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. Mint and tomatoes are strengthened in their vicinity. Nettle deters fermentation, keeping fruit trees free of mold.