Selective Herbicidal Action of Midsummer and Fall Applications of 2,4- Dichlorophenoxyacetic Acid

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Abstract
2,4-dichlorophenoxyacetic acid used as an herbicide in mid-summer as a water spray at 1,000 ppm. prepared in Carbowax 1500, resulted in drying out and killing of above-ground parts of bind-weed (Convolvulus arvensis). Below-ground parts proliferated, became spongy and water-soaked and decayed to a depth of at least 14 inches. Applied in warm water (110[degree]F), killing was more uniform and more rapid. In cool weather (45-75 [degree]F), the response was much slower and complete and killing occurred only after 3-4 weeks. Bindweed was killed by immersing tips in vials of soln., and also when foliage was sprayed exclusive of tips. Applied to sow thistle (Sonchus arvensis), leaf bases became much enlarged, flattened and etiolated; roots increased 50 to 300% in diam., and became soft and spongy; all plants were dead within 2 weeks. There was no visible effect upon quack grass, Kentucky bluegrass, yellow fox tail, green fox tail, wild oats, large crab grass, small crab grass, barnyard grass and goose grass. Bindweed, narrow-leaved plantain, dandelion, round-leaved mallow, lambs-quarters, and ragweed were killed following varying formative responses. Pigweed, milkweed, tomato, broad-leaved plantain, Pennsylvania smartweed, purslane, chickweed, and red clover showed varying responses, including epinastic curvatures, splitting of stems, swelling of stems and roots, browning of leaves, stems and roots, chlorosis of leaves and stems, elongation, chlorosis and enlargement of petioles and killing of some plants. Germinating seedlings of white sweet clover were completely killed. In a weed-infested bluegrass lawn, dandelion, plantain and mallow were killed and white clover mostly killed, whereas the bluegrass became dark green but was otherwise not visibly affected. Apple shoots showed minor curvature and slight chlorosis near tips; poison ivy, chlorosis, arrested development, and 50% killing of above-ground parts; red raspberry, formative effects 3-6 inches back from the tips; and dewberry and grape, formative effects and killing 4 inches back from the tips. Applications by the aerosol method, using 2% 2,4-dichlorophenoxyacetic acid in 10% motor oil SAE 30 and 88% dimethylether, proved effective against bindweed, pigweed, and purslane. To test the residual effect of 2,4-dichlorophenoxyacetic acid, 12 spp. of cereal, lawn, and pasture plants were sown on an area which had been sprayed 2 months earlier. The seed germinated and the young plants showed no curvatures or formative effects.