Sweltering heat is cooking your plants

Short of exporting your entire garden to a cooler, more hospitable climate, you can only do so much to protect your plants from the sweltering heat.

Most temperate zone plants grow best in a temperature range of 65-85 degrees F. Outside this range, plant growth slows to protect the plants and to help them survive extended periods of hot, dry weather.

For proof, just look at your vegetable garden. Most of the cool season crops that were planted in the spring are already done for the summer. The heat stress makes them tough and bitter, and the leaves on many of the leafy vegetables — including lettuce and spinach — have been burned. Cabbage, broccoli and cauliflower usually bolt and go to seed when exposed to high temperatures.

Warm-season plants grow at a slightly higher temperature range, but even those shut down when temperatures get too hot.

The most significant problem is how heat affects the flowering and fruiting of many vegetables. Tomatoes — one of the garden's most popular vegetable — stop producing when daytime temperatures get above 90 degrees F., or when night temperatures stay above 76 degrees F.

It is simply too hot for the pollen tubes to grow, so the blossoms abort and no fruit is produced. Even when fruits form, they are often misshapen and rough. Cucumbers and beans also drop their flowers if temperatures are too hot during pollination.

Certain insects, spider mites and some diseases become more serious when temperatures are elevated. Spider mites thrive when you have hot, dry, dusty conditions. Keep your plants clean by washing the backs of the leaves with a strong stream of water.

Verticillium and fusarium wilt are other serious diseases. They affect most vegetables and a host of garden plants and trees. The fungi grow inside the water-conducting tissues of the plant and plug them off so they cannot move water from the soil to the top of the plant. The plants then wilt, giving rise to the name.

Another problem that becomes worse with heat stress is blossom end rot. If you are lucky enough to have tomatoes already on the vine, they are still at risk. Gardeners first notice it on tomatoes as brown, leathery spots on the blossom end of the fruit.

Blossom end rot also affects peppers, eggplant, cucumbers, watermelons, squashes and pumpkins. This disease is caused when the growing point on the fruit doesn't get enough calcium to form properly, so the fruit tissue is damaged.