Stress is a normal condition for landscape plants. While plants can tolerate considerable stress, they may be weakened and made susceptible to attack by insects and diseases. A little TLC during the summer months will go a long way toward keeping trees, shrubs and perennials healthy and vigorous during the heat, drought and humidity of a typical summer.
The first and most important step in plant stress prevention is planting the right plant in the right place. Part of a good landscape plan is selection of plants suited to the various micro-environments of the home grounds. While a juniper will thrive in a hot, dry corner, a dogwood or Japanese maple planted nearby will probably show leaf scorch most years. But planting the juniper in a shady or poorly drained site is a waste of time. Even azaleas will grow poorly in very dense shade. Pruning up a few of the lower branches on over-story trees to provide a few hours of morning sun often dramatically improves the performance of light-starved plants.
When planting container grown trees and shrubs, tease the roots away from the root ball and spread them out in the planting hole. Teasing the roots is much different than taking out a knife and making vertical slices through the roots. Multiple vertical slices may sever up to 50% of the root system, setting the plant up for drought stress when hot weather comes. Root watering crystals (such as Agrosoke) can be a tremendous benefit when added at planting time. These crystals absorb moisture from the soil, but release it back into the soil between watering.
When planting trees with a soil ball (balled and burlapped), make sure that the burlap is not exposed to the air where it will wick water from the ball. Personally I like to remove the burlap entirely, once the plant is in the hole. Do not over-fertilize at planting time. This may stimulate excessive shoot growth at the expense of root growth, making the plant less drought tolerant. Mulching is essential for stress avoidance. It retains soil moisture, keeps the soil cool and eliminates competition from turf and weeds for moisture and nutrients. Compost makes excellent mulch since it provides a source of slow release nutrients, thereby promoting root growth. When mulching, do not pile the mulch around the base of the trunk (aka The Jersey Volcano). This will suffocate the plant, severely decreasing its lifespan. Instead, make a saucer with the mulch, building it up as you get to the edge of your planting area. This will allow water to drain directly to the root system where it is need most.
Don't hole up in the air-conditioned house during the hot summer and abandon your landscape. Check for signs of stress during the heat of the day. Even plants that wilt severely during mid-afternoon can recover by the following morning with a little TLC. But a few days of severe wilting can weaken a plant and interfere with fall root growth and cold hardiness development.
Other signs of summer stress include pale leaves and scorched leaf margins. These generally result from high leaf temperature and can occur even when soil moisture is seemingly sufficient. Careful watering, mulching and protection of Japanese maples and dogwoods from wind and afternoon sun will reduce the incidence of leaf scorch. Watch lilacs and dogwoods for powdery mildew in late summer. This can weaken plants as they head into winter, but can be prevented with a monthly application of Wilt-Pruf (an anti-dessicant that is normally used in the winter to prevent winter burn on evergreen leaves).
In general, the "supplement rainfall to make one-inch-per-week" rule for watering works well to prevent stress. Run a drip irrigation system with 1 gallon per hour emitters for an hour. Also stay away from overhead sprinklers – most of the water will evaporate before getting to the root zone, and is a waste of one of our precious resources. Although I would like to say we would have adequate moisture throughout the summer, our recent history proves differently.