Spring Weed Management - March 3, 2010
Jeff Schalau, Associate Agent, Agriculture & Natural Resources
University of Arizona Cooperative Extension, Yavapai County

Cool season weeds are busily taking advantage of unoccupied soil and available moisture. Bare ground and disturbed areas are becoming covered with cool-season weed seedlings. Many of these are common, easy-to-manage, garden variety weeds. Some, as I discussed last week, are invasive and more difficult to control. Like many pest management dilemmas, we are faced with choices and each of us will likely manage weeds differently.

The first step in any pest management strategy is to identify the species. Some of the more common cool-season weeds of our area are: London rocket (Sisymbrium irio); red brome or foxtail brome (Bromus madritensis ssp. rubens) along with other annual bromegrasses; red-stemmed filaree (Erodium cicutarium); sowthistle (Sonchus oleraceus); and cheeseweed (Malva parviflora). I have provided some links to photos and weed management information below. You may also bring a sample (with flowers) to the Yavapai County University of Arizona Cooperative Extension offices in either Prescott Cottonwood (call to confirm office hours).

Once identified, you will know whether it is an annual, biennial, or perennial. Small populations of annual weeds are easily controlled by hand-weeding, cultivation, or string trimmers. It is best to control them before they produce seed. Larger annual weed populations can be more difficult because this is often an indication that weeds have been present for a longer period and there are lots of seed that will continue to germinate in subsequent years. While the above mentioned control methods will work, larger annual weed populations could also be mulched to reduce seed germination in subsequent years.

Some homeowners and many commercial landscape managers choose to manage weeds with pre-emergent herbicides. These products prevent seedling survival and are usually applied twice each year. Pre-emergent herbicides have no effect on established weeds. Annual weeds can also be controlled with contact herbicides: killing the plant parts it contacts. Perennial weeds are often treated with translocated (systemic) herbicides.

There are two relatively new contact herbicides that are considered less toxic than some of the more traditional products. These are acetic acid-based and fatty acid-based products. Information indicates that these two products are only effective on annual weeds when they are very young (5-6” in height). These products are still pesticides and should be handled and applied in compliance with label instructions.

I have been battling annual brome grasses at my house for many years. In isolated areas where I have been diligently pulling them each spring before seeds are produced, they have largely disappeared. However, I live on a creek with shade, deep sandy soils, and seasonal moisture. The brome grasses love these conditions. For the first few years I used a string trimmer before they went to seed. Some survived, but most were killed. Still, these undesirable grasses were back each year presumably from a well-established seed bank in the soil. I noticed that I also had a few non-native perennial pasture grasses present which included orchardgrass, tall fescue, and timothy. My strategy for the last several years has been to allow these perennials to go to seed and use the string trimmer to strategically kill the annual bromes. This ecologically-based approach seems to be working! Yes, the perennials are non-native, but they occupy the space, provide forage and cover for wildlife, and best of all, do not produce the foxtail-like seeds like annual bromes.

I encourage you to seek balanced solutions to weed issues. Weeds grow in certain places because of the lack of competition from other plants, unoccupied soil, and excess moisture. Native plant communities stave off weeds because they are already occupied and utilizing soil, water and light resources present on the site. Weed problems are often exacerbated by trying to maintain unoccupied soil (think gravel landscapes) and excessive application of irrigation. View your property through an “ecological lens” and you will likely notice opportunities for simple changes that will reduce weeds without increasing labor and irrigation, or being overly reliant on herbicides. Remember also to visit the Backyard Gardener Website for weed identification resources and additional control strategies.

The University of Arizona Cooperative Extension has publications and information on gardening and pest control. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 ext. 14 or E-mail us at cottonwoodmg@yahoo.com and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site: http://cals.arizona.edu/yavapai/anr/hort/byg/.

Weed Management in Landscapes (University of California Cooperative Extension)

Photo: London Rocket (University of California Cooperative Extension)

Photo: Red Brome (Utah State University Extension)

Photo: Red- Stemmed Filaree (Utah State University Extension)

Photo: Sowthistle (University of California Cooperative Extension)

Photo: Cheeseweed (University of California Cooperative Extension)

Follow the Backyard Gardener on: Twitter

Back to Backyard Gardener Home Page

Arizona Cooperative Extension
Yavapai County
840 Rodeo Dr. #C
Prescott, AZ 86305
(928) 445-6590
Last Updated: June 21, 2010
Content Questions/Comments: jschalau@ag.arizona.edu

Legal Disclamer