Issue: September 6, 2008

It is time to divide and move irises
Continuation of article discussing herbicides to completely control vegetation in large areas


Is it too late to transplant my irises? They bloomed last spring, but not as well as in previous years, I think they have become crowded and need to be divided and moved.



Now is an excellent time to dig, divide, and move irises. I dug some of mine last weekend and saw new root development just beginning.

As you divide them, do not fear discarding the old rhizomes (swollen, horizontal stems from which the leaves and flowers are produced). Trim to a short, healthy piece of rhizome with a "fan" of leaves. You can trim the leaves back to about 4 inches if you wish.

Prepare the new planting bed by digging deeply, and by adding compost and phosphate fertilizer. Be sure to choose a place that is not too shaded by nearby trees and shrubs. Afternoon shade is good, but several hours of direct sunlight will result in the best growth and flowering.

Water the irises after you have planted them and maintain the soil at a slightly moist state until frost. During the winter water the plants about once a month.


This is a follow-up of last week's question from Jim L. regarding a chemical that will permanently prevent weeds from growing:



After sending the article stating that there were no recommended products still available to give long-term weed control, I received several e-mails informing me that such products still existed. One such product is prometon, a long-persistence herbicide that I alluded to when I wrote that weeds will return before desirable plants (vegetables, flowers, shrubs, and trees) can be planted in treated soil. This product was once more common, but was removed from shelves of some urban garden centers. Those who wrote to me said that this product is still available at some farm supply stores. This product is used for large areas in which no plant growth is desired (places where trucks and machinery are parked). It may harm trees and shrubs near enough that their roots can grow into the treated areas.

Another writer told me of some weed control products used in his area for maintaining bare ground. These include herbicides containing the active ingredients diuron, imazapyr, and bromacil. He stated that these herbicides greatly reduce the work of weeding in those areas. However, they may not kill all weeds. Read the label before purchasing the product to see that it will control the weeds that are on your property.

Some of the products mentioned above may be restricted-use herbicides, that means a homeowner may not purchase or apply these products. They must be applied by a licensed pesticide applicator who has successfully taken tests administered by the New Mexico Department of Agriculture to obtain the license. Contact your local farm supply store or a licensed pesticide applicator to see if there are products that may be used in your situation. Your local NMSU Extension Agent can also provide you with valuable information regarding these products.

With any of these herbicides as with any pesticide, be sure you understand the label directions and follow them carefully. Some of the products containing the chemicals mentioned say that they are not to be used around homes others are labeled for that use. One of the correspondents who wrote to me about this mentioned that he knows of law suits and neighbor conflicts from misuse of these products. I have also seen problems that developed when people used these materials (this is why some of these products are no longer easily available). Please use them carefully.

For more gardening information, visit the NMSU Extension publications World Wide Web site at

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!