Many are valuable soil conditioners whose vigorous roots penetrate deeply into the subsoil, transporting nutrients and minerals to the surface. They improve the soil structure through aeration and control of erosion. Many have a beneficial relationship with soil organisms which help the gardener, or with other plants, giving them increased vigor, taste, or greater resistance to insects and disease. My intent is to persuade you to grow some of these plants on purpose, and to seek to control the spread of others you consider undesirable without resorting to a program of total annihilation. Lamb's quarter and Sow thistle help melons, squashes, and cucumbers.
Lupine helps corn and many other crops.
Small amounts of Yarrow and Valerian give vigor to vegetables. Wild mustard is beneficial to grapevines and fruit trees. Dandelions stimulate fruits and flowers to ripen quickly. Stinging nettle makes neighboring plants more insect resistant. And the iron content of nettle helps plants withstand lice, slugs, and snails during wet weather.