I have lived in this region for more than 30 years, but each summer I have a hard time adjusting to our extreme heat. This year, I deluded myself into believing that our blissfully mild early summer would continue.
Scorching heat is not only extremely stressful on us, but also on our plants. Some ill effects are related to cultural factors, such as watering, but the heat itself can lead to "hot weather woes." One reason for this is thermoperiod.
Thermoperiod refers to the daily temperature change from daytime to nighttime.
Plant growth is best when daytime temperatures are about 80 degrees and nighttime temperatures are 15 degrees lower, although this varies with the type of plant. With this 15 degree thermoperiod, plants are making and building carbohydrates via photosynthesis at a higher rate than they are being broken down through respiration.
Higher temperatures lead to higher rates of photosynthesis, up to a point. Respiration also increases with higher temperatures. In extremely hot weather, carbohydrates are used up more quickly than they can be replaced. As a result, growth slows or stops.
That's why so many garden plants look stressed when 100-degree weather prevails. Even heat-tolerant annuals stop growing and flowering. Spent flowers shrivel, and no new ones replace them. If they are not drought stressed, they should bounce back just fine with cooler weather.
The failure of plants to produce fruit despite flowering is a common hot weather complaint of vegetable gardeners. Many crops are dependent on cross-pollination by bees. When temperatures rise above 100 degrees, bee activity and cross-pollination slows.
Even with plenty of bee activity, pollination and fruit set may still suffer because extremely hot, dry weather reduces the pollen viability of many crops. Blossom-drop, or the failure of flowers to set fruit, is a frequent complaint, even with plants not dependent on insects for pollination. Blistering weather is often the cause for poor fruit set on beans, tomatoes, peppers, squash, cucumbers and melons. As a result of hot weather, these crops may also develop deformed fruit because of incomplete pollination.
If you have trees, you may also note another hot weather phenomenon: seemingly healthy trees dropping an alarming number of leaves without warning. Some trees will suddenly drop some of their leaves in mid-summer, typically when hot, dry weather arrives. This is a physiologic adjustment because the tree cannot support all the leaves it developed when the weather was cooler and less stressful.
A tree can lose as many as 10 percent of its leaves without adversely affecting its overall health. However, it is important to make sure the leaf drop is not the result of other stresses, such as lack of adequate water, root problems or an insect infestation. You can help prevent drought stress on shade trees and harmful excessive leaf drop during hot weather by providing them a weekly deep watering.
Our summers are hot. There is no way to escape it, but we can avoid some of these garden woes by selecting plants, including flowers, vegetables and trees rated as "heat tolerant" and giving them the best growing conditions possible.
-- Marianne C. Ophardt is a horticulturist for Washington State University Benton County Extension.