| Abstract/Contents: | "Research has shown that it is easiest to control young seedlings of weeds as they germinate. Therefore, much of the research on principles of weed control has been directed to young seedlings weeds in all crops. And in turf, the toxicity of arsenic, chlordane, or other compounds is aimed at killing the crabgrass before it passes the seedling stage. Although seeds in the soil can or might be killed by chemicals, it is only when that seed sprouts that it is important, thus seedling kill is satisfactory. Research has also shown that to be satisfactory pre-emergence control must be almost perfect otherwise the few remaining plants may be as obnoxious as those in untreated areas. Arsenic toxicity. The arsenic susceptible plants include hairy and smooth crabgrass, common and mouse-eared chickweed, yellow and green foxtails, plus witch-grass and barnyard grass. Both established and young Poa annua plants are reduced in vigor by arsenic toxicity. Of the turfgrasses, established bluegrass, bentgrasses and redtop are quite tolerant of arsenic, and even young seedlings survive applications that are toxic to crabgrass. Ryegrasses, Bermuda and Zoysia are not susceptible as are weedy grasses. Many plants have not been observed. What industry offers. For many years lead arsenate and calcium arsenate have been available as fine powders for uses and sprays and dusts in insect control. Currently about 50,000,000 pounds of each are produced annually. Although much about arsenic toxicity has been known and was reported more than 20 years ago by Grau, Welton, Monteith and others, comparatively little arsenic was utilized for selective weed control. Research at Purdue University on Poa annua control with arsenics was started in 1952. Poa annua control was reported in 1952 and has been extensively observed on various test plots every year since then. Crabgrass prevention was recorded in 1954, and on 15 different sites replicated experimental tests have been provided reproducible date showing effective crabgrass control. In 1958 at Purdue, five forms of arsenic material, when used at recommendation rates in six test sites, averaged over 90% control of crabgrass infestations. In more than half the test (10 out of 17) four materials gave 98% crabgrass reduction. [Table 1]. Recent work on control of weeds, accompanied by extensive production research and development by industry, has resulted in products more easily handled and specifically prepared for pre-emergence weedy grass control for lawnowners and turf supervisors. At present, tentative reports show at least eight merchandisers will have trade name products available for spring market. All of these formulations attempt to apply enough material to be toxic whenever crabgrass or susceptible plants might emerge. After initial use, annual rates of one-third to one-fourth as much should be adequate to keep toxicity at the soil surface. Chlordane toxicity. For several years, variable crabgrass control has been observed on research plots in turf and other crops given heavy chlordane treatments. Sometimes perfect control is secured, but, percentages of from 100 down to 50% have been reported by several stations. Research results at Purdue, since 1951, have also been variable with 84, 80, 60% control reported. Chlordane's effect on young seedlings is to stop the seedling growth immediately after emergence and before true leaves develop. Further careful research on selectivity between species is needed, but most seedlings are susceptible. Some may wish to try chlordane in rose beds or elsewhere for use after weeds have been removed and plants are well established. As of January 1, 1959 there are at least two trade name products, plus the standard forms of chlordane products, on the market. Other compounds. Several chemicals, including 2,4-D, 2,4,5-T, Sevin, erbon, sesone, neburon, NPA and other related compounds and products, may prevent establishment, or kill new (less than 3 leaved) seedlings. Usually an early spring application (by the time dandelions bloom) may kill broadleaf seedling weeds, young knotweed and reduce early crabgrass infestation. Rates of 1 to 2 lb/A (standard rates) may be effective for 3 to 4 weeks. Some preparations combine weed killers with fertilizers or vermiculite for spreader applications as dry forms. These materials must be used with caution around shrubs, gardens and flowers. Furthermore, there is no protection against delayed or later crabgrass infestations unless repeat applications are made every 3 or 4 weeks until mid-summer. Pre-emergence control of crabgrass does offer a chance to enjoy good grasses with much less weedy grass infestation and competition. However, for satisfactory results the principle of toxicity should be understood and the materials carefully applied." |
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