Tidewater Gardening November 2006

Winter Hardiness


K. Marc Teffeau

     As we move into the late fall the deciduous plants in the landscape have gone into dormancy. In our temperate area, these woody plants need to go through this process to protect them from the sometimes harsh winter weather. My wife compares this to the stage men enter into as they sit in the mall waiting for their wives.
      By late autumn many woody and perennial plants are physiologically dormant and can withstand much lower temperatures than if they weren’t dormant. For instance a Norway maple can withstand a temperature of -30 degrees F when it’s dormant. During the growing season, however, temperatures just below freezing can cause extensive damage such as twig dieback. Some maintenance practices encourage plants to go dormant for example refraining from late summer tree fertilization combined with pruning.
      There are a couple of different factors that trigger plants to go into winter dormancy. One is the reduced day length or a reduced photoperiod. When the days begin to get shorter, a point is reached, usually in early October when the shorter day lengths trigger the plant to begin to go dormant. No matter how much heat is applied after this trigger has been pulled, the plants cannot be kept from going dormant. As you know, we usually still get some rather warm days in November (Indian Summer). The advantage to plants where day length triggers dormancy is that these sudden upswings in temperatures will not trigger regrowth or sprouting.
      Another factor that triggers dormancy is the drop in temperatures over time. However, plants whose cellular processes rely on temperature changes are more susceptible to warmer temperatures. If we have an abnormally warm fall, many plants that rely on cool temperatures to trigger dormancy can be killed when the temperatures suddenly drop. For example, when an early Canadian “High” weather system comes in unexpectedly in November, these plants suffer.
      Some plant parts are more cold hardy than others. Cold temperatures which destroy peach blossoms may do very little, if any, harm to the peach tree. The above ground parts of plants can withstand much lower temperatures than the roots. For instance boxwood stems may tolerate -10 degrees F where as the roots can be killed at 20 degrees F. Hardy woody plants and perennial flowers may be killed in containers left outdoors all winter.
      Some of our gardening practices may contribute to winter damage. If you fertilize wood ornamentals in the late summer, this may promote new leaf and twig growth that is not cold hardy. Late summer pruning may also promote new growth which is quite tender, and is easily killed since it has not become acclimated to the cold temperatures.
      The advantage to November in our area, however, is that it is usually fairly mild so we can still work in the yard. You can still do some late fall planting of trees and shrubs in the landscape. Woody plant roots will still grow as long as the soil temperature remains above 40 degrees F. In fact, one of a plant’s responses to oncoming winter weather is to store more sugars in the trunk and root system. These sugars are available to encourage new root formation.
      Avoid transplanting shrubs and trees on sunny or windy days however. On these days, the roots are exposed to too much light or drying winds, putting undue stress on the plant. Where circumstances necessitate very late planting of trees and shrubs, remember to mulch the area heavily to keep the ground thawed so roots can become established. “Mulching heavily” does not mean volcano mulching where the hardwood chips are piled 8 inches up the tree trunk! A three inch layer is sufficient to maintain even soil moisture and temperatures and promote late season root growth.
      Check the guy wires around newly planted trees to be sure hose sections still cover the supporting wires or ropes so they will not damage the trunks in windy weather.
      Some fall seasons in our area tend to be dry. If this occurs, it is very important to make sure that broad and narrow-leafed evergreens have enough soil moisture going into the winter. It is important to continue deep watering of evergreens until freezing weather occurs. This means that you will need to water them once every week or so until the ground freezes.
      Evergreens continue to lose moisture from their foliage all winter, but once the ground is frozen, they’ll be unable to take up enough water to replace it. Sending them into winter well watered reduces the potential for damaged foliage. Broadleaved and tender evergreens exposed to drying winds and sun may need to be shaded on the south and southwest sides to reduce moisture loss and foliage injury.
      If you use woody ornamentals as container plants, for example for patio plantings, don’t forget to move them to a protected location. Roots of woody ornamentals used as container plants may be killed if soil temperatures get very cold. Among the least hardy are aucuba, English boxwood, camellia, pampas grass, bearberry, cotoneaster, English holly, Japanese holly, star magnolia, and nandina. Their roots are killed when the soil temperature is 20 to 25F.
      After the woody plants have gone dormant, you can do some clean up pruning. Cut away suckers from the base of lilacs, forsythia, and crape myrtle. Leaf fall makes renovation of overgrown deciduous shrubs easier.
      Begin this year by removing all diseased or broken stems. Next, remove 1/3 of all remaining shoots, eliminating the oldest and tallest. If the bush is still too tall, cut the remaining stems to a side bud or branch. Repeat the process a second or third year to complete renovation. You can also trim hollies and other evergreens, such as magnolia, aucuba, boxwood, and pyracantha, to furnish material for Thanksgiving decorations.
      Clean up rose beds. Be sure all diseased leaves are raked up and destroyed. Spring (before the plants start active growth) is the preferred season for pruning roses. Do not cut off canes in the fall. It is better to stake and tie extra long canes in fall to prevent winter wind damage.
      Inspect trees and shrubs for bagworm capsules and the silvery egg masses of tent caterpillars. Remove and destroy them to reduce next year’s pest population. If there is any evidence of scale on trees and shrubs, spray with dormant oil in late fall and again in early spring.
      After several killing frosts have occurred in November, cut back dormant perennials to about 3 inches above ground. After the ground is frozen, plants can be mulched to guard against displacement due to soil heaving. These steps ensure a successful show of plant foliage and color next season. As soon as chrysanthemums are through flowering, remove the stalks to within a few inches of the ground. This will help root development and make plants send out vigorous sprouts in the spring. Some may be lifted and heeled into the coldframe.
      Plants for potting can be propagated from the side sprouts that will develop next May. Dispose of stems and all dropped and dried leaves and branches.
      Reduce peony botrytis blight and hollyhock rust by removing and disposing of all old stems this fall. This reduces the carry-over of the diseases during the winter, and you will have less trouble next year.
      Happy Gardening!!!