February 11, 2017

1 – Mustard weeds are problems in New Mexico, but there are things you can do to manage the problems.

Yard and Garden February 11, 2017

Q.

I have mustard weeds all over the place defying temperatures in the teens. What can I do?

- Sheron C.

Albuquerque

A.

Dr. Leslie Beck, NMSU Extension Weed Specialist, provided the following excellent information in answer to your question. This should also be helpful to other New Mexico gardeners.

Mustard weeds are very prolific and competitive weeds which invade multiple cropping systems, from lawn and landscape to agronomic crops, throughout the southwestern US. In New Mexico, these weeds are primarily comprised of London rocket (Sisymbrium irio), flixweed or tansy mustard (Descurainia Sophia), or shepherd’s purse (Capsella bursa-pastoris). Mustards, like most plants designated as weeds, have the ability to germinate quickly and mature earlier in the year than our desirable/native plants, which gives them a definite competitive edge. Additionally, London rocket is notorious for overwintering the curly top virus, which can then be transferred to nearby garden crops (i.e. tomatoes, chile) in the spring. This occurs as the beet leafhopper feeds on the mustard plant, carries the virus in its mouthparts which are then used to feed on the garden crops, thus effectively spreading the disease.

Mustard weeds have annual lifecycles, although they can germinate and grow as both winter and summer annuals depending on temperatures. Additionally, mustards like flixweed and shepherd’s purse have the ability to survive as weak biennials depending on temperatures and moisture availability. Plants with annual and biennial lifecycles must germinate from seed during each growth season. Thus, the key to controlling these plants is to target them when they are newly germinated before they have the ability to produce copious amounts of viable seed and disperse them into the soil to germinate later (in some cases decades later). Mulching (2-4 inches thick) or laying plastic over areas of bare soil will help to block sunlight and prevent germination of mustard seeds. I often get questions asking if allowing plastic covering to be exposed to the sunlight will heat the soil surface to a temperature that will kill dormant weed seeds. While this practice, known as solarization, may help some (which is better than none), generally the soil will not get hot enough at enough of a depth to really make a dent in the seed population. Using a weed burner or a propane flame torch may be effective on germinating plants, but again the soil acts as a great buffer against heat and will not damage seeds unless they are right on the surface. In addition, great caution should always be taken with using flame to control weeds. If you are not opposed to herbicides, a preemergence barrier with the active ingredient pendimethalin will also help to control the weed as they germinate.

If you are not opposed to herbicide applications for weeds that have germinated, you want to try and time these applications when the plant is young (the younger the better) and actively growing. Applications with products with active ingredients like 2,4-D, dicamba, triclopyr, and glyphosate have been reported to control mustard weeds. If organic options like acids or oils are preferred, make sure that the product that you use is labeled as a herbicide, such as enhanced vinegars. Home-use distilled vinegar is not labeled for use as an herbicide. Since these products only burn and damage the surface of the plant, it is also essential to try and make this application when the plant is young and most susceptible.

If you have mature mustard plants and it is late in the season (such as now) the plant may be too mature to respond completely to an herbicide application, synthetic or organic. In the southern portion of the state (Las Cruces, Deming, Hobbs, et.) temperatures have been so mild this winter that mustard plants are already beginning to produce seed heads with viable seed…at this point the most effective method of control is to cut the roots, rake, or hand-pull and remove the plants prior to dropping their seed. In some of the more northern cities (Albuquerque, Santa Fe, etc.) temperatures may still be cold enough to delay the production of seed heads in mustard plants. If this is the case, as the temperatures start to get a little warmer you can apply a postemergence herbicide, such as glyphosate, as the plant is actively growing and developing a seedhead.

As with annual weeds, all management efforts should focus on preventing the plant from producing viable seed and continually contributing to the population of the seedbank. While this method may take multiple seasons of effort, over time you will start to notice a decrease in the population of mustard weeds in your area and management will gradually become easier and less time consuming. Combining as many of the management options discussed in the above paragraphs as possible will also help you increase your success in trying to rid your lawn or gardens of this pesky weed!

For more gardening information, visit the NMSU Extension publications Web site at http://aces.nmsu.edu/pubs/_h, or to read past articles of Yard and Garden go to http://aces.nmsu.edu/pubs/periodicals.html

Send your gardening questions to Yard and Garden, Attn: Dr. Curtis Smith at cwsmith@nmsu.edu or leave a message at https://www.facebook.com/NMSUExtExpStnPubs.

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