This section is from the book "A Manual Of Weeds", by Ada E. Georgia. Also available from Amazon: A Manual Of Weeds.
Weeds cause a direct money loss to the farmer and to the nation. In the first place, the presence of weeds in such abundance as to attract notice, reduces the selling value of the land. A prospective purchaser who sees meadows thickly spangled with Daisies and Buttercups or looks over fields golden-yellow with Mustard, red with Sorrel, or white with the lace-like bloom of Wild Carrot, mentally subtracts the cost of cleaning the soil of these pests when estimating his offering price. And this is as it should be; for before a profitable crop could be obtained from such ground, much careful thought and expensive labor must go to the subjugation of its enemies; and the cost should very properly be borne by the neglectful husbandman who first allowed his land to be so abused. Rank growth of weeds may indicate fertility of the soil, and often the fitness of the ground for particular crops may be judged by the kinds of weeds found growing thereon; but, nevertheless, buyers are prejudiced and would-be sellers must submit to the embarrassment of debased values when land is infested to any considerable extent by these pernicious plants.
Weeds reduce the crop yield. It is this crop loss that is most considered when estimating the damage suffered from weeds. All living plants must have a certain amount of space for the circulation of air and moisture and to be open to the life-giving warmth and light of the sun. When crowded, even among themselves, they cannot thrive; and if this needed space is to any extent occupied by weeds, the returns from the crop must be correspondingly less. These obnoxious neighbors also steal from the soil a large share of the food and drink belonging to the rightful tenants of the ground. The robbery of soil moisture is one of the chief forms of injury. Weeds are notoriously more resistant to drought, more rapid of growth, more sturdy of habit, and more tenacious of life than the cultivated plants that they "shade down" or "starve out." It has been estimated by the United States Department of Agriculture that the average yearly loss due to weeds in the crop and meadow lands of the country is about a dollar an acre.
The presence of weeds not only decreases the yield, but also increases the expense of harvesting a crop. A field betangled with Bindweed or overgrown with the strong woody stems of Kinghead and Thistle enforces extra labor of draft-horses and extra wear of farm machinery, even sometimes compelling the task to be done by hand work - the most expensive form of labor in every occupation. Also, the labor and consequent cost of threshing and cleaning the seed from a weedy and inferior crop is much greater than for a heavier crop that is clean and thrifty.
The market value of the crop is reduced. A report from the Grain Inspection Department of the state of Minnesota shows the average dockage on wheat for two years to be nineteen ounces to the bushel. Minnesota produces yearly more than two hundred million bushels of small grain. A dockage of but one pound to the bushel means a loss of over two hundred million pounds, and if the money value be calculated at no more than a cent a pound it is two million dollars yearly; and this loss is in addition to decrease of yield and increased cost of harvest.
Some weeds serve as host plants for injurious fungi; and rust, smut, and mildew may be transferred from them to the useful crops. For example, the wild Barberry harbors the wheat-rust in one of its stages, and the fungus that causes the "club-root disease" of cabbage finds a host in several weeds of the Mustard Family. Weeds serve too as nurseries and feeding grounds for injurious insects. Wild relatives of the Potato, such as Ground Cherry and Horse Nettle, have been known to harbor the Potato Stalk-borer through the winter when all the ruined stems of the cultivated crop had been carefully burned in order to hinder its appearance another season. Weedy stubbles are often a breeding ground for cut-worms, flea-beetles, and other insect plagues.
Further, much serious loss is caused by a very bad class of weeds, possessed of other and much worse qualities than their mere presence where they are not wanted. Some, like the Death Camas and the Water Hemlock, or Cowbane, are poisonous, and cattle and sheep die from eating their young leaves or juicy tubers; even loss of human life is sometimes due to the deadly poison of the Hemlock, through the mistaking of its tuberous roots for harmless artichokes. In the Great Plains Region, horses and cattle are killed or made worthless by the "Loco Weeds." Some wild grasses, such as the Squirrel-tail Grass, or Wild Barley, and the Porcupine Grass, cause injury to the animals that graze on them by the lodgment of their barbed awns in the lining of mouth, throat, and stomach, causing painful inflammation, ulceration, and death. Milk, butter, and cheese are rendered unmarketable by the taint of Wild Onion or Garlic and the bitter Mustards. Cockles "cut the grade" of the wheat and spoil the flour if ground with it. Tick-seeds and burs yearly lessen the value of the wool-clip from the farmer's flocks. Altogether, the losses sustained by the American farmer from this cause are greater than he suspects or would believe. A needless loss, too; for there is no weed so vicious that it cannot be subdued, with profit to the owner of the soil, if its habits are well understood and sufficient determination goes to the battle.
But nothing in the world is so bad as to be entirely evil. It is only fair to admit that weeds do sometimes perform useful services to the land. Their presence compels tillage, and the most profitable farming is that which keeps the ground well tilled. They form the greater part of the covering which Nature promptly spreads over soil that the shiftless cultivator has left bare and neglected, keeping it from being blown about by winds, washed away by flood or rain, or baked into a barren desert by the sun. And such a weed-blanket, if turned under the ground in preparing it for a better crop, will supply the soil with green manure or humus, which it very much needs. It is not the best type of feeding crop for the land, but it is better than none. It is well that Nature is thus able to redeem the sins of slothful and selfish men, but her processes are too slow. The world grows no larger and its population increases very fast. The surest hope of its continued comfort and prosperity lies in better husbandry.