There are a number of natural pesticides available to gardeners for insect control, but relatively few natural herbicides for the eradication of weeds or other invasive plants. What ones do exist can do more harm than good if they are applied under the wrong conditions. There are plants that act as natural herbicides, however, such as black walnut, sunflowers, sagebrush, and spotted knapweed. These plants excrete chemicals that can kill off another plant species growing nearby. The process of certain plants acting in this way is called allelopathy.
Researchers are very interested in the allelopathic qualities of plants, since the chemicals responsible for natural herbicides can often be isolated and refined for commercial use. For example, scientists were able to extract an herbicidal chemical called catechin from the roots of spotted knapweed, an invasive weed found in the western United States. This chemical can be synthesized on a larger scale and applied to a number of other invasive plants. Many such herbicides are selective, which means that their chemicals only kill specific plants, not everything they touch.
Another popular species of natural herbicide is the black walnut tree. The oils extracted from the leaves of trees are often used in commercially-produced herbicides. Extracts of chemicals found in sunflowers may also be used by gardeners working organically.
Other natural herbicides are used primarily to control weed growth in commercial turf, such as golf courses and installed lawns. These herbicides are considered pre-emergent, which means that they destroy other plants at the germination stage, before the plant can establish roots. One is corn gluten meal, which was originally developed as a medium for growing fungus. Researchers discovered that it also inhibits the germination of other plants, especially weeds and grasses, so it is usually applied to lawns during the germination phase, weeks before the first blade of grass or weed stem appears.
Research to discover more natural herbicides is ongoing. Some agriculture experts have observed allelopathic phenomena in common crop plants such as winter rye and wheat straw. Rye plants have been known to affect the growth of certain vegetables, for example. Some researchers believe that a cover crop of rye, or at least a covering mulch containing rye, could work to kill weeds between soybean crops. Grain-based herbicides could be modified to kill off invasive plants while sparing the important crop plants.
Natural herbicides may sound more appealing than their chemical counterparts, but they are still poisonous substances that can affect humans and livestock. Their application may require the same safety precautions as applying commercial herbicides, so people who use them should never confuse the term "organic" with "non-toxic."