What is a Weed?
The concept of a weed has been around for a long time, but it is poorly defined. An example of an older book about weeds is A Manual of Weeds (1914) by Ada Georgia, while a more recent book on the same topic is Weeds of the Northern U.S. and Canada (1999) by France Boyer and Richard Dickinson.
There appear to be three dominant perspectives on weeds: agricultural-ranching, suburban, and ecological. The agricultural perspective considers any plant a weed if it competes with crops for the available nutrients and moisture in a field. Such plants can lower crop yield or cause crop failure. Closely related to this is the ranching perspective, which replaces the agricultural perspective in those areas where rearing of livestock is dominant. The ranching perspective considers any plant a weed if it presents a threat to livestock or reduces the quality of forage that is otherwise available. As a result, livestock gain weight less readily, or they may become ill or injured, sometimes fatally by the uneconomic properties of weedy plants. I combine the agricultural and ranching perspectives together because they are driven primarily by the same economic motives in rural areas.
The suburban perspective considers any plant a weed if it invades the lawns and gardens of suburban homeowners and other managed landscapes of mainstream society, which includes the lawns and gardens of banks, schools, churches, malls, government buildings, industrial parks, etc. In the context of this perspective, plants are valueless weeds unless they have been made available through the mass market and have price tags attached. The ideal plant of the managed landscape in suburbia has the following characteristics:
1) It is an introduced species that is poorly adapted to the surrounding environment.
2) It is a sterile hybrid or patented cultivar.
3) It is difficult to maintain and often short-lived.
4) It doesn't spread readily to places where it doesn't belong.
5) It has obvious aesthetic or culinary properties.
6) It has to be purchased at a store or through a catalog.
The weediness of a plant is defined by the absence of the preceding characteristics. Thus, suburbia considers any plant a weed that is well-adapted to its environment, prone to reproduce itself and spread, requires little or no effort to maintain, has no obvious aesthetic or culinary properties, and doesn't require money to acquire.
The ecological perspective considers any plant a weed if it is a pioneer species that thrives in a degraded habitat with a history of disturbance through human agency. Such weeds may be native or introduced. There are also superweeds that have the capacity to invade high quality natural areas. Superweeds are usually introduced plants with few natural enemies in the area of invasion. By forming dense colonies, they displace native plants and reduce biodiversity. The more ordinary kinds of weeds are not considered a significant threat to the natural environment because they tend to disappear after artificial disturbances have been removed, while superweeds are dreaded by nearly all ecologists.
In the United States, the agricultural-ranching perspective dominated people's opinions about weeds prior to World War II, and it is still very influential. Both of the books that are cited above have been strongly influenced by this perspective. One reason for the dominance of this perspective was that most people lived on farms, ranches, or rural areas that were economically dependent on such commercial interests. After World War II, however, the suburban perspective on weeds became more dominant as most people became homeowners in suburban areas that were not dominated by agricultural-ranching interests. The ecological perspective on weeds became influential during the 1970's and thereafter as people and government leaders became more concerned with environmental issues. Today, all three of these perspectives coexist among members of modern society, even though they are not entirely compatible with each other.
As an example of this lack of compatibility, consider Medicago sativa (Alfalfa). Alfalfa is highly regarded as a forage crop and a source of hay for livestock, particularly in the Western states. As a member of the legume family, it helps to replenish worn-out agricultural soil by fixing atmospheric nitrogen. From the agricultural-ranching perspective, Alfalfa is more often regarded as a valuable crop plant than a weed. However, suburban homeowners and ecologists have a different assessment of this plant. Alfalfa is just another weed to be eliminated from the suburban perspective because it can invade lawns and gardens. It is not particularly attractive, nor is it highly regarded as a source of food for humans, however acceptable it may be to livestock. Similarly, Alfalfa is regarded as a weed from an ecological perspective because it is an introduced plant from abroad that displaces native plants. It also has a tendency to thrive in habitats where there has been a history of disturbance, which is typical of weedy plants.
Other examples could be given in which a given plant species is highly regarded from a suburban perspective, but regarded as a weed from the agricultural-ranching and ecological perspectives, and similarly for those species that are highly regarded from an ecological perspective. It is well known that ecologists often find themselves in pitched battles against real estate developers, farmers and ranchers, and other commercial interests over the preservation of habitat containing rare species of plants. However, there are also plant species that are regarded as weedy pests from all three perspectives, one of which is Conium maculatum (Poison Hemlock). However, this does not mean that Poison Hemlock is devoid of value, as I will attempt to explain below.
Personally, I'm more influenced by the ecological perspective than the others, although I include many species that are regarded as weeds in fields and pastures, as well as lawns and gardens. Examples of common field weeds include Thlaspi arvense (Field Pennycress) and Abutilon theophrasti (Velvetleaf), while examples of common lawn weeds include Taraxacum officinale (Dandelion) and Plantago rugelii (Broadleaf Plantain).
One characteristic of all three perspectives is that weedy plants are regarded as having little value, and thus homeowners, business establishments, ecologists, farmers, and ranchers expend considerable time, money, and effort in getting rid of them. There are some ecologists, however, who have a higher opinion of weedy plants (I happen to be one of them) because they assume important roles in the functioning of the ecological system and are interesting subjects of intellectual inquiry. Furthermore, some weedy plants have produced practical benefits that further the interests of modern society.
Why are weeds more valuable than many people realize? I suggest the following reasons:
1) Some weedy species are pioneers of degraded landscapes where the soil is worn-out or nearly destroyed. Such weedy species are necessary to the healing process of the landscape, as their decaying organic matter improves the quality of the soil and the sets the stage for the succession of non-weedy plants.
2) Some weedy species are pioneers of disturbed landscapes where the soil is high quality, but exposed to erosion by wind and water. Such weedy species quickly cover the exposed soil and prevent erosion from occurring until they are replaced by non-weedy plants.
3) Weedy plants are often important sources of food and cover to various kinds of wildlife, including mammals, birds, and insects. For example, the caterpillars of Danaus plexippes (Monarch butterfly) feed on Asclepias syriaca (Common Milkweed), while the the caterpillars of Papilio polyxenes asterias (Black Swallowtail butterfly) feed on Daucus carota (Wild Carrot) and Pastinaca sativa (Wild Parsnip).
4) Weedy plants are potentially important sources of food, medicines, herbicides, and other products to modern society. For example, Fragaria virginica (Wild Strawberry) is one of the parent species of the hybrid strawberries in supermarkets. A powerful herbicide has been derived from Centaurea biebersteinii (Spotted Knapweed), while the fiber of Cannabis sativa (Marijuana) can be used to make paper, rope, or clothing.
For these reasons, I think weedy plants are entitled to greater respect than they have received from the past. Even professional ecologists have a tendency to underestimate their value to the environment. I hope that this website will rectify some of these misperceptions, even though it is clear that some weedy plants present a major threat to commercial interests, while others are superweeds that can invade natural habitats.