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Issue: March 21

Be careful when using fertilizer combined with post-emergent herbicide.

Q. I live in El Paso near the Franklin Mountains. I have two globe willow trees in my yard; they are located inside an area with lawn. They were already here when I bought the house 5 years ago.

To fertilize the lawn, I have used a well-known brand-name fertilizer with added ingredients for weed control. But last year, after the leaves of the globe willows turned green they started to fall; this happened in late spring and during summer, most of the inner leaves fell and some branches dried out. I tried to correct his situation first by watering the trees a lot, it did not work and then I tried watering the trees very little. This year I have not used any fertilizer so far.

My concern is how I can grow a healthy lawn and healthy globe willows at the same time and in the same place. This house was built 15 years ago and the trees have a circumference of about 50 inches, measured at about 3ft from the ground. Some roots are now apparent at the lawn level.

Please give me some guidance on how I can take care of both the lawn and the trees.

Javier A.

A. The product you mentioned above is a product that combines fertilizer and post-emergent weed control. This seems convenient, but it can cause problems. The globe willow tree is a broad-leafed plant. The herbicide contained in the fertilizer kills (or injures) broad-leafed plants (dandelions and trees). You may have witnessed damage from the application of the herbicide. However, it is possible that the late freeze last spring also caused some damage. The symptoms you described can be caused by a late freeze, and in much of New Mexico, and perhaps, El Paso, just such a freeze occurred.

However, it is important to consider the possibility of herbicide injury. The labels on most products containing fertilizer and herbicide warn homeowners not to use it in the root zone of desirable trees and shrubs. The roots of trees and shrubs extend well beyond the canopy of the tree or shrub (up to 4 or more times the height of the tree). So, in most urban and suburban lawns, there is no place to use this kind of product.

The alternative is to apply fertilizer separately from the herbicide. Water after fertilizing and water well to wash the fertilizer into the soil so the roots can absorb it. However, water before applying weed control and then delay watering for several days before watering again to avoid washing the herbicide into the soil where the tree roots can absorb it (the herbicide will be absorbed by the leaves of the weeds).

A non-herbicide method of managing weeds is to maintain a healthy, dense lawn that inhibits weed growth. Proper fertilization, irrigation, and mowing heights help discourage weeds and foster a healthy lawn (mowing height depends on the type of turfgrass). More information on turf management is available on the NMSU College of Agriculture and Environmental Sciences web site listed below.


For more gardening information, visit the NMSU Extension publications World Wide Web site at http://aces.nmsu.edu/pubs/_h.

Send your gardening questions to:

Yard and Garden, Attn: Dr. Curtis Smith
NMSU Agricultural Science Center
1036 Miller Rd. SW
Los Lunas, NM 87031

Curtis W. Smith, Ph.D., is an Extension Horticulture Specialist with New Mexico State University's Cooperative Extension Service. NMSU and the U.S. Department of Agriculture cooperating.