It was Emerson who identified a weed as ""a plant whose virtues have not yet been discovered."" Men Allen agrees, and here distinguishes the useful weeds from the destructive in concise, authoritative characterizations. Almost all make good compost, some are reliable indicators of soil acidity or alkalinity, and others make fine companions: garlic grown next to roses reduces black spot, yarrow absorbs nitrogen, thereby increasing the keeping quality of nearby onions. The dandelion gets a mixed review; hard to eradicate, it steals nutrients from soil--which enhances its value in salad. Most weeds, however, don't deserve encouragement: ragwort is poisonous to farm animals, bindweed roots strangle their neighbors, and buttercups take minerals from bordering plants and exude a noxious substance besides. Discerning.