May 14, 2016

1 - Trees that did not fully leaf-out this spring may have been injured by the spring weather or by herbicide treatments.

Yard and Garden May 14, 2016

Q.

The desert willow in front of our house has always been a great looking low tree. This year only half of it got leaves. The other half looks as if it is trying to stay alive but has leaves under-developed and sparse. What can we do to rescue it?

- Carole E.

From NMSU University-Wide Extension

A.

I have been looking around to see if I can see similar symptoms (I live in Albuquerque). I have found trees that have only partly foliated, even at this late date when most trees have a good covering of leaves. In the cases where I see the symptoms you described, I can think of a few possible causes.

Some trees in my neighborhood have leaves on part of the tree while the rest of the tree is barren. The barren portion may be at the top or on a side. In these cases, I know that the owners had their properties treated with weed killers. Broadleaf weed herbicides can also injure or kill many kinds of trees, including desert willow.

The variable weather this late winter and spring may also be a cause of these symptoms. After the trees have experienced sufficient cold weather (the appropriate amount of cold depends on the species or variety of tree), their buds loose dormancy and hardiness and prepare to grow. Warm weather in late winter can cause the buds to begin metabolism for growing and to lose their hardiness. Last year my apricot trees warmed too much in February and subsequent cold weather killed the flower buds. The lower parts of my apricot trees produced no flowers, but there were some flowers higher up in the tree where warmth from the soil did not induce early loss of hardiness. The trees produce numerous leaves, but many of the flower buds were killed and never opened, even though I had anxiously watched them swell and prepare to blossom. Apricot flower buds loose hardiness earlier than the vegetative leaf buds, so I had many leaves, but no fruit.

In the case of many trees, uneven soil moisture around the tree could be a problem, but in the case of the desert willow I was once taught that desert willow trees should not be irrigated during the winter as that could damage the trees. When I told this to a friend, he exclaimed that this explained why the desert willow trees in his lawn (irrigated in winter) had died while those away from the lawn and unirrigated had grown well. Normally winter precipitation does not seem to be a cause of problems for desert willow unless the trees are in a low place that accumulates water during the winter or if the tree receives supplemental irrigation during the winter.

Do any of these conditions fit the circumstances of your landscape? Perhaps this will explain what happened. There are other factors such as wind damage and physical injuries that could be a problem. Disease may also be a consideration. To test for disease you can take a sample to your local NMSU Cooperative Extension Service office. Collect a sample of the damaged branch cut back far enough to include some healthy tissue.

If any of the situations I described above apply, the tree may survive. You should cut away any dead wood when it is obvious that there will be no growth. It is proper to cut dead wood in the summer. Water the tree properly and see if it can reestablish. Do not fertilize the first year, wait until recovery is well under way. If herbicides were used, most of these chemicals decompose over time under the influence of sunlight or biological activity so the chemical, if it was not applied in too great a concentration, it should have reduced impact this year. Some herbicides have long residual effects and may continue causing problem if they persist. Some may be washed from the root zone by heavy irrigation, but others may remain and continue to be active. Herbicides should be used very carefully and strictly in accordance with the directions on the packaging.

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.

Links:

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the Desert Blooms Facebook page.

Please copy your County Extension Agent and indicate your county of residence when you submit your question!