April 16, 2016

1 - Purple mustard and white top weeds illustrate the strategies to manage annual and perennial weeds.

Yard and Garden April 16, 2016


I have some plants growing in my backyard and have some plants I would like to ask about. I have attached pictures of each to my e-mail. The plant with white flowers has taken over the flower bed and is spreading. I think the purple flowered plant is a verbena or vervain? They are both voluntary plants. Can you tell me what they are? I live in the East Mountains east of Sedillo Hill.

- Dolly C.


Thank you for sending the pictures they are helpful. The purple flower is called "purple mustard", but it has several other common names. Its scientific name is Chorispora tenella. It is a weed, but considered a wildflower some people. It is listed as a noxious weed in some states. It is in the mustard family, Brassicaceae (formerly the Cruciferae family - a more descriptive name because the flowers have 4 petals in a cross shape). It is an herbaceous annual plant that develops new plants from seeds each year. It can be managed by removing the plants before seed formation. Preventing the formation of seeds will reduce the number of plants that appear in subsequent years. There are probably seed from previous years remaining in the soil, so you will probably see plants next year, but you should see plants in diminishing numbers each year if you prevent development and dispersal of seeds.

The white flower appears to be a much worse weed called "hoary cress" or "white top". Its scientific name is Lepidium draba, previously Cardaria draba. It is also in the mustard family, but this weed is a very aggressive, competitive, perennial that spreads from numerous seeds and underground rhizomes. It is not native to the U.S., but was introduced in the 1800s. It crowds out native species and can reduce yields of crop plants. It is definitely one to eliminate whether by physical removal (digging every time it is observed) or by chemical treatment.

These two plants are interesting in that while both are weeds, they illustrate the difference between annual weeds and perennial weeds. Annual weeds must return from seed each year and often produce seeds in extremely large quantities. The seeds often have varying degrees of dormancy so that they do not all germinate in the same year. This creates a "bank" of seeds in the soil to produce new weeds over several years. By preventing seed production, you can reduce the size of this seed bank and in time eliminate or greatly reduce the weed population. The perennial weeds may also produce seeds, so seed production in perennial weeds should also be prevented. However, perennial weeds have underground structures that can overwinter and produce new plants quickly in the spring. Unless these underground structure are removed, the weeds will persist and spread. Hoeing to shop the weeds may actually increase the number of weeds because the underground structures are often able to produce new plants from even small pieces. If you dig these plants and their roots up persistently before seeds form, you can eliminate or reduce the population of these weeds over time. Some gardeners prefer to use chemical control methods, using a systemic herbicide that translocates into the root system to help kill more of these plants, but even chemical management of perennial weeds may take several years. Whether you use physical removal (manually digging) or chemical treatments to manage perennial weeds, persistence is required. If you delay either treatment and allow storage of food in the root system and growth of the underground reproductive structures, the weeds will continue to increase and you will lose the battle. Be persistent and consistent in treating these weeds to eventually eliminate them or at least reduce the population to a level that is more easily managed.

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: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.


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