Issue: July 16, 2005

Black walnut or other shade trees

Question:

I am trying to establish some deciduous shade trees for my home. I have a Black Walnut tree that came up on its own. It is about 15 feet tall now, and I love its color, shape and mottled shade qualities. However, I have read that when it matures the walnuts will be horrible to have to clean up before mowing. I would like to plant more of them, but am worried.

I am having trouble choosing alternative trees that are not going to be too difficult to live with. I want the leaves to drop during the winter to let in the sun and don't mind mowing them up. However, I don't want root problems or fruit problems (i.e. nuts to break the mower). The Chinese pistachio, Chinese elm, and crape myrtle are looking good.

I was hoping that you would advise me since you encourage tree planting for the urban area. I am a high school teacher who enjoys gardening and has the summer time to engage in it.

R. Larson
via Internet

Answer:

The black walnut is a nice, very large shade tree for appropriate landscapes if its negative characteristics are considered. You mentioned one negative characteristic - the production of nuts. For some people the nuts are desirable, but the black walnut has a very hard nut that is difficult to crack and can be dangerous when mowing. When struck by a rapidly spinning rotary mower blade, the mower may be damaged, but a greater risk to the mower operator and children, adults, and structures nearby is that the nut may become a high speed projectile. It is possible to rake the nuts before mowing. This will be necessary only after the nuts have enlarged. Early in the season, they are small and not as dangerous.

A second negative characteristic is the allelopathic properties known to be associated with black walnut trees. These trees create chemicals that inhibit the growth of many other plants. This will result in poor plant performance in the vegetable and flower garden. If you are growing other landscape plants, the black walnut may not be a good choice.

Of the alternative trees that you mentioned, I prefer the Chinese pistache because of its fall color. It produces a deep root system in soils that allow good root depth and may cause fewer problems with landscape paving (sidewalks, driveways, patios, etc.). The female Chinese pistache does not produce large nuts so it will not be a problem in that sense (but will not produce nuts for eating). The Chinese elm in New Mexico is confused with the Siberian elm. The Chinese elm is a much nicer tree with an attractive exfoliating bark, and for that reason often called lacebark elm. It has a shallower root system than the Chinese pistache and can cause problems with hardscape areas in landscapes as well as septic systems. The crape myrtle is rarely a tree large enough to create shade in New Mexico. It makes a nice, large shrub or small tree that produces clusters of attractive flowers in the summer. In areas where it is not killed by low winter temperatures, it may freeze to the ground but return in the summer to form a shrub. In other areas where it does not freeze to the ground, it makes a small tree with a very beautiful, smooth bark. There are many other trees from which to choose, but this is a discussion of those you mentioned. I hope this helps you make an informed decision.

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Send your gardening questions to:
Yard and Garden, ATTN: Dr. Curtis Smith
NMSU Cooperative Extension Service
9301 Indian School Road, NE, Suite 112
Albuquerque, NM 87112

Curtis W. Smith, Ph.D., is an Extension Horticulture Specialist with New Mexico State University's Cooperative Extension Service. New Mexico State University is an equal opportunity/affirmative action employer and educator.