Most people assume that established trees and shrubs can handle drought conditions. Often this is true, but the severe weather extremes that we've had in recent years has depleted reserves and stressed many plants.
Please don't ignore your older trees and shrubs. Mature woody landscape plants often do not exhibit symptoms from underwatering for several months; or, some cases it takes 3-5 years to see the impacts of a major environmental event.
Younger plants show symptoms much sooner. So when deciding which trees and shrubs to water, focus on young trees or shrubs that were planted within the last two years.
Since most people cannot feasibly water all plants on their property you might need to make a choice of which older plants to water. Select those with the most value to you. Is that plant crucial to how you use and enjoy your yard? Woody plants cost much more initially and are harder to replace than annuals and perennials.
Most plants need about at least 1 inch of water every week to be healthy. Plants growing in sandy soil or experiencing a heat wave might need up to 3 inches per week. The best water comes from rain, but sometimes we need to provide supplemental water.
Before watering, think about how a tree or shrub grows. Most of their functioning roots are in the upper foot of soil and extend beyond the plant's drip-line. Therefore, water those plants at the drip-line to be most effective, not at the trunk. The drip-line is the farthest point from the truck where leaves grow. Water should reach at least 12 to 15 inches deep around the plant. And, always water thoroughly and all at once.
There are many ways to water trees and shrubs. You can hand water with a watering can or hose, but it may take some time to thoroughly soak the drip line area of large plants. The better way is to use a watering system such as an in-ground irrigation system or hose end sprinklers. When watering overhead with sprinklers, use a container to measure how much water is reaching the intended plant, then run the irrigation until there is 1 inch of water in the container.
For trees and shrubs, it is often better to direct water directly to their roots where it is needed most. That can be done with soaker hoses or tree water bags. Root feeders that attached to the garden hose are also a great option.
Don't just water during the summer, some trees may need supplemental water in the fall as well. This is particularly true for evergreens. If water is not available in the plant to replace that lost through leaves on a sunny winter day, evergreens may turn brown. Evergreens often benefit from a late fall, early winter-feeding as well as deep fall watering.
You can see examples of all these watering methods on my new YouTube video at go.illinois.edu/ferreevideos.