July 30, 2016

1 - Cottonwood and other trees may naturally drop leaves to minimize summer stress, but proper irrigation can help minimize this leaf drop.

Yard and Garden July 30, 2016

Q.

I have a cottonwood tree that I planted about 12 years ago, so it is approximately 16 years old. Yesterday it shed about half of its leaves which it seems to do every year about this time. I know the heat and lack of rain this year is probably aggravating the situation but like I said it seems to happen every year. I water it once per week for about 45 minutes with a sprinkler that has a fairly heavy flow covering the general area of the drip zone. I would guess this would amount to about 3 / 4 inch of water, but as I said that is a guess. Grass is in that area and is nicely green, if that matters. I have other types of trees that seem to be doing fine. The question is, is this enough water and / or is there anything else I can do to help it keep its foliage? It seems normal before it loses its leaves. Is there anything I can do going forward this summer?

- Mike H.

Alamogordo

A.

Cottonwood trees are valley / flood plain trees. They need soil that is constantly moist and can tolerate periods of waterlogging. While they will grow on upland sites, they do not perform as well in those locations. Even in the valley locations they will drop many of their leaves in the summer. This is also true for the mountain cottonwood trees as well, since they grow in drainage areas of the mountains that often have subsurface moisture. Cottonwood trees tend to produce numerous leaves in the spring to maximize photosynthesis and food production, but they have a survival mechanism to drop surplus leaves as the heat and dry weather develop in the summer. The heat, dryness, and winds this year may indeed have exaggerated the leaf drop. By dropping surplus leaves (and sometimes even twigs and branches), the trees reduce their water demand and are better able to tolerate the stresses of summer. There are other trees with this characteristic, but many of them do not have as profound a summer leaf drop. The fact that the leaf drop was sudden was because the leaves had been developing abscission zones (a specialized zone formed to allow the clean removal of the leaf when it drops) to protect the tree and twigs when the leaves fell. In many cases, the abscission zones form but do not activate until moisture relieves the stress so that the trees can remove some nutrients from the leaves before discarding them. (This is my hypothesis observed in plants that undergo stress and then suddenly exhibit leaf drop after the stress is removed by precipitation or irrigation.) There were some precipitation events in Alamogordo and in the mountains above Alamogordo in late June and early July. These may have been sufficient to activate the leaf drop process is such a sudden manner. Leaves would have dropped anyway, but the stress release because of precipitation may have synchronized the leaf drop, causing the sudden loss of so many leaves. (Mulberry trees often exhibit synchronized leaf drop after a hard freeze in the autumn.)

Your irrigation practices apparently are sufficient to maintain the trees, but I have some suggestions. Remember the cottonwood trees are trees that perform best when there is consistent moisture in their root zone. This root zone extends from the tree outward to at least 4 times the height of the tree. The roots most actively absorbing moisture are the very small root hairs and hair roots that are found in the region under the drip line and outward to the end of the root system (as much as 4 times the height of the tree away from the tree). The more of this region in which you can maintain moist soil, the better the tree will grow. Absorptive roots of trees are most common in the top one foot of soil where they are in competition with grass roots. Grass roots are more efficient at absorbing precipitation and irrigation water than tree roots and compete with the trees for the water. This explains why the grass is doing well while the tree is losing leaves. The trees have deeper roots that allow them to compete more effectively compete with the grass. The deeper roots are effective to a depth of 2 to 3 feet, if the soil is moist to this depth. While there may be even deeper roots, most of these are large structural roots that help hold the tree upright, they have fewer absorptive roots since the metabolically active absorptive roots require oxygen to grow and absorb water (and minerals). At greater depths in the soil, oxygen is a limiting factor, so ideal irrigation should moisten the soil to a depth of 2 to 3 feet once every 2 weeks. More frequent irrigation will be necessary to sustain the grass if you wish to grow grass in competition with the trees, but the interim irrigations need only moisten to a depth of about one foot.

Your irrigation providing 3 / 4 inches of water once a week will moisten the soil to a depth of approximately 10 inches if the soil is very sandy and to a depth of about 2 inches if the soil is clay. You may need to irrigate significantly longer to moisten the soil to the depth the tree prefers. To determine how deeply you are moistening the soil when you irrigate, you should wait a day after irrigation then probe the soil with a long metal rod (or dig a hole, without injuring the tree roots). The metal rod should penetrate moist soil relatively easily, stopping when it hits the dry zone (or a rock, root, or pipe). After you learn how deeply the soil was moistened by your irrigation, you can calculate how long you should irrigate to provide the proper depth of moist soil. Once every two weeks you should irrigate to provide this depth of moisture over the extent of the roots from the drip line outward to moisten the soil for a significant portion of the lateral extent of the root zone. During the winter you should irrigate this zone once a month unless natural precipitation has provided sufficient moisture. Periodically irrigate more deeply to prevent accumulation of salts and changes in soil characteristics that may occur at the bottom of the moist zone if you always moisten to the same depth. Cottonwood trees and many other trees are able to tolerate and even prosper when they do not receive ideal irrigation, but the closer you come to meeting their preferred needs, the better the tree will perform for you. The cottonwood tree will probably still exhibit summer leaf drop, but perhaps not as severely as you have experienced this year.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

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