Watering Your Trees

A mature tree can lose hundreds of gallons of water a day through its leaves, especially in hot or windy weather. If trees don’t get enough water, they will experience stress when they can’t replace the water lost through their leaves. Signs of water stress are wilting, leaf scorch (browning of leaf edges), and even dieback of twigs and branches. The older leaves of some types of trees, such as sycamore, turn yellow and brown and then drop from the tree in response to water stress... without the appearance of the wilting or leaf scorch. Here are some things you should know about watering your trees.

Know How Much Water You are Applying

You should know how much water you’re applying when you irrigate your trees. Check the output of your irrigation system. If you haven’t done it yet, I want you to do it this week. Take a bunch of tuna fish cans, soup cans, or any other straight-sided tin cans and put them out in your yard. Turn on the sprinkler system for the amount of time you typically let it run. After the irrigation has run for that set amount of time, check the depth of water in the cans. That’s the amount or depth of water you’re applying. A wide variation in the depth of water between cans, tells you that water is being applied unevenly and you may want to adjust your system for more even application.

How Much to Apply

Apply one to two inches of water when you irrigate your trees. (If you checked the output of your system, you know how long you need to run your sprinklers to apply this amount of water.) Apply the water slowly enough so that it soaks into the soil without running off. This means you may have to start and stop the system several times to allow the water to soak in, especially if you have a system that puts out a lot of water in a short amount of time or if you’re applying water to a sloped area.

The often repeated adage “water deeply, less frequently” holds true. When you apply water in frequent, light applications, you encourage shallow roots. Shallow roots are more susceptible to summer heat stress, winter cold injury, and drought stress..

Check Soil Moisture

Most of the fine feeder roots that are responsible for the uptake of water are located in the top 12 to 18 inches of soil. It’s that area that should be kept evenly moist, letting only the top three to four inches dry out before re-watering. If the soil in that top 12 to 18 inches becomes too dry, the small feeder roots will die, impairing the tree’s ability to absorb water when it becomes available again. Be sure to check the soil moisture to determine when water is needed. You do this with a trowel, shovel, or soil-sampling tool. Don’t rely on the appearance of the soil surface, dig down and feel the soil several inches below the surface.

If your trees are growing in a lawn area, keep in mind that the grass may be using a large portion of water that’s being applied through a sprinkler system. Even if you think you’re applying enough water, check the soil moisture in the tree root zone. Make sure adequate moisture is reaching the soil and tree roots. Consider supplemental watering if your trees need more water.

Apply Water in the Root Zone

Water should be applied in the “root zone”. The fine feeder roots that absorb water are not located close to the trunk of established trees. Most are situated in the area beyond the “drip line”, that’s the area beneath the furthest spread of the branches. If the soil is shallow or compacted, the root zones may be wider and shallower than anticipated.

Water applied at the trunk of a tree is wasteful and can be harmful too. Frequent, excess water at the base of the tree trunk can lead to collar rot and a tree’s demise. Apply water away from the trunk.

Mulch Helps Conserve Water

Mulching trees with a three to four inch layer of bark or compost decreases the amount of water that evaporates from the soil, aids in water and air penetration, and cools tree roots. The greater the area of root zone that’s mulched, the better your tree will perform. However, mulch mounded around the trunk of the tree can also lead to collar rot and eventual tree death. So keep mulch six inches away from the trunk.

Too Much Water Can Kill Trees

Too much water is just as bad as too little water. Excess soil moisture excludes oxygen from the soil. Roots die without “air”. One symptom of too much water is the yellowing of leaves starting on the lower part of the tree on the inside of the crown and then moving outward. Other symptoms include leaves turning a lighter shade of green and then yellow, wilting of young shoots, and brittle green leaves. The soil should be moist, not soggy and squishy for any length of time.

Newly Planted Trees

Newly planted trees are watered a little differently than trees that are well established in the landscape. That’s because they haven’t been able to grow new roots into the surrounding soil yet. Newly transplanted trees have a very limited root system. The root ball and the surrounding area should be kept evenly moist and not allowed to dry out. The root ball can dry out much more quickly than the surrounding soil, especially in hot or windy weather.

Don’t expect your normal watering practices for established trees to be adequate for newly planted trees. If the new tree is located in the lawn area, don’t rely on lawn irrigation to provide enough water. Check the moisture in the root ball, keeping in mind that the root ball can dry out within a day or two in hot weather.

This special attention should be given to trees for at least two or three years until they become established and develop adequate root systems out into the surrounding soil. It may take even longer for larger transplanted trees. Trees that are dug from the ground and transplanted lose over 90 per cent of their root system. Once a tree becomes established with roots growing out into the surrounding soil, it will need less attention.

Note: Many newly planted trees are often killed from too much water. Young trees drown when excess water is applied at the base of the tree. Remember they can’t swim, so only apply water often enough to keep the root ball evenly moist.

If you water your landscape by hand, use a faucet timer and soaker hose to water your trees.

If you water your trees with drip irrigation, don’t place emitters close to the trunk. Check soil moisture periodically to insure that the root zone is being kept uniformly moist. Add emitters to expand the area watered as the tree grows. Use micro-sprinkler emitters to enlarge the wetting pattern and avoid puddling of water around the base of the trunk.

Soaker hoses are a great way to apply water slowly and deeply to the soil in the root zone of trees.

Wonder what time of day is best to water? Irrigate when water is available to you. You may find gardening books that advise against watering at night to avoid foliar fungal disease. However, this is not really a problem in our dry climate and you can water any time of day or night.. Watering in the middle of the day is not harmful to most plants, but is less efficient due to evaporation. Just be careful not to water tree and garden plants with hot water from a hose sitting out in the sun.