This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
Many growers do not appreciate the quantity of water that is stolen from the soil by weeds. After all, weeds are plants - outcasts of the horticultural world, but they must live, if allowed to remain on the ground. Being more vigorous in their nature than cultivated plants, they need large supplies of moisture to keep them going, and they transpire through their leaves at least as freely as do cultivated crops. It therefore follows that if an acre of ground, carrying, say, 40,000 Lettuces or 10,000 Cabbages, is allowed also to carry a crop of weeds between, the amount of water taken up from the soil and evaporated in the course of the season will be probably twice as great as if no weeds were allowed to grow. The commercial gardener should therefore decide whether it is cheaper and better for him to allow weeds to grow and steal the moisture and food from his Cabbages, Carrots, Beets, Lettuces, and fruit trees or bushes, or whether it is more remunerative to spend money in keeping the weeds down, and thus conserve the moisture and food for his crops. The sensible grower will, of course, spend money in labour for hoeing by hand or machine between his crops, because he knows he will not only keep his crops clean, healthy, and steadily growing with the moisture he is conserving, but also because he knows that freshening up the surface of the soil means more food for the roots of his plants, fewer insect pests in the soil, greater absorption of rain and dew, and consequently crops that are likely to sell more quickly and fetch higher prices than those that have been neglected.
During the summer months it is not at all uncommon to see the ground between rows of fruit trees and bushes, and between vegetable crops, full of weeds. These are not only robbing the air of carbonic acid gas (see p 108). but also the ground of moisture, and leaving it in a parched and cracked condition. The weeds also harbour the grubs of various fruit and vegetable pests that sleep in security until nature calls them forth again to plague the grower who despises knowing anything about them. The prevalence of weeds, therefore, in any garden indicates a penny-wise saving in labour and a pound-foolish extravagance in other directions.