Gardeners and farmers have been dealing with the adverse effects of weeds in gardens since the advent of agriculture. Weeds compete with food and garden plants for nutrients. They reduce available soil nutrients, resulting in slower growth and lower crop or flower yields. Some weeds can grow tall enough to shade shorter flowers and crops.
In the past, weeds were removed by pulling them from the ground. Although this is still an effective weed management technique, weed pulling is labor-intensive. As farmers began trying to grow larger areas of crop, manual weed pulling became impractical. The last decade of the 19th century saw the introduction of chemical herbicides in the form of broadleaf herbicides for cereal crops. Sodium nitrocresylate started seeing use as a selective weed killer in 1934. 1945 saw the introduction of the first growth-regulating herbicide, 2,4-dichlorophenoxy acetic acid (2,4-D). This eventually lead to the development of 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), or Agent Orange, the defoliant used in Vietnam, that was produced using highly toxic dioxins, or 2,3,7,8-tetrachlorodibenzodioxin.
There are many kinds of herbicides for different purposes. 2,4-D is the systemic herbicide introduced in 1945. Atrazine is a widely used modern herbicide that targets broadleaf weeds. Diquat is a general aquatic herbicide, and glysophate is another broad-spectrum herbicide used in to control weeds in food and non-food crops.
How They Work
There are two general types of herbicides: Systemic and contact herbicides. Contact herbicides affect only plants on which the chemicals are placed. Systemic herbicides target certain types of plants and are sprayed over large areas. They are taken up by plant roots and kill the targeted plant types through systemic damage.
Herbicides are toxic to plants. Although they sometimes are less toxic to animal life, herbicides also have a level of animal toxicity. 2,4-D is toxic to animals in concentrations between 300 and 1,000 mg per kg of body weight. Diquat is toxic in concentrations of 230 mg per kg. Paraquat, an herbicide used in grasses, is toxic in concentrations of 138 mg per kg. Atrazine is a known carcinogen and has been associated with sex changes in frogs. Atrazine has been found in a number of municipal drinking water systems.
Organic Weed Killers
The safest organic way to kill weeds is to either pull them or to mulch around your plants. Mulch is a method of covering the ground around your vegetables with dried grass, wood chips or shredded bark. This cover prevents the sun from reaching the weeds, thus starving them of nutrients. Other weeds can be controlled with common household substances like bleach, vinegar or salt. However, buildup of those substances, especially salt, in the soil can harm some other plants.