Proper acclimation is critical for plants to survive over winter. Plants develop the ability to survive winter by exposure to shortening days and lower temperatures. Plus, a plant is only as hardy as its roots and roots are much less cold hardy than the shoots. If exposed to consistently lower temperatures, without sudden damaging ups or downs, many plants are able to tolerate very cold temperatures. Cultural practices such as fertilizing, watering, and pruning stimulate late season growth and should be avoided.

Nurseries continually try to lengthen the growing season to meet the need for readily available plants. This has led to plants being dug earlier and later than normal and more container and B&B stock being stored outdoors over winter. These plants left exposed to the elements for even short times can suffer root injury from cold temperatures.

Although people are accustomed to using USDA hardiness zones for evaluating the survival potential of a mature plant in a given region, this is not a very useful approach for container grown stock. The reason is that root hardiness, a key factor for overwintering container plants, does not consistently correspond to shoot hardiness. Plants with similar USDA hardiness ratings often have widely divergent root hardiness. Therefore, the USDA ratings may be of little use for predicting their success in containers. For example, flowering dogwood, Cornus florida, is hardy to Zone 5, but its roots die in the range of 11° to 21° F (-11° to -5° C), depending on root maturity. Juniperus conferta, with roughly similar shoot hardiness, will not start to lose roots until soil temperatures drop to around 12° F (-10° C), and larger roots can survive down to about -10° F (-23° C). Further, plant material derived from a warmer region may have less cold tolerance than specimens of the same species from a colder region.

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 the chart below are relatively accurate when applied as a hardiness rating of B&B stock.

Water Requirements

Desiccation is the number one stress related to freezing. Plants stored outdoors with their root balls protected but with their tops exposed can suffer wind desiccation. Reduced watering schedules can result in a failure to provide adequate water to the plants in mid-winter. Cold or frozen soils inhibit water movement to plant tops that need water on a warm winter day. Therefore, be sure to water plants when put into storage and check for water need throughout the storage period.

Desiccation is also a threat to container plants in winter. Ground temperatures below the top few inches of soil generally do not drop below the 20° to 30° F (-6° to -1° C) range, which many plants easily tolerate. However, in containers the soil more closely reflects air temperatures. Therefore, it gets much colder and may freeze completely. Freezing, per se, does not necessarily kill roots, but it does prevent them from taking up water.

Many evergreen conifers may tolerate winter water stress better than most broadleaf evergreens. Deciduous plants are less vulnerable, though not immune, to winter water stress. Be sure to check container soil periodically and add water if necessary. Watering is important because water provides a source of heat. Liquid water has a temperature above 32º F (0° C) and when it freezes it gives off heat. Because of this, dry soil freezes much more quickly than wet soil.

Other Hardiness Factors

Larger containers are less prone to temperature swings than smaller containers. Larger mature roots tolerate much lower temperatures than small roots. Thus, older and larger specimens should survive better than younger plants. Furthermore, containerized plants cannot take advantage of the temperature buffering effect of the soil. In a nursery, push all the containers close together and provide protection on the perimeter; then cover all the root balls with mulch.

Winters vary in temperature, precipitation, snow cover, and wind, all of which can affect a plant's survival. Therefore, do not be surprised if a container plant that has made it through a few winters finally succumbs in an especially harsh one.

The key to overwintering is keeping the plants cold and alive but not actively growing. The degree of winter protection should be based on the expected minimum temperatures at a particular site and the root hardiness of the species being grown. Unfortunately, root and crown hardiness in most herbaceous perennial plants has not been clearly identified by researchers. Much of the current information is based on limited grower experience. As a result, most growers provide more protection to plants than is necessary. Until root hardiness is identified, growers must take steps to avoid losses of plants.

Preparing plants for successful overwintering begins when plants are potted. Most plants should be potted up by early October and allowed to establish themselves for several weeks (early November) prior to the ground freezing. The better its root system, the better the chance a plant will survive the winter.

