What Are Weeds?

Japanese knotweed growing in a colony near a road.
David Beaulieu

What are weeds? There's no answer that will satisfy everyone. That's because "weed," rather than being a botanical classification, is a subjective term: It reflects a value judgment; it's a pejorative. People disagree on its meaning.

The Many Definitions of a Weed

A variety of definitions of the term exist. Many of them allude to a kernel of truth that we can agree on but then go too far and lose our support. The most pervasive definition is that a weed is "a plant growing in the wrong place," which can mean either of two things, depending on the speaker: One, it dares to grow in a place where humans live or go about their daily business, as opposed to a wild place where it is out of the way. Two, it's growing in a wild place, but it's an alien spreading vigorously and displacing native plants.

The problem with this definition, while it may be useful on a strictly practical level, is its focus on location. If you consider a particular plant a weed when it grows in your yard, for example, then you don't stop regarding it as such when you see it in a wild setting: Its status as a weed is a constant, a trait that is transportable from one place to another. You may not pull it out of the ground when you find it growing in a wildlife preserve, but you still recognize it as a weed.

Moreover, the bad qualities you perceive in it may be just as relevant in a wild setting as anywhere else: A brush with poison ivy (Toxicodendron radicans) in the wild is just as bad as one that happens in your backyard. Its location doesn't change its status as a noxious weed.

Thus the necessity of keeping the definition as basic as possible, distilling it down to its bare essence: a weed is a plant that a person finds undesirable and deserving of distain.

This definition excludes a plant that you simply don't care to grow (because its flower color doesn't appeal to you, for example) or that inconveniently pops up in a garden bed not intended for it. For example, if you have a vegetable bed adjacent to your lilac bushes (Syringa vulgaris), some of the suckers from the latter may pop up next to your vegetables, causing you the extra work of removing the suckers. You may grumble about having to remove this "plant growing in the wrong place," but only by using the most artificial of definitions would you designate lilacs as weeds; you'd be hard-pressed to find anyone who thinks them contemptible.

What Is a Weed?

A weed is a plant that a person finds undesirable and deserving of distain. A plant that one person calls a weed will not necessarily be called a weed by another person.

Invasive Plants Are the New "Weeds"

Native plant activists — i.e., people who are proponents of growing plants that are native to that geographic area — have essentially replaced the "weed" with the invasive plant as an object of scorn. These advocates for native plants are the same people who would agree that the definition of a weed is an alien plant growing vigorously and displacing native plants. Such a plant is alright if it stays in its own native land but becomes contemptible when it invades foreign shores.

Far from finding all of the plants traditionally considered weeds contemptible, native plant activists tend to tolerate, or even champion, some of them that grow in their yards. The result is the emergence of something of a great divide on the issue of which plants to look down upon, a divide between the average gardener and the native plant activist. For instance:

  • While average gardeners consider wild violets (Viola sororia) weeds and remove them from lawns, native plant activists are more likely to forgo well-manicured lawns and allow them to stay.
  • Native plant activists promote the growing of common milkweed (Asclepias syriaca) as a way to support butterfly populations. Due to their "weedy" appearance, many average gardeners have contempt for them.
  • Average gardeners perceive many invasive plants as desirable to grow due to their beauty. One is burning bush (Euonymus alatus), a fall standout that's one of the most popular landscape plants in North America. By contrast, native plant activists have contempt for it.

But there's common ground between the two camps because some of the plants that average gardeners find contemptible also happen to be invasive.

Common Characteristics of Weeds

There's widespread agreement that certain traits can make a plant weedy, including that it is such a prolific spreader that it tends to get out of control; it survives on its own, without human help; and it is noxious in some way.

While poison ivy and common ragweed (Ambrosia artemisiifolia) are two plants that possess all of these characteristics.

Examples of Common Weeds

  • Crabgrass (Digitaria spp.): Creeping annual with multiple stems that plagues lawns and exploits cracks in hardscape
  • Japanese knotweed (Polygonum cuspidatum): Tall (9 feet), bamboo-like, herbaceous perennial with fleecy fall flowers that spreads vigorously by rhizomes
  • Stinging nettle (Urtica dioica): Herbaceous perennial (3 to 7 feet tall) with pointed, serrated leaves that are covered in fine hairs, contact with which causes rashes

Beneficial Weeds

Besides native plant activists and average gardeners, there's a third group that often finds itself in disagreement with both, feeling weeds are just "plants." This live-and-let-live group tends to agree with the definition for "weed" famously offered by the philosopher, Ralph Waldo Emerson: "A plant whose virtues have not yet been discovered." As long as a plant isn't clearly noxious or such as bully as to make gardening impractical when it's around, there's nothing contemptible about it, especially if it offers benefits. Examples of beneficial weeds are:

  • Dandelions (Taraxacum officinale): Boil the fresh spring greens for a spinach substitute.
  • Purslane (Portulaca olearacea): Eat the stems of this succulent raw in salads.