It's too darn hot! Few gardeners would disagree, and if plants could vote they would also cast an affirmative vote. Temperatures over 100 degrees F. have forced gardeners to retreat from their favorite abode and relegate strenuous gardening tasks to early morning or evening hours. Plants have no such options, and many are suffering from the heat.
Most plants grow best in a temperature range of 65-85 degrees F. Outside this range systems slow down to protect the plants and help them survive extended periods of hot, dry weather. Plants do not manufacture as much food when temperatures get too warm, so your garden does not grow as well. Warm-season plants have a slightly higher temperature range, but even those shut down when temperatures get too hot.High temperatures also have other harmful effects on plants. With many cool-season crops, such as spinach, it induces premature flowering or bolting. Other cole crops such as broccoli and cauliflower go to seed if exposed to high temperatures. Heading is delayed, and yield is badly affected. Tip burn affects many of the cole crops, including cabbage, collards and kale. The leaves burn on the ends making leafy vegetables unattractive.
Heat induces premature blossom drop in tomatoes when daytime temperatures get above 90 degrees F. and night temperatures stay above 76 degrees F. Cucumbers and beans also drop their flowers if they are too hot during pollination.
Many crops can be planted now but show poor seedling vigor when temperatures are warm. Carrots, lettuce and other small seeded vegetables germinate poorly when it is hot and dry. Temperature and water relationships are also critical during hot weather. Blossom end rot affects tomatoes and causes brown, leathery spots on the blossom end of the fruit. It also damages peppers, eggplant, cucumbers, watermelons, squashes and pumpkins.
Large trees are also badly affected by the hot, dry weather. Summer leaf scorch is common on trees with large, broad leaves. Problems show frequently on poplars, maples, horse chestnut and catalpas, although all species can be damaged. In many cases soil moisture is adequate, but the tree cannot move water from the soil to the top of the tree fast enough to prevent scorch. It is usually worse on trees with pavement around the tree, as this restricts the roots and their ability to absorb water. Every two to three weeks water trees deeply so the moisture penetrates two feet into the soil.
Fruit crops suffer from warm temperatures, although much of this damage occurred earlier in the season. Apples are not as sweet nor is the flavor as good when the fruit ripens during extremely hot weather. Peaches develop split pits, which cause the pit to break and allows insects inside the fruit when it is hot early in the season. The other common problem is scorched leaves and fruit discussed previously.
I wish I could say that the warm temperatures have slowed the insects down, but in my opinion, it has done exactly the opposite. I have never had so many insects that require control in my garden. At the top of the list are earwigs and grasshoppers, although I am not sure I can blame those on the hot weather. Many insects prefer to attack plants that are damaged or under stress, and spider mites become prolific in warm, dry weather.
One disease that prefers the hot, dry weather is powdery mildew. Powdery mildew, unlike most other fungal diseases, does not require free moisture to grow. For this reason, powdery mildew is showing up much earlier than normal. It is rampant on crab apples and susceptible fruiting apples, euonymus and squashes as well as zinnias and many other kinds of flowers. Normally this disease does not become a problem until so late in the season that we don't control it. This year the problem may require intervention on your part with an appropriate fungicide. Funginex has some edible crop registrations and can be used on apple trees. Halt and benomyl are also available for ornamental crops.
Avoid damage to the base of the tree from weed whips, lawn mowers or other mechanical equipment. Avoid flowers around the tree as frequent, shallow irrigation encourages problems from water molds and other pathogenic diseases.
It is impossible to list everything that the hot weather can do to your plants, and it is impossible to list every remedy. Common sense is the most important way to help plants survive. Don't overlook the positive affects of plants as they cool the city, making a refuge from the extreme heat. The best remedy for these warm temperatures is a beautiful shade tree with a tall grass of lemonade. Although it won't help your plants, it will make the heat less noticeable until the problem passes.