With commercial growing small margins are very important. A few percent improvement in yield, or a similar decrease in rejects, can make a crop more profitable. But only if the additional costs to obtain that change at the margin are less than the increase in profit.This ebook is a compendium of crops and the other plants that may have an effect on their performance by encouraging beneficial insects and discouraging pests, or the converse, and those plants that are not just weeds but also have a deleterious effect either by altering the soil biome or through the harbouring of joint pests and diseases.