The effects of cold on plants is immediately recognizable. Extreme cold can kill a plant immediately. Heat, on the other hand, is trickier. Death can come after a lengthy decline, sometimes measured in years. The plant may become sickly, chlorotic and stunted no matter what measures you take to improve its condition. This is an apt description of trying to grow delphinium in western Kentucky. No matter what you do or where you plant it, it is only acceptable-looking one season. Then it declines and eventually dies. Those of us who felt we couldn't live without it, grew it as an annual. It never reaches the gorgeous stature I have seen in Duluth.
Scientists have discovered that physiological damage to many plants begins to be experienced at 86º F. The longer the period of time spent at temperatures above 86º F, the greater the effects of the damage. The American Horticultural Society has taken data on temperature maximums around the country like the USDA did with the minimums and has produced the Heat-Zone map with 12 zones representing "heat days" - the average number of days over 86º F. They range from zone 1 (no heat days) to zone 12 (more than 210 heat days). My home in western Kentucky was, unhappily, in a pocket of zone 8 (90 to 120 heat days), while my Minneapolis home is in zone 5 (30 to 45 days). This week it seems as if I have gone back to Kentucky. We are in the midst of our heat days and I am no more tolerant than the delphinium.
As with cold, heat damage can be modified by consideration of microclimates in our landscapes. The effects of heat are greatly exaggerated by drought and minimized by surrounding vegetation. As an example, in Kentucky, you could not grow a lovely container of trailing lobelia for very long on a sunny patio. The reflected heat would kill it quickly. However, that same container sitting in a grassy lawn would survive all but the worst of the summer if careful attention was paid to the watering too.
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