Agriculture Department

Chesapeake Master Gardener Tips

January 2000

Virginia Cooperative Extension, Chesapeake Office

Some Like It Hot ... Some Like It Not!

When we look at the catalogs that are coming in the mail these days, we are mesmerized by the beautiful pictures and forget that there are many things that a plant requires. One of these is the ability of a plant to survive in the weather conditions of the area in which it is to be planted. These conditions should be considered when selecting a plant.

Most gardeners are aware of the U.S. Department of Agriculture's Plant Hardiness Zone Map. This is the map that gives an indication that plants may or may not be able to "winter over" in the area. In Tidewater, we are on the edge of the Hardiness Zones 8 and 7. Zone 8 has the warmer temperatures. While the clever gardener may select a sheltered spot and so successfully grow a plant that is considered to be a zone 8/9 plant, a watchful eye should be kept on the winter temperatures and the plant in question.

For a plant to survive and prosper in any given region, it must acclimate (prepare itself) to withstand the coldest temperatures of the winter AND it must initiate the process and acclimate rapidly enough to tolerate any unseasonably cold weather that occurs early in the winter. Again, in the early spring, when there are unseasonably warm spells, the plant must retain sufficient hardiness to endure freezing conditions that may follow. This acclimation is triggered by the decreasing length of day in the latitude and elevations in which the plant is grown or the seed produced. The original source of the plant, called its provenance, is very important in influencing its time of acclimation and ability to tolerate the winters in any given location. Thus, when ordering a plant from a catalog, it is important to consider where the nursery from which you are ordering is located.

Cold isn't the only factor determining whether plants will survive and thrive. The effects of heat damage are more subtle than those of extreme cold. Cold can kill a plant instantly. Plant death from heat is slow and lingering; a plant may survive in a stunted or chlorotic state for several years before it finally dies.

The American Horticultural Society Heat Map is used in the same manner as the USDA Plant Hardiness Zone Map. The 12 zones of the map indicate the average number of days each year that a given region experiences "heat days", temperatures over 86 degrees F., the point at which plants begin to suffer physiological damage from the heat. The zones range from Zone 1 (no heat days) to Zone 12 (210 heat days). The Tidewater Area of Virginia is in Zone 7 (60 - 90 heat days a year).

One of the interesting aspects of this map is the difference between the USDA Hardiness Zone Map and the AHS Heat Zone Map. Areas that are in the same Hardiness Zone are not necessarily in the same Heat Zone. As gardeners in the southern United States, we should be aware of these differences.

There are other factors that can cause stress and make it difficult for a plant to thrive. Oxygen, light, air movement, surrounding structures, soil pH, and nutrients all contribute to the health of the plant. Some of these factors are more controllable than others. In selecting a plant for the yard, a homeowner needs to consider all these factors. First of all, consider the provenance, cold hardiness and heat tolerance of the plant.

Helpful Hints, Events, and Garden Scouting Tips:

• Using mulch around the plants helps insulate their roots from the colder temperatures. Mulch should never be over 3-4 inches deep and keep it away from direct contact with the woody trunk of a shrub or tree. Mulches are also beneficial because they help in conserving water, keeping down weeds, and reducing water runoff. Mulches also promote faster growth of trees and shrubs than grass or groundcovers.
• If you have invited birds to visit or live in your yard, do not forget they will need water as well as food for the cold winter. Treat them right and they will provide you with plenty of insect control and entertainment when the weather warms up.
• If you use ashes from your fireplace in your garden make sure you get a pH test before planting. Wood ash can raise the pH of your soil.
• Winter is the time to apply dormant oils to control over-wintering insects such as mites, aphids, and scale on deciduous trees and shrubs. Make sure you read the label and only apply the pesticide according to the directions.

Much of which appears in the helpful hints for gardener's concerns the practice of integrated pest management. Everyone can incorporate integrated pest management (IPM) practices into their gardening. One way is to scout for pest problems and only treat as needed for problems. Make a promise to yourself to incorporate more IPM into your gardening for the year 2000.

January 2000, Jean Holton, Chesapeake Master Gardener and Lynnette J. Swanson, Associate Extension Agent, Environmental Horticulture, Chesapeake