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When dry weather continues for an extended period, landscape trees depend on homeowners for water. According to the Texas Forestry Service, more than 5 million urban and landscape trees in Texas alone have been lost due to drought, so it's important to take care of surviving trees and nurture replacement trees with proper watering.
The amount of water a tree needs depends on many factors, including the age and species of the tree, the time of year, weather and soil type. As a rule, newly planted and young trees require more frequent watering than older, well-established trees. But during extended periods of drought, all trees benefit from supplemental watering.
According to Skip Richter, County Agent with the Texas AgriLife Extension Service in Houston, during periods of drought, "the goal is to provide just enough supplemental irrigation to maximize growth on young trees and to keep older, established trees healthy. We don't want to water so much or so often that we encourage more canopy growth than the soil, climate and tree species can support during normal rainfall years. Excessive watering can make a tree dependent on irrigation rather than resilient enough to survive on what nature normally provides."
For the first several months after planting, most of a tree's roots are still within the original root ball, with some roots beginning to grow beyond this area. The root ball and the surrounding soil should be kept evenly moist to encourage healthy root growth. After a few months, expand the watering zone to cover the entire area under the canopy. It can take two or more growing seasons for a tree to become established — for roots to venture into the soil well beyond the planting hole. It's vital to provide supplemental moisture in those early years, if nature doesn't provide regular soaking rains. During hot, dry weather, new trees may require water as often as three times per week to ensure that the root ball doesn't dry out.
It's a common misconception that a tree's roots are a mirror image of the aboveground canopy. In reality, an established tree's roots usually extend well beyond the edge of the canopy, or drip line. Although some anchor roots may reach deep into the soil, most tree roots are concentrated in the upper 12" to 18" of soil. When watering established trees, provide a deep, soaking irrigation to the entire area beneath the tree canopy and extending several feet beyond the drip line. Ideally, you should moisten the soil to a depth of 10" each time you water. To prevent rot, don't apply water to the area directly around the trunk.
The easiest way to check soil moisture is to take a long (8"-plus) screwdriver and poke it into the soil. It will pass easily into moist soil, but be difficult to push into dry soil. If you can't poke it in at least 6", it's time to water. This technique works best in clay and loam soils.
Overhead sprinklers are the easiest way to cover large expanses, but they're inefficient, losing up to half the water to evaporation. Trees are better served by watering methods that apply water slowly, right at soil level. It may take several hours to properly water a single mature tree.
Soaker hoses are an efficient way to water trees because they're porous and release water slowly. Encircle a tree with a spiral of soaker hose and run it for an hour or more — as long as it takes for water to penetrate 6" or 8", using the screwdriver test.
A Pressure Regulator improves the efficiency and prolongs the life of soaker hoses.
Bubblers are hose-end devices that reduce the velocity of the water, so it soaks in rather than running off. Because it waters one spot at a time, you'll need to move the bubbler around.
Our Snip-n-Spray Garden and Landscape Sprinkler System has ground-level sprinklers suitable for watering trees.
If possible, avoid watering during the hottest part of the day — 10 a.m. to 6 p.m. — to conserve water.
How much water does my tree need?
As a general rule of thumb, apply an inch of sprinkler irrigation or enough water to moisten the soil to a depth of 10" or more for mature trees. A common mistake is to apply frequent shallow waterings that don't soak deeply into the soil.
My irrigation system waters my lawn regularly. Isn't that enough for my trees?
Probably not. Most irrigation systems are programmed to apply frequent, shallow waterings. Trees do better with less frequent but deeper soakings — a heavy soaking once a week is much better than a shallow watering every few days. That's because shallow waterings encourage tree roots to remain near the soil surface where they're prone to drying out. Watering deeply, on the other hand, encourages deep, drought-tolerant roots.
Should I mulch under my trees?
Yes. Grass growing under trees will intercept much of the water you apply, keeping it from reaching plant roots. It's best to keep a large (3' plus), turf-free circle around the trunk. A 2" to 3" layer of organic mulch, such as shredded bark or pine straw, helps conserve moisture and keeps weeds at bay. To prevent rot, don't pile mulch against the trunk.
Should I fertilize during a drought?
As a rule, drought-stressed trees should not be fertilized. When water supplies are limited, trees naturally slow their growth. Applying fertilizer can encourage a flush of growth that causes the tree to require more water than is available. And the salts in many fertilizers can harm drought-stressed roots.
Is the technique the same for those of us living in desert climates?
According to Phoenix-based author Cathy Cromell, in desert conditions irrigation should soak the soil at least 3' deep. "We have such salty water and salty soil; deep watering helps by leaching salts past the root zone. Salt burn is a very common here, especially with non-native trees." Deep irrigation also encourages roots to grow deeply, as opposed to frequent light watering which leads to shallow roots that are more vulnerable to drying out.
Even if your municipality imposes watering restrictions, it's likely you'll be able to properly water trees. If you must choose between turf and trees, remember that trees are a bigger investment. And it will take years, if not decades, for a newly planted tree to take the place of a mature tree that has been lost to drought.
Last updated: 8/5/19
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