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.
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.
Q.) I have mustard weeds all over the place defying temperatures in the teens. What can I do?
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.
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.
Sheron C., Albuquerque
Herbicides can also be effective in controlling wild mustard. There are several different types of herbicides that will work against wild mustard, but there are some that the weeds have grown resistant to and that will no longer work.
Unfortunately, there are no other cultural or biological control methods for wild mustard. Burning does not help, nor does allowing animals to forage. The seeds of wild mustard can actually be toxic to livestock.
Because it’s so tough, getting rid of wild mustard can be a real project. If you do not want to use chemicals in your garden, the only way to eliminate this weed is to pull it out. The best time to pull mustard weeds is when they are young. This is because they will be easier to pull out, roots and all, but also because removing them before they produce seeds will help limit future growth.
Controlling Wild Mustard Plants
Wild mustard control can be a challenge because this is a tough weed that tends to grow and create dense patches that out-compete other plants. Wild mustard is a pain, but it is a bigger problem for farmers than for home gardeners. You can use both physical and chemical strategies to manage or eliminate wild mustard in your yard or garden.
If you have too many to pull, you can mow down wild mustard before seed production, during the bud to bloom stages. This will limit seed production.
Wild mustard (Sinapis arvensis) is an aggressive weed native to Europe and Asia, but one which was brought to North America and has now taken root. It is an annual that grows to about three to five feet (1 to 1.5 meters) and produces yellow flowers. You will often see these plants growing densely by the roadside and in abandoned areas. They are mostly problematic in cultivated fields, but wild mustard plants can take over your garden too.
There are different varieties of wild mustard, so first determine which type you have and then ask your local nursery or university agricultural department to help you select the right chemical.
Charlie Gray is a fan of growing mustard — the plant, not the hotdog spread — as a soil bio-fumigant. This Newbury, Vt., farmer knows that to make it work, you need to persevere.
He also learned that mustard must be incorporated within 20 minutes of mowing, or 80% of the gas is lost. “We had a lot of bees, so we’d chop it early in the morning before bees got active. Then we’d wait a few hours to till it in. We were waiting too long.”
It takes time to learn how to use it effectively, he says. “Some people, in the first year, don’t do some of the things they should, then don’t see the benefit, so they don’t do it anymore.”
“I’d love to learn more about what’s going on in the soil. We’ve been planting strawberries on this farm for 35 years. We try to rotate. But when you have five acres of beautiful sandy, south-facing land, you don’t want to seed it down for 10 years.
Gray uses no fumigation herbicides, but does use pre- and post-emergence herbicides to help fill in the gaps where they don’t use mustard. “We’ve noticed that we don’t have to use as strong pre-emergence herbicides after mustard fumigation,” he says.