Winston C. Dunwell & Robert E. McNielUniversity of KentuckyDepartment of Horticulture

More container and B & B stock are being stored. Continuing attempts to lengthen the season has lead to plants being dug both later and earlier than normal. B & B or container plants left exposed to the elements for even short times can suffer root injury from cold temperatures. Year-round landscaping has compounded the problem with the need for readily available plants regardless or the time of year.

Plants develop the ability to survive winter temperatures following exposure to shortening days and lower temperatures (acclimation). In order to develop maximum tolerance to cold the plants must be exposed to freezing temperatures. Ultimately, if exposed to consistently lower temperatures, without sudden damaging drops or swings up and down, many plants are able to tolerate very cold temperatures. Of course, temperatures do not consistently drop in our area but vary up and down dramatically, often with devastating results, such as the fall damage to azaleas where the bark separates from the stem. Cultural practices, such as fertilizing, watering, and pruning impact a plants ability to acclimate. Any practice that stimulates late season growth should be avoided.

Above ground portions of selected trees when properly acclimated have been known to survive when submersed in liquid nitrogen (-320F). The USDA Plant Hardiness Zone Map or the Arnold Arboretum Map define shoot hardiness. While shoot hardiness of temperate plants is important when making overwintering decisions, roots rarely, if ever, will survive temperatures below -10F, therefore, root hardiness should be our primary concern related to making overwintering decisions. Young roots of some plants do not seem able to acclimate at all, mature roots normally are able to tolerate more cold than young roots. Dr. John Havis reported that the loss of young roots may not be critical to subsequent plant survival. Studer, et al divided roots into two groups, young and mature, and found mature roots were 5-10F more hardy than young roots. All the known research done to measure root hardiness has been done with container stock and may not give exact temperatures when considering B & B stock dug and left in the field, but observations have indicated that the temperatures reported in Table 1 are relatively accurate when applied as a hardiness rating of B & B stock. Table 1 has been compiled from the articles by John Havis, Peter Steponkus, et al, and Studer, et al. It can be used as a guide, but as genotypic differences in hardiness can occur some data may only be valid for the plants tested. Different geotypes occur, therefore, the location of the plant from which seed or cuttings were collected can effect hardiness significantly within species. Ideally, plants that are dug, or received by the Garden Center or Landscape Contractor, should be from an environment similar to theirs and when received should be protected if temperatures near or below freezing are expected.

Experienced nurserymen often find certain balled and burlapped plants cannot be left in the field overnight if the temperature is going to be below freezing, while other plants such as junipers can be left out in the field. Holding or staging areas for loading trucks can have straw or mulch available to put around the root balls stacked closely together. With the current trend to load trailers directly in the field, using a Tree Boss, covers should be put over plants as soon as possible.

The nurserymen has to deal with several questions when considering overwintering nursery crops. Some of the questions are: what type of plant are we trying to store? is it a container plant, balled and burlapped, bareroot? is the plant sensitive to cold? what part of the plant is most likely to be damaged and how do we protect it?

Containers are typically put in quonset-type greenhouses. The more northern the location of the greenhouse the narrower, or if wide, the taller the house in order to ensure the snow will slide off an unheated overwintering house. Overwintering quonsets traditionally have been 14 feet wide and hundreds of feet long, then 20 feet wide seemed more frequently used, but some nurserymen use standard greenhouses 28-30 wide by 96 feet or longer. Attempts are made, where feasible, to orient the houses south to north. Plants in the southeast corner, may receive more light and day length, requiring more frequent observation. With plants that are particularly cold sensitive a poly blanket should be set in the house so that if abnormally cold temperatures are encountered (below 0o F or less) it can be pulled over the plants to provide a second layer of cover. Ultimately, for sensitive plants some heat may be required. Balled and burlapped plants are traditionally heeled-in. Bareroot plants for winter storage have definite requirements. Proper acclimation is critical. Perennials, while know to be tough, can be difficult to store bareroot for spring mail-order sales or potting, especially the "evergreen" types, until experience is obtained

Another question is what market is the plant destined for when removed from storage. Azaleas are a plant that is placed in a controlled environment so that plants are shipped to market in bud or starting to bloom. Azaleas past peak are hard to sell. Other spring flowering plants may not be as critical, but they do sell better when in bud or early to peak bloom at the retail market.

Watering is important. Desiccation is the number one stress related to freezing. Reduced watering schedules can result in neglect due forgetfulness because of the lack of regular schedules, leading to failure to provide adequate water to the plants in mid-winter. Cool or frozen soils inhibit water movement to plant tops that need water on a warm day. Ice between cells pulls water to it reducing the water available in the cells. Plants stored outdoors with the containers or root balls protected but the tops exposed can suffer wind dessication. Be sure to water plants during acclimation and when put into storage. Check on plants for water need throughout the storage period and be especially diligent in watching those plants nearest the walls of the greenhouse. Clay pots are especially susceptible to drying out to the point where I would not recommend they be stored in the Microfoam with a poly cover directly over plants placed on the ground because they will require numerous winter irrigations.

Methods of Overwintering

1. Push pots together with protection at the edges2. Mulching3. Microfoam with a poly cover directly over plants placed on the ground4. Greenhouse with single-layer poly (white poly)5. Greenhouse with double-layer poly6. Greenhouse with double-layer poly and a poly blanket7. Greenhouse with double-layer poly and a microfoam blanket8. Greenhouse with double-layer poly and heat

References for Table 1.

Havis, J. R. 1976. Root Hardiness of Woody Ornamentals. HortScience 11 (4):385-39.

Steponkus, P.L., G. Good, and S.C. Wiest. 1976. Root Hardiness of Woody Plants. American Nurseryman 144 (6):16

Studer, E. J., P.L. Steponkus, G.L. Good, and S.C. Wiest. 1978. Root Hardiness of Container Grown Ornamentals. HortScience 13:172-174.

References of Interest

Cuny, Holly. 1997. Container Solutions through Efficiency. NMPro, August, pp. 49-53.

Cuny, Holly. 1997. Southern Hoop Houses. 1997. NMPro, September 1997, pp. 41-43.

Gouin, Dr. Francis R. 1974. A new concept in overwintering container-grown ornamentals. American Nurseryman December 1, 1974, pp. 7-8, 45-50.

Lindstrom, Orville M. and Michael A. Dirr. 1989. Acclimation and Low-temperature Tolerance of Eight Woody Taxa. HortScience 24(5):818-820.

Morgan, David. 1997. Seedling Survival. NMPro, October 1997, 37-39.

Sakai, A., L. Fuchigami, and C. J. Weiser. 1986. Cold Hardiness in the Genus Rhododendron. J. Amer. Soc. Hort. Sci. 11(2):273-280.