For plants, it's too hot to grow!

After last summer, I think we all have a good idea what drought stress looks like.  But did you know that plants here are especially prone to heat stress here?  Because we have so many frost-free days each year, it's logical to assume that we must have a very long growing season.  But we really have two shorter growing seasons, separated by the horrific summer, where even the low temperatures are too high.

Plants grow best when the temperatures are between 68° and 86° -- when it gets hotter, they have a hard time balancing the energy they consume (respiration) with the energy they create (photosynthesis).  Different plants have different heat thresholds, which is why we use pansies and snapdragons here in winter, and replace them with zinnias and vincas in summer.

Signs of heat stress include brown leaf edges, slowing of growth, or growth stopping altogether, yellowing or withering of leaves, and leaf drop.  Sometimes high temperatures will reduce the sugar content in fruits.  This can be good -- I like my tomatoes on the acid side.  Or it can be bad -- timing the grape harvest gets tricky!  Hot weather also makes flowers smaller and less brightly colored or fragrant.  Remember how big the Knockout rose flowers were in April?  I imagine they're quite a bit smaller now.

The gardening world is used to thinking about plant hardiness in terms of cold tolerance, but more and more growers and breeders are beginning to understand that some of us require heat tolerance from our summer plants.  The American Horticultural Society has published a heat-zone map to distinguish between areas of the country that may have the different summer weather.  It's very similar to the USDA Zone map we're more familiar with, but there are noticeable differences.  Look at the Pacific Northwest:  warm in winter and cool in summer.  No wonder they have great gardens there!


What can you do to help your plants survive the hot summer?  Not much.  Make sure they get adequate moisture, so you don't add drought stress to heat stress.  I don't like to fertilize too much in the middle of the summer.  It's enough for me that my plants merely survive the summer.  I'm okay if they don't grow for a while.