Issue: May 3, 2003

Which cottonwood makes cotton?


I know some cottonwood trees make cottony fluff and others make pollen. Which is which? Gary Albuquerque


Male cottonwood trees produce pollen, while the female trees produce the cotton. That cotton is an appendage to help disperse the cottonwood seeds so they do not fall at the base of the mother tree. Since the seed are the potential offspring, they are produced from the mother (female) tree. Because of their pollen, the male varieties of poplar (the genus Populus, which includes the cottonwoods) are banned in Albuquerque. (There are a few native species of poplar that are not banned.) The male varieties are banned because of the human allergy problems associated with the pollen from poplars. Some people do not like the cotton from the female trees. While it doesn't cause allergies, it can clog car radiators, swamp coolers, and swimming pool heaters. This cotton is very flammable, so it can be hazardous near flames. This same relationship is true for the junipers. Male junipers make pollen, which causes allergies, while the females make the berry-like cones (after the female flowers are pollinated). To avoid the allergies, don't plant the male junipers near your home. Select varieties that produce the berry-like cones. This is all based on the fact that these plants are dioecious. Dioecious is a word that means the male flowers and female flowers are produced on different plants. This is not true of all plants. Most plants have both male and female parts in a single flower (e.g. apple), and a few have separate flowers on the same plant (e.g. pecan).

Irises water conservative


I have noticed a lot of beautiful irises in several towns I visited in New Mexico. I thought they required a lot of water. Shouldn't people avoid such high-water plants in New Mexico landscapes?


There are some irises that need considerable water (Louisiana iris), but there are others that are extremely well adapted to New Mexico. The bearded iris, aril, and spuria irises do well here. They are adapted to the soil and their growing season is in the early spring when the weather is cool, so irrigation water isn't quickly lost to evaporation. The spuria irises are interesting because they don't need to be divided as often as others, grow taller, and are very elegant looking. The aril varieties are from very arid regions and especially well adapted to soils and moisture conditions in New Mexico. As I speak to gardeners around the state, I continue to learn. I once complained to a Master Gardener that the problem with irises was that they only bloomed once a year. She has since shown me that there are bearded iris varieties that bloom two or more times a year. Irises are extremely beautiful, easy to grow and, if the proper types are chosen, very well adapted to New Mexico.

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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:, 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, or at the Desert Blooms Facebook.

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