Warm Season Weed Control - March 30, 2005
Jeff Schalau, County Director, Agent, Agriculture & Natural Resources
Arizona Cooperative Extension, Yavapai County

If you are plagued with summer weeds in driveways, ornamental landscape plantings, orchards and gardens, or other areas, then now is the time to plan for summer weed control. Hoeing, mowing, and mulching work well for isolated annual weeds and where weeds have not become well established. Post-emergent herbicides are also an effective tool to control large weed populations or deep-rooted perennial weeds. However, if you have had lots of weed seedlings in past summers, then you may want to consider an application of pre-emergent herbicide.

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 where weeds tend to flourish. They should not be used in vegetable gardens and flower beds where plants are grown directly from seed.

Examples of pre-emergent herbicides include: benefin (Balan and Balfin), DCPA (Dacthal), dithiopyr (Dimension), isoxaben (Gallery), metolachlor (Pennant), napropamide (Devrinol), oryzalin (Surflan, Weed Stopper), oxadiazon (Ronstar), oxyfluorfen (Goal), pendimethalin (Pendulum, Pre-M), prodiamine (Barricade), and trifluralin (Bayonet). DCPA, dithiopyr, oryzalin, napropamide, pendimethalin, and prodiamine control annual grasses and many broadleaf weeds and can be used safely around many woody and herbaceous ornamentals. Metolachlor has become popular because it controls yellow nutsedge as well as most annual grasses. Isoxaben is used for control of broadleaf weeds.

There are many summer annual weeds that are of special concern in the Verde Valley area. Field sandbur (Cenchrus incertus) is a relatively recent arrival and spreading rapidly in pastures, along roadsides, and waste areas. It is a grass that has seed heads composed of spiny burs. Punturevine or goathead (Tribulus terrestris) is a prostrate annual weed with five petalled yellow flowers. The seeds are enclosed in spiny burs that frequently cause flat tires on bicycles. Both of these weeds are easily transported when the burs stick to vehicles, clothing, animals, shoes, etc. Pre-emergent herbicides recommended for control of these two weeds are pendimethalin, trifluralin, and a combination of oryzalin and benefin (Helena XL 2G).

Other warm season weeds in the Verde Valley area include Russian thistle or tumbleweed (Salsola iberica), pigweed (Amaranthus spp.), lambsquarter (Chenopodium album), cocklebur (Xanthium strumarium), purslane (Portulaca oleracea), and many others. These are easily controlled by the use of pre-emergent herbicides. You should read the entire label prior to purchasing any pre-emergent product as each label list species that it effectively controls and which species may experience injury by exposure to the herbicide.

Some pre-emergent products are designed to be made into a solution and sprayed on while others are granular and can be applied with a rotary or drop-type spreader. The application method should be known before you purchase any products. The other option is to hire a professional to apply a pre-emergent herbicide. Professionals working in ornamental landscapes and other non-agricultural settings should be working under a current license with the Arizona Structural Pest Control Commission.

Soil sterilants (products containing imazapyr or prometone) are not recommended for home landscapes due to 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. Like I say, it is best to avoid soil sterilants in residential areas.

Naming of companies or products is neither meant to imply endorsement by the author nor criticism of similar companies or products not mentioned. Always read product labels and MSDS.

The University of Arizona Cooperative Extension has publications and information on gardening and pest management. 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 mgardener@verdeonline.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/.

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Arizona Cooperative Extension
Yavapai County
840 Rodeo Dr. #C
Prescott, AZ 86305
(928) 445-6590
Last Updated: March 24, 2005
Content Questions/Comments: jschalau@ag.arizona.edu
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