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

1. Experiments were conducted in the immediate vicinity of Geneva, New York, between July 14 and October 14, 1944, using 2,4-dichlorophenoxyacetic acid as a herbicide applied as a water spray at a concentration of 1000 p.p.m. in Carbowax 1500 at the rate of one part acid to five parts Carbowax. 2. Applications to bindweed in rows of nursery stock on July 14 and July 31 resulted in drying out and killing of above-ground parts. Below-ground parts proliferated, became spongy and water-soaked, and decayed to a depth of at least 14 inches. Applied in warm water (110⚬ F.), killing was more uniform and more rapid. In relatively cool weather (45⚬-75⚬ F.) the response was much slower, and complete killing occurred only after 3-4 weeks. Bindweed was killed by immersing the tips in vials of the solution, and they were also killed when the foliage was sprayed exclusive of the tips. 3. Applications to sow thistle growing in a garden plot resulted in lighter green color and wilted appearance within 24 hours. The bases of the leaves became much enlarged and flattened, and the roots increased 50-300% in diameter and became soft and spongy. All plants were dead within 2 weeks. 4. Applications were made to field mixtures of common weeds. There was no visible effect upon quack-grass, Kentucky bluegrass, yellow foxtail, green foxtail, 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 smart-weed, 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. 5. Germinating seedlings of white sweet clover emerging 3-7 days after the soil surface had been sprayed were completely killed. 6. Applications were made to a blue-grass lawn infested with dandelion, buckhorn plantain, round-leaved mallow, and white clover. All plants of dandelion, plantain, and mallow were dead and disintegrated within 30 days; most of the white clover was dead, and that which was not dead was killed back to the main stolons; and the bluegrass became dark green in color but otherwise not visibly affected. 7. A few woody plants were treated. The apple showed minor curvature and mild chlorosis near the tips; poison ivy showed chlorosis and arrested development and 50% killing of above-ground parts; red raspberry, formative effects 3-6 inches back from the tips; and dewberry and grape showed formative effects and killing 4 inches back from the tips. 8. Applications by the aerosol method, using 2% 2,4-dichlorophenoxyacetic acid in 10% motor oil SAE 30 and 88% dimethyl ether, proved effective against bindweed, pigweed, and purslane. 9. The residual effect of the acid in the soil after application as a herbicide was tested. Twelve species of cereal, lawn, and pasture plants were sown on an area which had been sprayed 2 months earlier. The seeds germinated and the young plants showed no curvatures or formative effects.