activities.

Sources of Additional Information

Principles, Practices and Coniparative Costs of OverMntering Container-Grown Landscape Plants. David J. Beattie, Editor. Southem Cooperative Series Bulletin 313. May, 1986. Pennsylvania State University, Agricultural Experiment Station, University Park, Pa.

Weather and Climate in North Carolina. North Carolina Cooperative Extension Service, Raleigh, NC 27695. Bulletin AG-375. $2.50.

Wright, R.D. 1987. The Virginia Tech Liquid Fertilizer System for Container-Grown Plants. College of Agriculture and Life Sciences. Information Series 86-5.

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