I may be able to recognize heat damage, but I can't lower the outdoor temperature, so what can I do about it besides water more? You can do a number of things. One is mulch your plants. Mulch keeps the soil cooler and retards the evaporation of soil moisture. The presence of other plants in a garden or woodland means that the transpiration of all lower the ambient temperature a bit as well as raising the humidity to offset the drying effect of the heat. If you have container plants in full sun on masonry patios and porches, move them to a spot in your garden or where they will receive shade during the hottest part of the day. Plants surrounded by rock mulch will suffer greatly if they are in full sun. If you cannot remove the rock mulch until the hot spell is over, try covering it with a white cloth such as a sheet or frost blanket or even layers of newsprint to lower the heat absorption.
If all this seems a tempest in a teapot given our season so far, file it away for future reference. The hot spells are becoming statistically more frequent and are part of the concern over global warming. These are techniques we may well need yet this summer. Long term planning for heat means looking at the AHS Heat Zone Map and choosing plants that will do well in our microclimates. Here in Minneapolis, we usually have many places that are at least one zone warmer than the area as a whole, so the heat zones will be similarly distorted. Research on heat tolerance of plants is still in its infancy compared to cold tolerance and breeding for tolerance even further in the future. Many catalogs and garden centers are beginning to offer heat zone numbers for plants and more will be coming all the time. In their publication The American Gardener, the AHS prints a listing at the back of each issue with both the hardiness range and the heat range for all the plants discussed in the issue. If you are a record keeper, and most of the best gardeners are, add a section on heat effects and start to learn how it affects your garden. Look up the AHS map here. If you are a member you can download a copy or there is a toll free number to call and order a copy if you are not. In any case, we should learn to recognize heat symptoms in our garden plants as easily as we recognize it in our children.
