Recent high temperatures, ranging from the low 90’s to the upper 90’s has put extreme stress on not only our containerized plants, but also on all of our perennials and trees that are planted in the ground. Normally plants that have become well established in the ground do not need much care as their root system has spread out enough to get moisture from deep in the ground. But when you combine no rain along with past high temperatures, then you have a combination for root and leaf damage.
Most folks worry about their tomatoes and other vegetables in the garden but ignore their trees and shrubs, and these are the plants that need your attention most. During these extreme conditions you should slowly soak your most stressed trees (not all of your trees will be stressed) once a week, or until we get a good, heavy rain. Simply lay the garden hose on the ground by the tree and run the water slowly occasionally moving it from one side to the other, and remember that the roots go out as far as the drip-line.
Hydrangeas, even the ones that have been there for several years, seem to be the flowering perennial plants that need the most water during these hot and dry times. I have seen the need for watering hydrangeas as often as two times per day. Once the temperatures drop and the clouds block the hot sun, you must stop all of the extra watering immediately or you could quickly create a rot situation. Fertilizing stressed plants is extremely dangerous. When a plant is under stress, switch to using only plain water and try to anticipate the need for watering and do it before the stress sets in.
Most of your other perennials will be happy with only an occasional watering, maybe once every five days or so. If you have damage due to stress, it is very important to cut-off all damaged leaves and branches as these bad areas tend to draw insects and fungus like magnets. Once weather conditions are favorable, i.e.; cooler and more moist, then the use of fertilizers are important. Perennial plants need to be fertilized during the spring and summer seasons while they are actively growing and then none during the late summer and fall just before dormancy.