Hot weather is tougher on plants than it is on people. It’s easy to understand why, when you consider that our bodies contain about 60 percent water and most plants are 85 to 90 percent water. So when temperatures rise, plants get even thirstier and sweatier than we do.
As with people, some plants tolerate heat better than others. Knowing which plants like it hot and which would prefer air conditioning, you can help your vegetables and flowers survive — and even thrive in — hot weather.Vegetables
Tomatoes, peppers, eggplant, corn, melons and squash actually need at least a month of 80 to 90 degree weather to develop a flavorful and abundant crop. As long as they don’t run out of water, these sun worshippers are well equipped to survive the heat. On hot days they conserve energy and moisture by slowing down. While resting, their foliage may appear to be wilting from lack of water, but as evening approaches they’ll perk up again. Heat loving plants are thirsty– the average tomato plant needs more than 30 gallons of water in a season. Using a combination of mulch and drip or soaker hose will ensure these plants stay healthy and well hydrated.
In the vegetable garden, it’s the cool weather crops — lettuce, spinach, arugula, broccoli, kale, cauliflower, peas, cilantro — that suffer in hot weather. Even with an abundant and consistent supply of water, when temperatures rise over 80 degrees, these plants tend to stop growing, go to seed, or just give up the ghost.
Cool-weather crops should be grown on either side of midsummer heat. The plants will be much healthier and they’ll taste better, too. Look for varieties that are well matched to spring or fall production. Plan to sow salad greens every couple weeks to maintain a steady supply of high-quality leaves.
In hot weather, heat-sensitive crops want protection from both heat and sun. Shade netting helps these plants in several ways. Most importantly, it keeps soil and air temperatures as much as 10 degrees cooler (lettuce seeds germinate poorly in soil temperature higher than 70 degrees). Shade netting also protects tender foliage from being scorched by intense sunlight and reduces moisture loss. For best results, suspend the shade netting several inches above your crops, letting it rest on wire or fiberglass hoops, or a wooden frame.
Perennials that bloom in mid and late summer, such as asters, echinacea, rudbeckia, sedum and daylilies, are usually unfazed by heat. They may need extra water when the thermometer hits 90 degrees, but the heat itself doesn't bother them much.
Cool-weather perennials that flower in spring and early summer often have a hard time with heat. These get their blooming work out of the way early so they can kick back when temperatures gets hot. In fact, some cool-weather perennials, such as Oriental poppies and bleeding heart, avoid summer altogether; they die right to the ground after blooming and don’t reappear until next spring. But for the most part, these plants just struggle along, looking gangly and unkempt. You can help them out by removing spent blossoms and stems as soon as they’ve finished flowering. Trim back foliage that's not fresh and healthy. If you haven't done so already, mulch around the plants to keep the soil cool and water as needed during hot weather to keep the soil from drying out. A piece of shade netting, laid right on the foliage, will provide instant relief for plants that have been damaged, transplanted or are otherwise ailing.
Annuals span the full range from heat lovers (gazania and portulaca) to heat haters (pansies and sweet peas), so it's impossible to generalize about their care. With so many annuals to choose from, the best strategy is to match plant to place. If the snapdragons and petunias along your front walk are struggling in the heat, next year consider plants that have a higher tolerance for heat and drought, such as zinnias or gaillardia.
For annuals grown in pots, hot weather can be deadly. Ignore them for a couple days and they may never revive. When you water, make sure that the entire root ball gets moistened. Daily watering quickly leaches nutrients from the soil, so either use a slow-release fertilizer or apply a water-soluble liquid fertilizer every other week throughout the season. Nip off spent blooms and trim back rangy stems to encourage reblooming.
Many annuals and perennials now have a USDA heat zone rating as well as a hardiness rating. When a plant is not cold hardy, it will simply die, but the signs of heat stress are usually more subtle. Plants that can't tolerate high heat may stop blooming, the leaves may turn pale, and the plants may become more susceptible to pests.