Whew, has it been hot! Scorching temperatures and dry conditions in the last few weeks have made it difficult for both people and plants. Our trees are still recovering from the last two years of severe drought and are not fully recovered yet. Fall watering is one of the best things homeowners can do to prevent further stress and help their trees continue to recover.
But the drought’s over, why are trees still stressed?
During severe drought, as we experienced in 2022-2023, trees were damaged in several ways as listed below.
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Reduction in photosynthesis and, consequently, production of energy for growth, defense and storage.
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Reduction in stored energy reserves.
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Death of mycorrhizal fungi. This symbiotic tree root-fungal root association greatly expands a tree’s root system and the tree’s ability to take in nutrients and water from the soil.
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Death of tree root hairs and eventually progressively larger roots.
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Leaf loss.
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Loss of functional xylem, the water moving conductive tissues within a tree’s roots, trunk and branches.
Trees must regrow xylem, roots, mycorrhizal fungi and replace stored energy reserves to fully recover. For this reason, woody plants often experience a 2-5 year period of biological lag time in their recovery from drought.
Yes, many parts of Nebraska have had good amounts of rain this year, but those rains have been followed by some long dry periods and hot weather which causes plants to need large amounts of water for normal growth and function. According to the
U.S. Drought Monitor, Lancaster county has normal to abnormally dry levels of soil moisture. But that’s not too bad, right?
Additional stress can slow down a tree’s recovery or may be enough to push it over the edge into a downward spiral. As the proverb goes - the final straw which broke the camel’s back.
Aren’t tree roots really deep?
“Trees have roots as deep as the tree is tall” – this is a common misconception related to trees and watering. For those operating under this misconception, it’s easy to think trees can draw moisture from very deep layers of the soil - but it’s not true. Yes, some tree species do have tap roots or anchoring roots which grow deeply into the soil, but these are not water-absorbing roots.
Research has demonstrated 90% of tree water-absorbing roots are in the top 24-inches of the soil. In fact, 50% of water-absorbing roots are in the top 12-inches of the soil. If these relatively shallow layers of soil are dry, the tree is very limited in its ability to pull up water.
How can I tell if my trees are drought stressed? Symptoms of drought stress often include the following.
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Thin canopy of foliage. Few leaves or smaller than normal leaves.
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Browning or scorching around leaf edges. Evergreen needles turn brown from the tip, resulting in half brown-half green needles.
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Development of early fall coloration, reddening or yellowing of leaves.
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Early leaf drop.
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Smaller amounts of new growth.
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Twig or branch dieback.
Of course, these symptoms can be caused by other problems, too. If you need help determining the cause of problems in your tree,
contact your local Nebraska Extension office.
How often should I water?
For mature, established trees when conditions are dry, deep soaking every 2 weeks in August and September is usually adequate. Apply water deeply, moistening the soil to a depth of about 12-18 inches. Use a long-bladed screwdriver or piece of rebar to check the depth of water penetration. Once you've reached dry soil, it will be much harder to push the probe into the ground. Water the entire area underneath the tree’s dripline if possible.
If using a sprinkler, let it run in a low pattern in one area until the top 12-18 inches of soil is moistened then move it as needed to water the entire area underneath the tree’s canopy or over a shrub’s root zone. Or coil a soaker hose several times around the tree from the trunk to the dripline, and let it run until the soil is moistened.
If dry conditions continue into late fall, irrigation can continue monthly from October through December or until the ground freezes.
What else can I do to help my tree?
Apply a 3 to 6-foot diameter ring of mulch around the base of your trees and shrubs, using 3-4 inches of an organic material, like coarse wood chips. Make sure the mulch is applied in a flat layer, like a pancake, not piled up like a volcano against the plant's bark. Keep it far enough away that it doesn't touch the tree's trunk.
Research has shown trees grow roots significantly better under wood chip mulch than they do under grass. Giving the tree a small area where it doesn’t have to compete with grass roots for moisture and nutrients can help the tree more quickly regrow roots lost during the drought.
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- Smaller amounts of new growth are an indicator of tree stress. Less than 6" of new growth each year on a well-established tree may indicate there is a problem.
- Development of early fall coloration, reddening or yellowing of leaves is a common stress symptom, especially in maples.
