Figure 2. Wild mustard seedling. Notice the broad kidney-shaped cotyledons indented at the tip.
Wild mustard plants have from 10-18 seeds per pod and from 2,000-3,500 seeds per plant. During harvesting operations, shattering may result in large quantities of seed being left on the ground, or seeds may be transported into other fields by harvesting machinery or as impurities in crop and forage seed. Some wild mustard seed is capable of germination as soon as it is mature. However, these seeds may also remain viable in the soil for as long as 60 years, particularly those that are buried at considerable depths. Due to the longevity of wild mustard seed in the soil it is important to control this weed and reduce the amount of seed returned to the soil. This minimizes potential economic losses in current as well as future years.
Wild mustard is an annual plant that exhibits erect growth. The seedlings have broad kidney-shaped cotyledons (seed-leaves) that are indented at the tip. (Figure 2) Older plants have alternate leaves that are somewhat hairy, especially on the lower surface of the veins. The lower leaves are usually stalked, deeply lobed with a large terminal segment and a few smaller lateral lobes. (Figure 3) Upper leaves are stalkless, generally undivided but coarsely toothed. Plant height can range from 30-100 cm with either simple or much-branched stems. The stems usually have stiff downward pointing hairs, especially on the lower parts, and are green or somewhat purplish. Flowers are produced in small clusters at the ends of branches; these clusters elongating as the seedpods develop.
Mature pods usually remain intact until the crop is harvested. The seeds of wild mustard are spherical, 1.5 mm in diameter, black or purplish, and appear netted under high magnification. Wild mustard reproduces only by seed and requires 2½-3 months to produce mature plants from seed.
Fruit: Fruit is a silique, 5/8 inch long, tapering to a conical beak, appressed against the stalk of the raceme as it matures; petiole of silique (or flower) is about 5/16 inch long; seeds are dark brown or black.
Leaves: The alternate leaves are 2 to 10 inches long, 1 to 6 inches wide, becoming smaller as they ascend the stems; lower leaves are pinnately lobed and obovate in outline, tapering to a long and rather stout petiole (not clasping), terminal lobe much larger than the lateral lobes, upper surface, often bristly with scattered hairs that are stiff, short, and white, lower surface usually glabrous, except for a few hairs along the central vein; upper leaves often lanceolate, broadly elliptic, or some other odd shape, 1 to 2 lobed or none.
Flowers: Flowers May to July; narrow racemes of yellow flowers, 6 to 24 inches long when fully mature; flower up to 5/16 inch across, consisting of 4 sepals and 4 yellow petals. The sepals are initially green, but become yellow while the flower blooms. The petals are well rounded toward their tips.
Reproduces by seed.
Black mustard occurs in dry disturbed sites such as waste places, pastures, and along roadsides and railroad rights-of-way within elevations that generally range below 7,000 feet.
Erect annual, taprooted forb, 2 to 8 feet tall; stems usually glabrous and glaucous, sometimes with scattered stiff hairs toward the base; upper stems terminate in narrow racemes of yellow flowers.
Native to Eurasia; black mustard seeds and foliage have a pungent taste. Black mustard grows profusely and produces allelopathic chemicals that prevent germination of native plants; in addition, the seeds contain an alkaloid and the sinapina the glucoside sinigrin. This species generally occurs as a weed in wildland areas of the Southwestern Region rather than as an invasive plant.
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.
Sheron C., Albuquerque
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.
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.
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.
Q.) I have mustard weeds all over the place defying temperatures in the teens. What can I do?
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.