Sunday, 15 August 1999 John Begeman Monsoon rains intensify battle against weeds Special to The Arizona Daily Star The abundance of monsoon rains we have been experiencing this summer comes with few negatives. One downside, however, is the emergence of a bumper crop of weeds. Gardens and landscapes are literally being choked with a host of summer annual weeds that have sprouted and continue to sprout with each rain. Weeds compete with desirable garden and landscape plants for water and nutrients. They also harbor insects and diseases that can spread to surrounding plants. Pollen produced by weeds greatly contributes to the discomfort experienced by allergy sufferers. It's too late for effective use of preventive weed control, but control is necessary because maturing weeds will soon produce seed for next year's crop of weeds. Mechanical control is probably the best method to eliminate weeds now, especially for weeds that have grown large. Many of the weeds growing currently are annuals and only come up from seed. Unlike perennials, these annual weeds will not re-grow from the roots. So killing the above-ground, leafy portion of the plant will eradicate the weed. Cutting weeds off with a sharp garden hoe is most effective. Simply shave the weeds off at the soil surface. If you're careful, powered string trimmers can also be used to cut down weeds. String trimmers should not be used in rock mulched areas, however. Gravel and rocks can be dangerously thrown by the flailing string line. Hand pulling weeds is effective when weeds are young. Larger, more mature weeds will have an extensive root system very difficult to pull out of the soil. And if you do, the roots from nearby desirable plants will be damaged. Also young weeds will pull much easier if the soil is moist. Gas- or electric-powered tillers, both large and small, can be used to cut weeds in cultivated gardens. A shallow setting of the tines is best. This will cut or shave off the weeds without turning too much soil over and exposing more weed seed. Chemical control of weeds may be used in lieu of mechanical control measures. Because a majority of weeds growing now are of the annual variety, as with hoeing, only the above-ground portion of the weed need be killed. Chemical weed killers that can accomplish this fall into the category of contact or locally systemic herbicides, such as Diquat or Finale. The systemic herbicide ``glyphosate'' sold under the trade name Roundup may also be used. These products can be purchased at most garden centers and nurseries. It's important when using either of these products to spray the entire weed. It is most effective to treat actively growing weeds. Don't wait until the weeds get large and mature. At that point, it will be more difficult to kill them. When using any chemical, weed killers included, follow all label directions. Mix the concentration of spray recommended. Mixing a stronger or weaker solution of spray than that recommended will result in poor control. Wear a long-sleeved shirt and pants, use chemical resistant gloves, and wear goggles or safety glasses as a minimum. If the label calls for further protective equipment, wear it. When spraying weed killers, use only a sprayer designated for that use. Even when sprayers are properly washed, some chemical residue may remain. If you then mix an insecticide in that same tank to spray, let's say, tomatoes for aphids, the tomatoes can be injured. Spray post-emergent herbicides when the air is calm. Early morning is a good time because temperatures are lower and the wind is generally down. Never spray under windy or breezy conditions! Spray may drift onto valuable plants and cause damage. If you must spray weeds close to plants, use a shield made of a large piece of cardboard to place between the weeds and surrounding plants. Finally, to prevent weeds from re-growing, consider using pre-emergent herbicides. Amaze, Surflan XL and Portait herbicides all come in easy-to-use granular forms. They provide control of weeds for four to six months. To be effective, pre-emergent herbicides are best applied prior to rainy periods in the summer and winter. They also must be watered into the top layer of soil. John Begeman is the horticulture agent for the University of Arizona's Pima County Cooperative Extension. Visit the University of Arizona Cooperative Extension Web site.