Soil temperatures should be in a range of 30° to 34º F (-1° to +1° C) for most plants. Temperatures cooler than 30º F may kill some sensitive species. Use a soil thermometer to verify that plants are in this range. Fluctuations always occur during winter. If temperatures warm up for several days above 40º F (4° C), ventilation should be provided. The more that plants are allowed to freeze and then thaw, the greater the risk for plant loss.

A plant is capable of surviving even if it loses most of its immature roots to winter injury, but it cannot lose its mature roots. Delayed spring growth, stunted spring growth, and death of branch tips are all indications that plants have suffered the loss of immature roots. Repeated loss of primary roots from winter injury also means that a plant's ability to recover and produce normal growth can be significantly reduced.

Sources

• Dunwell, Winston C., and Robert E. McNiel, "Overwintering Nursery Crops" University of Kentucky, College of Agriculture, 2005.

• Gouin, Dr. Francis R., "Better Growing" University of Maryland, College Park, August, 2005.

• Krause, Charles, "Root Hardiness and Influence of DNA Herbicides in Overwintered Containers" USDSA Research project, Research 2003 – 2008.

• Liskey, Eric, "Researching Maintenance", Grounds Maintenance Magazine, July 1, 1998.

• Smith, Tina M., "Overwintering Containerized Perennials", UMass Extension, Amherst, MA, 2003.

This chart applies to container-grown ornamentals only.

Root-killing temps
Botanical Name	° F	° C
Acer palmatum 'Atropurpureum'	15	-9.4
Buxus sempervirens	27	-2.7
Cornus florida	22	-5.6
Cotoneaster adpressus var. praecox	12	-11.1
Cotoneaster congestus	25	-3.8
Cotoneaster dammeri	23	-5
Cotoneaster horizontalis	17	-8.3
Cryptomeria japonica	17	-8.3
Cytisus x praecox	16	-8.9
Daphne cneorum	23	-5
Euonymus alatus	19	-7.2
Euonymus fortunei 'Carrierei'	15	-9.4
Euonymus fortunei 'Coloratus'	5	-15
Euonymus fortunei 'Vegetus'	23	-5
Hedera helix 'Baltica'	15	-9.4
Hypericum species	23	-5
Ilex crenata 'Convexa'	23	-5
Ilex crenata ' Dazzler'	25	-4
Ilex crenata 'Helleri'	23	-5
Ilex crenata 'Hetzii'	23	-5
Ilex crenata 'Stokesii'	23	-5
Ilex glabra	16	-8.9
Ilex opaca	23	-5
Ilex 'Nellie R. Stevens'	23	-5
Ilex x merserveae	23	-5
Juniperus conferta	12	-11.1
Juniperus horizontalis 'Douglasii'	0	-17.8
Juniperus horizontalis 'Plumosa'	12	-11.1
Juniperus squamata	12	-11.1
Kalmia latifolia	15	-9.4
Koelreuteria paniculata	16	-8.9
Mahonia aquifolium	10	-12.2
Mahonia bealei	23	-5
Magnolia x soulangiana	23	-5
Magnolia stellata	22	-5.6
Pachysandra terminalis	15	-9.4
Picea glauca	-10	-23
P. omorika	-10	-23
Pyracantha coccinea 'Lalandei'	23	-5
Rhododendron carolinianum	0	-17.8
Rhododendron catawbiense	0	-17.8
Rhododendron prunifolium	20	-6.7
Rhododendron schlippenbachii	15	-9.4
Rhododendron "Exbury Hybrid"	17	-8.3
Rhododendron 'Gibraltar'	10	-12.2
Rhododendron 'Hinodegiri'	10	-12.2
Rhododendron 'Hino Crimson'	19	-7.2
Rhododendron 'P.J.M.' hybrids	-9	-23.3
Rhododendron 'Purple Gem'	15	-9.4
Taxus x media 'Hicksii'	-4	-20
Taxus x media 'Nigra'	10	-12.2
Thuja occidentalis	10	-12.2
Viburnum carlesii	15	-9.4
Viburnum plicatum var. tomentosum	20	-6.7
Vinca minor	15	-9.4

References for Chart

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

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

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

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