   Summer Weed Management - July 18, 2012 Jeff Schalau, Agent, Agriculture & Natural Resources University of Arizona Cooperative Extension, Yavapai County Slight variations in summer temperatures and spring precipitation always bring a slightly different palette of warm season weeds. This year, I’ve seen lots of camphorweed, pigweed, dogbane, and sandbur in pastures. We always have bumper crops of bermudagrass, johnsongrass, giant reed, nutsedge, and other perennial grasses and grass-like plants in irrigated landscapes and pastures. The most critical step in pest management is proper identification of the pest. I’m not going to dwell on identification or scientific names here, but I have included photos and brief descriptions of many common Verde Valley weeds immeadiately following this column. After you have identified the weed, the first question you should ask is: am I dealing with an annual or perennial? Warm season annuals are usually fast-growing, and can be pulled by hand if the soil is moist. Perennials are more permanent residents and hand pulling results in them breaking off in the ground or a losing tug-of-war. Puncturevine, tumbleweed, and pigweed are warm season annuals that can be hand pulled. Annual weeds with taproots can also be dispatched with a “stirrup” or “scuffle” hoe. Sometimes weeds have been present for many seasons. Areas that are regularly disturbed and/or compacted tend to stay weedy. Establishing desirable warm season perennial plants in these areas creates competition for annual weeds and will choke them out in time. Organic mulches also work well to minimize annual weed growth. Line trimmers can be very useful, but should be used on annual weeds prior to seed set. Line trimmers are also used to trim grass around trees – remember not to girdle the base of trees by getting too close. It is often better to manage the grass growth near trees with mulches or herbicides. In large garden spaces, soil solarization can be employed to “cook” emerging weeds and kill some disease causing organisms. In brief, soil solarization uses solar energy to kill weeds and seeds under a clear plastic sheet covering the soil and is buried along the edges. The soil is moistened prior to covering to create a steamy environment that kills harmful weeds and pathogens after six weeks or more under the plastic. The treated area must be in full sun. Landscape fabrics are sometimes used under inorganic mulch to prevent weed growth. This works for annuals, but established perennial weeds should be killed prior to mulching. A drawback to mulching on top of landscape fabrics can also arise when fine soil particles flow or blow into the mulch after a few years’ time. This thin layer of soil allows weeds to grow on top of the fabric and defeats the purpose. Inorganic mulches also work best when combined with herbicide applications (pre- and post-emergent). I prefer organic mulches for use in home landscapes. They need to be replenished every few years as they decompose, but organic mulches are cooler, increase biological activity, and add some organic matter to the soil. Many people want to control weeds without using herbicides. This can be done, but if the weed in question is a well-established perennial, it will require much time and labor. Johnsongrass can be removed manually, but established bermudagrass is very difficult to manage without herbicides. Most people use glyphosate herbicides to manage persistent perennial weeds like bermudagrass. Success depends on the weeds growing vigorously prior to the herbicide application. This means applying the herbicide during active growth. Translocated herbicides, such as glyphosate, will not kill weeds if applied when plants are dormant. For best results managing bermudagrass, irrigate and fertilize for a couple of weeks prior to a summer glyphosate treatment. Each gardener will approach weed control differently – I’m hoping that people will learn that large areas of bare ground will eventually have a weed problem. Why not establish a native or drought-adapted landscape that occupies these areas without regular irrigation. These desirable plants will effectively compete with weeds, provide a natural look that is beneficial to wildlife, and will survive on natural precipitation once established. Also, see the links below – there are many photos to help you identify your weeds. Follow the Backyard Gardener on Twitter – use the link on the BYG website. If you have other gardening questions, call the Master Gardener help line in the Camp Verde office at 928-554-8999 Ext. 3 or e-mail us at cottonwoodmg@yahoo.com and be sure to include your name, address and phone number. Find past Backyard Gardener columns or provide feedback at the Backyard Gardener web site: http://cals.arizona.edu/yavapai/anr/hort/byg/.
Note: The Texas A & M photos were linked from their "Weeds of Field Crops and Pastures" database at http://twig.tamu.edu/keyindex.htm. Many other weed photos can be accessed there. The SEINet photos were linked from http://swbiodiversity.org/seinet/. This database include data and specimens from herbaria across the southwest and Colorado plateau. Many of the photos referred to were taken by celebrated local botanist Max Licher.  |
