1 - Cottonwood (Poplar) trees drop leaves due to mid-summer heat stress, but not all do so equally for various reasons.

Yard and Garden August 19, 2017

Q.

I have 2 cottonwoods in Belen. I read why they shed but it seems to be mine are the only ones.

I water all my trees three times a week also 6 or 7 years ago my tree were hit with aphids and other kinds of bugs due to my neighbor’s tree, but now I treat them.

Am I doing something wrong?

- Berna U.

A.

I will answer with some possibilities since it is impossible for me to say exactly what is happening. If you need to contact me again, please feel free to do so.

You said you have read that cottonwood trees shed leaves, and this is true. Environmental conditions such as the transition from cooler spring conditions to our really hot late June and early July can be such an environmental trigger. Your questions as to why your tree shed leaves and the neighbors did not is challenging. It may be that they have a different species of poplar (cottonwood tree). There are the native poplar trees, poplars from other parts of the country with different environmental conditions, and hybrid poplars. All can shed leaves under environmental stress, but may not all respond at the same time. Perhaps your trees are a different type of cottonwood.

Irrigation is important as you indicated. It is important for gardeners to know that poplars have relatively shallow, but very wide-spreading root systems. Irrigation to a depth of approximately 2 feet every 2 weeks over a large area from the dripline of the tree outward for many feet should help maintain a healthy tree, but may not prevent heat induced leaf drop.

Different soil types can affect how irrigation water impacts the trees. Soil types can change over relatively short distances, so perhaps your soil is different from where your neighbor’s trees are growing. One soil more sandy and drying more quickly and the other soil containing more clay and able to hold more water for a longer period of time.

Disease such as “slime flux” (wet wood disease) may also influence the growth and leaf retention of the tree. Slime flux disease does not quickly kill a tree, but does impact the health and vigor of a tree.

Insects may also be a problem. Aphids and scale insects can weaken a tree and cause premature leaf drop.

This is a fairly long list of possible causes of leaf drop. There are other possibilities as well. Your local NMSU Cooperative Extension Service agent and NMSU Master Gardeners working with some county agents can help you narrow down the list and may even be able to diagnose a specific cause. They also have access to the NMSU Plant Diagnostic lab that can help identify problems.

Once again dear readers: For over 22 years I have had the honor of writing the Yard and Garden column as NMSU Extension Horticulture Specialist and in retirement, but that time is drawing to a close. Dr. Marisa Thompson has assumed the duties of NMSU Extension Horticulture Specialist and will soon be answering your questions in the Yard and Garden column. I will continue writing for a brief period as Dr. Thompson gets settled into her office and the many aspects of her job, but you will notice that it is her contact information provided at the end of this column. I will soon begin writing a garden blog, so I hope I will be able to see my friends online as I do that. I hope to see you at SouthwestGardenSmith, but please also use the resources of NMSU’s County Extension offices and rely on the great expertise of Dr. Thompson. I remain your garden friend, Curtis Smith (SouthwestGardenSmith).

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:

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