Extremes of heat or cold are stressful to both plants and gardeners. Heat Stress Extreme heat is stressful, and, like extreme cold can kill both people and plants by causing physiological changes that can't be reversed. Unlike freezing, the results of which can usually be seen immediately, heat stress can weaken plants, causing effects that aren't noticed until much later, such as flower bud drop the spring after a hot, dry summer. Heat stress can also be seen as it occurs. Plants droop, leaves crisp and fall prematurely, new growth, particularly on coniferous trees, shrivels and turns brown because it isn't actively involved in transpiration and can't cool itself during hot spells in late spring or early summer.
Much plant growth seems to stop.
Most of these symptoms are also drought related. Drought Heat and drought go hand in hand. Evaporation from moist soils and plants cools the land just as perspiring cools us when it's hot. Drought stress results when water loss from the plant through transpiration exceeds the ability of the plant's roots to absorb water, reducing the water content in the plant enough to interfere with normal processes. Continued drought stress can alter or damage photosynthesis, leading to long-term reduction in vigor.