Managing Warm Season Annual Weeds - May 25, 2011
Jeff Schalau, Associate Agent, Agriculture & Natural Resources
University of Arizona Cooperative Extension, Yavapai County

Last summer was a good year for Russian thistle or tumbleweed (Salsola iberica). I’ve talked to many people that were overrun by them. It must have been the timing and amount of summer precipitation that caused them to become so large and numerous. Small populations of annual weeds such as Russian thistle are relatively easy to control. However, when large areas have dense populations of weeds, the job becomes more difficult. Perennial weeds are more difficult to manage and often require multiple management methods and more time to control. Let’s discuss some strategies that are appropriate for warm season weed management.

The best method of weed prevention is to occupy the site with a plant that is more desirable than the weed. In non-irrigated areas, this can be accomplished by establishing native perennial grasses. These grasses should be planted in early summer and will require some irrigation to become established. In the Verde Valley, sod-forming natives like buffalograss (Bouteloua dactyloides), tobosagrass (Pleuraphis mutica), or vine mesquite (Panicum obtusum) can be established by seed and spread by above ground stolons to occupy larger areas. Larger warm season bunchgrasses such as deergrass (Muhlenbergia rigens), alkalai sacaton (Sporobolus airoides), and sideoats grama (Bouteloua curtipendula) can also be used and will add diversity and dimension to the planting. Sand dropseed (Sporobolus cryptandrus) is a fast establishing native grass that will help fill in between slower growing species and occupy the space more speedily. Wildflowers can also be interspersed with grasses to add color and interest.

Bermudagrass (Cynodon dactylon) is a non-native grass that is very aggressive and common in many irrigated areas of the Verde Valley. It will choke out most weeds, but creates challenges of its own. I do not suggest seeding it where it does not exist because it can be difficult to manage and contain. Johnsongrass (Sorghum halepense) is another aggressive non-native that is not desirable for planting. Some people plant weeping lovegrass (Eragrostis curvula) in droughty situations. This South African native is not as difficult to control as bermudagrass of Johnsongrass, but it does spread and may require some management.

Mechanical weed management is often necessary even when trying to establish desirable natives. Hoeing, mowing, and mulching work well for isolated annual weeds and where weeds have not become well established. Soil solarization is another weed management strategy that uses clear plastic sheeting that is laid over moist soil and sealed around the edges. The intense heat kills most seeds and some of the perennial plant roots. It is left in place for four to six weeks and also controls soil pathogens, insects, and nematodes.

Post-emergent herbicides are also an effective tool to control large weed populations or deep-rooted perennial weeds such as bermudagrass. Glyphosate is the most commonly used post-emergent herbicide active ingredient for managing warm season perennials. The plants targeted for control should be healthy and growing prior to the herbicide application. If plants to be treated are dormant or drought stressed, glyphosate is not translocated to the roots as effectively and, hence, it is not as effective at killing the targeted weed.

Pre-emergent herbicides are only effective when applied before weeds germinate on soil that has been recently cultivated, irrigated, and allowed to settle. Pre-emergent herbicides kill germinating seedlings of both annual and perennial weeds. Once applied, they are incorporated by rainfall or irrigation and are effective for about six months. Pre-emergent herbicides work well in ornamental landscapes and other areas having ample bare ground. They should not be used in vegetable gardens and flower beds where plants are grown directly from seed.

Soil sterilants (products containing imazapyr or prometone) are not recommended for home landscapes due to their long period of activity and the potential for harm to non-target plants. These chemicals move downward and laterally with water in soils and can injure or kill desirable plants. Nearby trees and ornamental plantings can also be injured or killed when roots grow into soil treated with soil sterilants. I have seen this happen in residential landscapes. Furthermore, when a soil sterilant damages a neighbor’s plant, the applicator is liable for the damage. I think it best to avoid soil sterilants in residential settings.

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Arizona Cooperative Extension
Yavapai County
840 Rodeo Dr. #C
Prescott, AZ 86305
(928) 445-6590
Last Updated: May 17, 2011
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