Weed is a subjective term used in a variety of senses, usually to describe a plant considered undesirable within a certain context. The word—commonly applied to unwanted plants in human-controlled settings, such as farm fields, gardens, lawns, and parks—carries no botanical classification value, since a plant that is a weed in one context is not a weed when growing where it is wanted. Indeed, a number of plants that many consider weeds are often intentionally grown in gardens and other cultivated settings. Less commonly, the term is applied to any plant that grows and reproduces aggressively outside its native habitat. The term is occasionally used to broadly describe species outside the plant kingdom that can live in diverse environments and reproduce quickly, and has even been applied to humans.
weed: "A herbaceous plant not valued for use or beauty, growing wild and rank, and regarded as cumbering the ground or hindering the growth of superior vegetation... Applied to a shrub or tree, especially to a large tree, on account of its abundance in a district... An unprofitable, troublesome, or noxious growth."
Weeds generally share similar adaptations that give them advantages and allow them to proliferate in disturbed environments whose soil or natural vegetative cover has been damaged. Different types of habitat and disturbances will result in colonization by different communities of weed species.
Naturally occurring disturbed environments include dunes and other windswept areas with shifting soils, alluvial flood plains, river banks and deltas, and areas that are often burned. Since human agricultural practices often mimic these natural environments where weedy species have evolved, weeds have adapted to grow and proliferate in human-disturbed areas such as agricultural fields, lawns, roadsides, and construction sites. The weedy nature of these species often gives them an advantage over more desirable crop species because they often grow quickly and reproduce quickly, have seeds that persist in the soil seed bank for many years, or have short lifespans with multiple generations in the same growing season. Perennial weeds often have underground stems that spread out under the soil surface or, like ground ivy (Glechoma hederacea), have creeping stems that root and spread out over the ground.
Some plants become dominant when introduced into new environments because they are freed from specialist consumers; in what is sometimes called the “natural enemies hypothesis,” plants freed from these specialist consumers may increase their competitive ability. In locations where predation and mutual competitive relationships no longer exist, some plants are able to increase allocation of resources into growth or reproduction. The weediness of some species that are introduced into new environments can be caused by the introduction of new chemicals; sometimes called the "novel weapons hypothesis," these introduced allelopathyic chemicals, which indigenous plants are not yet adapted to, may limit the growth of established plants or the germination and growth of seeds and seedlings.