When you buy a plant from a mail order catalog or read a description in a magazine, you often find words like “Zones 3-8” or “Zone 4.” What does this mean?
The numbers refer to the U.S. Department of Agriculture’s Cold Hardiness Zone Map. Updated in 1990, the map is based on average annual minimum temperatures recorded throughout North America. By using the map to find the zone in which you live, you can determine which plants will “winter over” in your garden because they can withstand these average minimum temperatures.
Zone 1 is the coldest, most northerly region and Zone 11 is the warmest, most southerly. We live on the border between Zones 4 and 5, meaning our average annual minimum temperature can get down to -10 to -30 degrees F.
If a plant is described as “Zones 4-8” or “hardy to Zone 3” you can expect it to do well in our area. That said, there are climatic variations within each region and even within each garden. Many factors including altitude, wind exposure, proximity to bodies of water, terrain and shade can cause variations in growing conditions by as much as two zones in either direction.
For example, you may be able to grow plants on the south side of a stone fence that should survive only as far north as Louisville, Kentucky, while on the north side of that same wall you may have trouble with plants that theoretically should do fine here.
However, 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, which will kill a plant instantly. Flower buds may wither, leaves may droop or become more attractive to insects, chlorophyll may disappear so that leaves appear white or brown, or roots may cease growing. Plant death from heat is slow and lingering.
The American Horticultural Society has developed a Heat Zone Map similar to the Hardiness Zone Map. The twelve zones of the map indicate the average number of days each year that a given region experiences “heat days” – temperatures over 86 degrees F. That is the point at which plants begin suffering physiological damage from heat.
We are in Zone 4 on the Heat Zone map which means we have an average of 14 to 30 days each year with temperatures above 86 degrees F.
Thousands of plants have been coded for heat tolerance, with more to come in the near future. You will see the heat zone designations joining hardiness zone designations in garden centers, reference books, and catalogs. On each plant, there will be four numbers. For example, a tulip may be 3-8, 8-1. If you live in USDA Zone 4 and AHS Zone 4, you will know that you can leave tulips outdoors in your garden year-round. An ageratum may be 10-11, 12-1. It can withstand summer heat throughout the United States, but will overwinter only in the warmest zones. An English wallflower may be 5-8, 6-1. It is relatively cold hardy, but can’t tolerate extreme summer heat.
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