What does a heat zone map have to do with us? We diligently study the USDA Hardiness Zone Map in this climate. The winter survival of our gardens consumes much of our thinking and planning. This long, cloudy, wet spring that, at this moment, is again too cool has even put thoughts of heat damage further from our attention. But that is why it is important to us...our experience with heat damage is far more limited than most. That means during periods when our temperatures rise above 86ºF (the point where plants begin to suffer physiological damage from heat), we need to know what can happen and how to appropriately respond to it. The Heat Zone Map is divided into 12 zones where the average number of days over 86ºF is similar ranging from less than one day in zone 1 to more than 210 days in zone 12. Minnesota falls into zones 3, 4 and 5. Zone 3 has less than 7 days over 86ºF and zone 5 has between 30 and 45.
How do you treat a plant for heat prostration? Do they actually get heat prostration? How would you know? What are the symptoms? What should you do? See what I mean? We are not used to dealing with heat on any continuing basis. Heat damage symptoms are often mistaken for other conditions but are best understood as increasing the plant's susceptibility to those conditions. Flower buds may wither or drop, leaves can wilt or become more attractive to insects, the chlorophyll may break down leaving large patches or whole leaves appearing white or brown, and growth may come to a standstill as the roots stop growing. In areas where the heat damage is long term, plants may survive but become chlorotic and stunted, dying a slow and lingering death. If this is not recognized for the heat problem it is, the gardener will try treating the symptoms unsuccessfully or worse, blame him or herself. The only way that gardeners can know that heat could be the cause of their problems is if they are aware of it. Here in our northern zones, a lingering heat spell is noticed. Take a look at your garden and notice the signs of stress the plants are showing.
Heat damage is always linked to an insufficient amount of water being available. At higher temperatures available water is used up rapidly. During any period of temperatures above 86ºF, gardeners should pay careful attention to providing enough. All good tomato growers know that wilting at any time can cause the next fruits formed to suffer from blossom end rot. Heat wilt can also cause plants like fuchsias and gardenias to abort their flower buds. Above 86ºF many members of the Solanaceae family which includes tomatoes, peppers, eggplants and potatoes slow down or stop producing new blooms. This directly affects their yield.
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