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mustard seed weed

Planting date: Later planting will reduce wild mustard populations.

Chemical

Mechanical

Longevity: Low persistence – 50% of the seed bank is reduced in less than one year, and it takes seven years to reduce the seed bank 99%.

Tillage: Seedlings are readily killed by tillage.

Winter/summer annual. Emerges in late summer, early fall or spring. In Michigan, several populations of wild mustard act as a summer annual. Flowering peaks in June and July, but can continue until the first frost.

Management:

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.

Wild mustard can represent a serious weed problem in canola and spring cereals. Germination of wild mustard seed, and rapid early seedling growth under cool spring and fall temperatures, allow wild mustard to compete effectively with crop plants for light, water and nutrients. Populations of wild mustard left uncontrolled throughout the growing season can reduce potential yield and seed quality of the harvested crop.

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.

Economic Importance

Wild mustard also has beneficial aspects. Flowers of wild mustard are a prime source of pollen and nectar, making them a desirable site for pollinating insects. In Europe, wild mustard is used as a leafy vegetable, and oil from seeds is used for making soap, cooking and as a lubricant.

The flowers are bright yellow, about 1.5 cm across with 4 small sepals, 4 petals arranged in a cross formation, 4 long and 2 short stamens (total of 6), and 1 slender pistil. Flower stalks are thin and short (3-5 mm), becoming thicker but not longer as the seedpods develop. The seedpods, called "siliques," are 3-5 cm long, usually hairless, often with lengthwise ribs, erect and pressed to the stem or spreading out. (Figure 4) Each pod has a flattened terminal beak about 1/3 the total length of the pod having 1 or 2 seeds in its base, and a main section containing several seeds that are released when the 2 sides or valves of the pod split apart from the bottom end and entirely fall away. Wild mustard can be confused with other annual yellow-flowered mustards. It is distinguished by having

Wild Mustard (Sinapis arvensis L., Brassica kaber (DC.) L.C. Wheeler var. pinnatifida (Stokes) L.C. Wheeler) is an aggressive weed indigenous throughout most of the temperate regions of Europe, Asia minor, southwest Asia and North Africa. It was introduced into North America and now occurs throughout all Canadian provinces, as well as in the MacKenzie District, Northwest Territories.

Figure 2. Wild mustard seedling. Notice the broad kidney-shaped cotyledons indented at the tip.

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

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 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?

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.