Trees and Drought

Portions of the United States are struggling from the impact of excessive rain and snow melt as others are dealing with the effects of long-term drought. For the people with houses under water, there may be minor envy of the dry regions, and for those of us living in the parched environs there is a whispered desire to have the worries of too much rain. The extremes of weather make comrades of the victims at each end of the spectrum.

The U.S Drought Monitor tracks the effects of prolonged periods without precipitation and shows the entire southern portion of the United States as suffering from at least “abnormally dry” conditions. Nearly the entire state of Texas is designated as “exceptional” drought, the highest level. Every week brings us new record temperatures. Yesterday Phoenix, Arizona, set a new record high of 118F degrees.

While we perspire and curse the heat, drought stress on plants, particularly trees, can be fatal (see my blog, “The Life of a Tree”). Trees need water to sustain their lives, like every living organism, but they have the ability to survive on energy and moisture reserves stored in their roots. Like camels and their humps, trees can survive fairly well without regular water, but only to a point. For survival, most landscape trees need supplemental water.

My Aspens look good, but they're thirsty

That means that you must water your trees when natural precipitation fails to deliver enough life-sustaining moisture. Many people think that occasional rain, even when far below normal, is enough to sustain trees. They remember the television science program where deserts came alive after a single downpour. While some desert plants are able to store vast amounts of moisture for long periods of time, the typical trees we have in our yards need regular irrigation and periodic light rains during a drought aren’t enough.

As with most garden plants, begin by checking your soil. Dig random holes a few inches deep under the outermost branches of your trees. If the soil is retaining moisture, if you can form it into a ball that holds its shape, the roots will be able to absorb it and the tree is okay. If you have a soil moisture meter insert it at different levels. If the soil is dry at two inches, at three inches, and deeper, you need to water deeply.

Most tree roots are located in the upper six to 24 inches of soil. The shade tree in your backyard hasn’t tapped into a deep water reserve with penetrating roots. It’s trying to soak up the moisture in the top few inches of soil. The smaller feeder roots are the ones you encounter when you dig a hole for a new plant and they’re the ones seeking water. If you encounter dry soil with a test hole, the tree is encountering the same dry soil.

The solution is deep watering. A tree requires gallons of water and bigger trees need more. Trees obtain water best when it soaks the soil to a depth of at least 12 inches. A general rule is 5 to 10 gallons of water per inch of tree diameter, per watering. A three-inch wide tree needs up to 30 gallons of water. You should water this deeply at least twice a month during drought when no other natural precipitation occurs.

The key difference in the amounts of water required are due to the individual needs of the tree. Some trees are more drought tolerant and can survive with less water, others need more water more often. Most Maple, Hawthorn, Goldenrain Tree, Coffee Tree, Juniper, Pine, and Oak trees can handle dry growing conditions.

How you water is important too. Drip lines next to the trunk don’t work well. Watering within the dripline is good, but may not be enough. You have to look at the size of the tree and try to determine where the feeder roots are.

The roots of an established tree extend well past the confines of the branches. Normal root growth extends two to four times the diameter of the tree crown. A tree that is 15 feet wide at the top may have roots throughout a 30- to 60-feet circle. You can expect the root spread to be at least equal to the height of the tree. Under very dry conditions some trees can send roots out 10 times the crown diameter; there might be roots 100 feet away from large trees.

Younger trees have roots much closer to the trunk so watering with a soaker hose curled around it can be effective. Using a deep-root feeder needle attached to a hose, inserted about eight inches deep, may also work on young trees. But neither of these methods can adequately cover the root zone of a large, established tree.

Bigger trees need water throughout the span of their root zone. Oscillating sprinklers and impact rotors can broadcast water over larges areas. Think about the kind of sprinklers you see in parks and golf courses.

You’ll need to determine how much water your hose and sprinkler put out. One easy way is to take a bucket and direct the sprinkler spray into it (yes, you’ll probably get wet). Time how long it takes to fill the bucket. If a one gallon bucket fills in 30 seconds, then your hose and sprinkler are distributing two gallons per minute. If you use that sprinkler to cover a tree’s root zone, you can expect about 30 gallons in 15 minutes.

That’s the first part of watering. Soil is always key. You still need to be sure the water is soaking the soil. So after you’ve moved the sprinkler to a new location, and after you’ve let any puddles on the surface drain, go back to your test hole and see if water is making it down to the level of the roots. If you find only the top inch or two is wet while it’s dry below that, you will need to water for a longer period of time even if the math says you’ve distributed the appropriate number of gallons. If the soil is wet at six inches it should fine below that.

After you’ve figured out how much water your sprinkler puts out and how effectively the soil absorbs it, you can set up a schedule of regular watering at two or three week intervals. Much depends on the weather. If you’ve had no rain, water more often. If thunderstorms and rainstorms start occurring, you may not need supplemental water at all. Check your soil’s moisture content to be sure.

Soil is still dry after a soaking thunderstorm

Another consideration is the tree’s location. If a tree is near a lawn that is on a regular irrigation cycle, the roots on that side of the tree are soaking up moisture under the grass. You should account for that and may only need to do a partial watering on the tree’s dry side. Trees surrounded by turf may not need additional water at all, even in drought.

You take the time to water your vegetable garden, your flowers, and your lawn. Remember the trees. A drought can be devastating and with trees you may not see the impact for two or three years. If you wait for stress indications, it will be too late. If it’s dry now, water now. Your trees will appreciate it.

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2 responses to this post.

  1. You hit the nail squarely on it’s head.
    Most people truly fail to grasp the important need to ‘Deep’ water both newly planted and older trees.
    Many seem to think trees never need watered in winter months and don’t grasp the fact that a tree is putting down a good root system as much or more in winter months as they are in summer months.

    Reply

  2. Thanks. You’re absolutely correct. Winter watering is critical, especially after a dry summer and fall. Until the ground is frozen solid, roots continue to grow.

    Reply

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