Killing Weeds

In testing growth-regulating substances as weed killers, scientists learned first that when a compound like 2,4–D was placed on the leaves or stems of a plant its effect showed up several inches or even a foot or two away in another part of the plant. That discovery meant that 2,4–D was an entirely new type of herbicide; its effect traveled through the plants both upward and downward from the place where it was applied. Next, they learned that the presence of 2,4–D on a young leaf prevented it from expanding, and that the new leaves curled up and became distorted so that they were of little value to the plants.

Finally, the scientists found that some plants treated with 2,4–D were stimulated to burn up a large part of their reserve food supply, so that the sprayed plant actually starved to death. That is the reason why some weeds, including dandelions, plantain, and annual morning glory, can be killed by spraying their leaves with a water mixture of 2,4–D. The effect of the chemical results in a quick check in growth of the leaves already there, while the stimulus travels through the plant and prevents the formation of new leaves and causes the plant to burn up the food reserves in its leaves, stems, and roots.

Department scientists found that by judicious use of 2,4–D it is possible safely, easily, and cheaply to kill many kinds of weeds that grow on lawns, pastures, golf courses, or other grassy areas without noticeably hurting the grass. Various formulations that contain 2,4–D are on the market. The manufacturer's directions should be carefully followed.

A Department scientist, J. W. Brown, found an easy and inexpensive way of cleaning 2,4–D from spray tanks. The spray tank is first rinsed with water and then filled almost to capacity with hot water (under boiling or about 180° F.) , and household ammonia is added at the rate of about 2 teaspoonfuls to a quart of water. The filling with water is then completed, the ammonia solution stirred, and the sprayer closed. A small portion of the solution is then sprayed through the nozzle and the remainder allowed to remain in the sprayer for about 18 hours or overnight. The equipment is then drained and rinsed twice, a portion of each spray being sprayed through the nozzle.

The killing effect of 2,4–D is gradual. When it is sprayed on a weed, some time is required for the effect to travel to the roots and some additional time is needed for the 2,4–D to cause the plant to burn up its food reserves. In summer, it generally takes about 1 to 3 weeks before the sprayed plants die and rot away. In this respect it differs from the better-known plant poisons, such as sodium chlorate or arsenical preparations, which generally kill the above-ground parts of plants quickly.

Little is gained by using 2,4–D with one of the more common plant poisons because the latter generally kill the tops of plants so quickly that the 2,4–D does not have time to take effect and kill the roots as well as the above-ground parts of the weeds. The addition of a conventional plant poison to 2,4–D, therefore, merely dilutes the effect of the plant growth regulator.

Although old and relatively mature weeds can sometimes be killed with 2,4–D, more rapid and complete effects are had when the plants are sprayed during or just after an active stage of growth, after they have all their leaves or just before the flower buds open. Low temperatures and low soil moisture reduce to some extent, however, the effectiveness of 2,4–D as a weed-killer.

Tests on some farm animals showed that 2,4--D was not poisonous to cows and sheep even when the animals were fed the acid, or when they ate pasturage sprayed with it. The acid is considered to have little, if any, bad effect on humans. I stress, however, that pure 2,4–dichlorophenoxyacetic acid was used in the tests; they do not prove that all 2,4–D formulations on the market are nontoxic to animals and humans because they contain materials other than the acid.