The American Horticultural Society identifies 86 degrees F. as the temperature at which plants start to suffer damage from high temperatures. When you combine high temperatures with prolonged, unrelieved drought, what do you get? Well, looking around my garden, I can tell you. You get plants that are normally considered "tough" that are being stressed to the limits of their endurance. Some of them aren't going to make it.
I have already lost more plants to the drought and heat than I lost to the cold in our last two winters. As I was hand-watering some plants in my yard a few days ago, I was surprised to notice that my oldest beautyberry shrub had several brown limbs and appeared to be in dire stress. Yet, about ten feet away, another beautyberry - my white-berried shrub - looked perfectly fine and was full of blooms. Thinking about it, I decided that the old shrub's stress is probably caused by the fact that it is between my apple tree and a cherry tree on my neighbor's side of fence. It's having to fight it out with those two trees for water and it is losing the battle.
When temperatures are above 86 and plants are aspirating water through their leaves at a high rate, the battle for water to replenish that moisture can become a fight to the death, and having already hit 100 degrees F. here in early June, that is what I am seeing in my garden.
I went looking for more information about plants' ability to withstand heat and found something that, in my ignorance, I didn't know existed. We are all pretty familiar, I think, with the USDA's plant hardiness zone map. Any gardener worth his/her salt can tell you in which zone he/she resides. But did you know that there is a heat zone map also? You won't usually see it referred to on plant tags, but it does exist, and the zones are based on the average number of days in the year when the temperature is above 86.
There are twelve zones as indicated by this list, and here's what Southeast Texas looks like on the map:
Our entire area is in zone 9, which means that we generally have between 120-150 days in the year that are above 86 degrees. I suspect we are going to hit the higher mark or maybe exceed it this year.
How does this knowledge help us in our gardening plans and practices? Gardeners could be generally divided into two categories: traditional or daring. Traditional gardeners try to choose plants that they know are suited to their zone and will do well there. Daring gardeners love to push the envelope a bit (or sometimes a lot) and grow plants that are really not meant for their area. Considering those two catergories, I suppose I would be considered traditional, (although in other areas of my life, I like to think of myself as a daring rebel!) but at times like these, even a traditional philosophy of gardening will be tested.
I suppose the Anazazi of the arid Southwest, who certainly adhered to a traditional philosophy of farming, must have been tested in much the same way when the long drought that put an end to their blossoming civilization ground on without relief year after dry year. I've been thinking about those Anazazi farmers a lot lately.
We certainly have access to a lot more information and a lot more tools than the Anazazi ever did, and no matter what type of gardener you are, looking around a local nursery can lead you to believe that your choices are endless. But knowing something about your hardiness and heat zones can guide you to make wiser choices. And observing the plants in your current garden, the winners and the losers, can give even more guidance when making your future plans. I know I'm going to have a lot to consider before adding any new plants to my garden.