Weeds are commonly defined as plants that are growing in a place where they are not desired. Although that definition has some practical utility, it does little to further understanding of how weeds operate as species or how to use an understanding of weed biology to improve management of garden weeds.
From an ecological point of view "weeds are plants that are especially successful at colonizing disturbed, but potentially productive, sites, and at maintaining their abundance under conditions of repeated disturbance" (Liebman, Mohler and Staver 2001, Resources). Recognition that weeds are plants that specialize on disturbed habitats makes sense of a great many aspects of their biology. For example, the generally small seed size of annual weed species follows from the fact that they must produce many seeds in order to have some seedlings survive repeated disturbance. Similarly, persistence in the soil seedbank (Seed longevity) and ability to recognize environmental cues associated with tillage (Tillage & germination) allows weeds to pop up after many years of absence when sod is turned to prepare for a new garden.
In general, weeds differ from the crops with which they compete in a variety of ways. Their seed weight is usually low whereas many crops have large seeds (Seed size). The size of the two types of plants at establishment is further exaggerated since small seeded crops are often transplanted. Although their initial size is often less than that of the crop at the beginning of the season, common garden weeds have some of the highest relative growth rates recorded for any plants. Weeds usually also have very high rates of nutrient uptake. They are adapted to take advantage of the brief pulse of nutrient release that accompanies the breakdown of organic matter when soil is disturbed. Consequently, weeds often have 1.5 to 3 times higher concentrations of N, P, K, Ca, and Mg in their tissues than the crops with which they compete. This not only tends to deprive the crops, but, in combination with the weed's higher relative growth rate, allows the weeds to eventually overtake the crops in size.
There are many ways to be a weed: different combinations of characteristics allow a plant species to specialize on disturbed habitats. Some species are annuals. They complete their lifespan within less than a year and survive between years as seeds in the soil. Lambsquarters and barnyardgrass are good examples of these. Other species are perennials that are fixed in place and unable to spread vegetatively except by human intervention. Dandelion and perennial populations of annual bluegrass are examples. Most of these species are primarily weeds of grasslands (lawns, pastures, hayfields) rather than gardens, because turning the soil kills them. Stationary perennials found in gardens are partial exceptions to that rule (e.g., dandelion). Finally, some of the worst garden weeds are the wandering perennials. These spread by horizontal roots or underground shoots (rhizomes) from which daughter plants emerge above-ground. They are notoriously hard to kill, and because they renew themselves by vegetative reproduction; individual clones are essentially immortal if left alone. Some of the contrasting characteristics of these three types of weeds are summarized in the following table.
(Modified from Liebman, Mohler and Staver 2001)
|Character||Annuals||Fixed perennials||Wandering perennials|
|Vegetative lifespan||< 1 year||2 to a few years||Long, indefinite|
|Seed longevity||Years to decades||Years to decades||A few years|
|Energy allocated to seed production||High||Medium high||Low|
|Usual means of dispersal||In soil, manure||Soil, wind, feces, crop seed||In soil|
|Examples||Lambsquarters, barnyardgrass||Dandelion||Quackgrass, hedge bindweed|