Issue: August 31, 1998

Pecan problems

I have two Western Schley pecan trees planted seven years ago. Last year both produced very well. This year one suddenly dropped all of its pecans, but the other didn't. They are only 20-feet apart and are managed the same. What happened and how do I prevent it next year?

Last year our 20 year old pecan tree had pecans with black shucks that stuck tight. When we removed the shuck,the pecan meats were also dark and shriveled. This year the tree doesn't have a dozen nuts on it. Before last year the nuts were large and very good. We put pecan food on it every year and water often. Please advise us.

I spoke with Dr. Esteban Herrera, NMSU Extension Horticulturist working with grapes, tree fruit crops, and pecans. He feels that the answer to the first question involves the hot dry weather this year and that irrigation is the ultimate cause of the problems described.

He stated that the maximum water needs for pecans occur during August in Las Cruces, at a physiological point of pecan nut development called "water stage." Pecan shells harden in late August. Water deficiency in August, before the shell hardens, causes the trees to drop of most of the crop as described in the first question. This drop follows a good rain or irrigation following the dry period, especially in sandy soil. Since it follows a rain, drought often doesn't seem to be the cause.

Insufficient water late in the season will make the trees weak and the shucks will not open, producing "stick tights." An early freeze will do the same, but the pecans will be well filled. However, there are years when pecans are affected by other factors at the time the pecan shell (not the shucks) are hardening in late August. Wind or insect damage before shell hardening will make the nut drop, but if it happens at the end of shell hardening, the pecan will not drop, but it will not fill, producing the empty nuts called "pops." Pops are stick tights with no meat inside.

He also thinks that a characteristic of pecans, alternate bearing, is also involved. Some pecan varieties, like many other fruit crops tend to produce heavy crops one year followed the next year by light crops. If there is a problem with orchard management(especially water and fertilizer), he said this cycle can go wild. According to Dr. Herrera, established pecan trees need 150 to 200 gallons of water on the hottest day of the summer. In very dry weather, especially in a lawn setting in which there are not many trees around to break the wind, it is difficult to provide sufficient water to the trees. He also observed that few pecan trees in a lawn are provided that much water in the first place. Remember that in a lawn, the trees compete with other plantings and the grass for the water provided, so all that is provided does not go to the pecan tree. You did not describe the landscape in which these trees were growing, but consider the amounts of water that Dr. Herrera said the pecans need.

So, why did two trees only 20-feet apart have different responses? There are several factors to consider. Soil conditions can change over that distance, so one had a soil which held water better than the other, contributing to the problem, or one was a variety which tolerated the conditions better. Pecans are grafted onto seedling rootstocks, and each is different from the others. This can influence a better root system for one tree, allowing a better start when the tree was planted.

Many other factors also affect the development of pecan crops. Insect problems, disease, mineral deficiencies, especially micro-nutrient deficiencies, can affect the crop in pecans. There are many good NMSU Extension publications written by Dr. Herrera available to you at your County Extension Office and on the World Wide Web at http://www.cahe.nmsu.edu/cahe. You might wish to get some of the publications to read for more information.

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.

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

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

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