Few people like 90+ degree temperatures. Plants aren't far behind. On hot days, plants can lose water faster than roots absorb it, even if sufficient soil moisture is available. In these conditions, you can watch the plants wilt, leaves droop, and stems seem to flop. If the soil is moist, the plants start to recover as the sun sets. By morning, they look turgid, only to begin the cycle again when the sun shines hard. This is what we are currently seeing in Illinois.
It's only when the plants consistently lose water faster than they can absorb, and don't recover, that serious issues arise.
Larger leafed and shallow rooted plants are the most affected. An example would beHydrangea macrophylla, the showy blue to pink to purple flowering shrub. Many annual vegetables and flowers also suffer more from heat as the upper soil surfaces dry out faster than those deeper.
Trees and shrubs may wilt, but are more likely to shed leaves. Leaves turn yellow and/or brown, and fall from the tree. Seldom do green leaves just drop. Leaf drop occurs throughout the plant, though older leaves and those closer to the trunk seem to be shed first.
Birch (Betula), cottonwood and other true poplars (Populus), maples (Acer) and tuliptree/yellow poplar (Liriodendron) commonly drop large amounts of chlorotic-looking leaves, though less if plants are watered heavily during the hot periods. Typically, this occurs with lowland trees or those trees who natural habitat tends to be near streams, rivers or floodplains. This is a defense mechanism by these plants to lessen the water stress on the entire plant. The tree's canopy may look thinned in severe conditions.
Some woody ornamentals start exhibiting autumn colors. Red maples (Acer rubrum) and burning bushes (Euonymus alatus) may show hints of red but leaves remain attached.
Many of the warm-season vegetables will flower, but flowers abort with no resulting fruit production. This is common on tomatoes and peppers.
Corn and tomatoes develop leaf roll, where the edges of the leaves roll inward or downward as a means of reducing water loss from the plant's stomata. As temperatures cool, the leaves generally resume their normal shape.
Another condition associated with excess temperatures isscorch. Leaves of many trees and shrubs turn brown on the edges and progress toward the veins, or turn brown between the veins. With scorch, the symptoms tend to be consistent throughout the plant, though it's possible more damage may be exhibited on those sides experiencing excessive heat from paved hardscapes such as driveways, streets, sidewalks and parking lots that can radiate the heat back toward the plant, or sides of the plants with root damage or restriction. Scorch can also be exacerbated by dry soil or droughts.
Annual and perennial flowers may be smaller and colors may fade quickly. Flower longevity may also be shortened in high temperatures, even with sufficient moisture.
Cool-season turfgrasses such as Kentucky bluegrass, bentgrass, perennial ryegrass and the fescues enter dormancy, with a gradual yellowing and then browning of the leaves. If you peel back the dead leaves, the crowns usually are still green. However, heat stress on lawns may allow diseases to invade.
Long, slow, deep waterings are the best for preventing heat stress, preferably in the morning when water loss from evaporation is less. Water at the driplines of trees and shrubs, not at their trunk. Water slowly so there is no run-off, and the water slowly permeates into the soil with the goal of wetting the ground to a depth of 8 to 12 inches. This may mean keeping sprinklers on for 60 to 120 minutes.
Shallow and frequent waterings, such as turning on the irrigation system or sprinklers for 15 minutes every day, only keeps roots close to the surface, resulting in their drying out in the heat. (David Robson)