Weeds and Weediness: A Look at the Origins of the Plants We Love to Hate

Volume 25 Issue 3, Summer 2020

by Nan McCarry

What is a weed? Many of us may have heard that Ralph Waldo Emerson defined a weed as “a plant whose virtues have not yet been discovered.” However, there are any number of definitions of weeds and weediness. Some definitions depend upon how humans feel about weeds. A “plant in the wrong place” is one, but other definitions have more to do with how the plant functions in the landscape: “pioneers of secondary succession” – that is, plants that grow back after disturbance due to fire or other clearing of the vegetation.

Some of the properties of weedy plants that allow them to colonize disturbed landscapes include having many small seeds and/or tough taproots and being able to germinate quickly when exposed to sunlight. Many of them spread clonally as well. Many weed seeds can remain viable for decades in the soil seed bank. That’s why we get weeds whenever we turn over the soil! Some thrive in freshly tilled soil, while others are adapted to the compacted soil that humans create by their activities.

I particularly like the definitions that describe weeds as plants which thrive in landscapes disturbed by humans. It is said that Mother Nature abhors a vacuum. Weeds are opportunists that will fill an ecological niche left open by disturbance. Anytime we cut or burn existing vegetation, create paths, roads, or railways, abandon a field, or turn the soil, we are creating “disturbed” landscapes. For this reason, some definitions of a weed focus especially on weeds as plants that “follow” humans around. We may not want them around, but it is our activity that creates habitats in which they flourish. Botanist Edgar Anderson in his book Plants, Man, and Life called weeds “artifacts” — that is, things that would not exist if humans hadn’t produced them. In Anderson’s words, “though man did not wittingly produce all of them, some are as much dependent upon him, as much a result of his cultures, as a temple or vase or an automobile.” Anderson said that “the history of weeds is the history of man.”

Weeds have been around for a long, long time. Archaeologists have found what they call “proto-weeds” from a hunter-gatherer settlement in Israel dated to 23,000 years ago. With the evolution of agriculture, around 10,000 years ago, weeds really took off. In fact, the presence of certain weed species indicates to archaeologists that agriculture was occurring at a site. At this time humans began disturbing ever larger portions of the landscape by fire and other means. The acts of clearing, cultivation, and collection of crop seeds favored not only the crops but the weeds. If you think about organisms in terms of their strategies for survival and reproduction, being a weed that is unconsciously gathered and spread by humans is a good one, given how many human beings there are. No wonder we gardeners can’t get rid of them!

In many cases, a crop and its weed relative evolved from the same progenitor wild plant, and there are many crops with a closely related weed species or variety that remain with us today. In many cases, these relatives, which continue to evolve in the wild, hold precious genetic diversity needed by breeders, since crop plants hold very little genetic diversity. There is a need to conserve these “crop wild relatives” worldwide. Like crops, these weeds are dependent upon the disturbed areas humans create for their continued existence.

It’s helpful to think of weeds and weediness as being on a spectrum — from those plants with the “weediest” habits, which prefer disturbed sites, to those on the other end of the spectrum, which do best in more “closed” habitat — such as a forest with long-lived tree species and little opportunity for sunlight to reach the ground.

There is also a spectrum of what we humans consider weeds and are willing to tolerate or even value in our landscapes. These landscape preferences are changing, largely because of the efforts of organizations such as Loudoun Wildlife and Audubon at Home, which encourage homeowners to value plants such as common milkweed because of the wildlife value they offer. Weeds perform many ecosystem services in the landscape. For example, when weedy pioneer species move into a newly disturbed landscape, they hold the soil in place, preventing further erosion.

Nature makes disturbed habitats, as well. Weedy pioneer species thrive on riverbanks, steep cliffs, and where trees fall in a windstorm, leaving an open, sunny site. It’s been suggested that many of the weeds we know today descended from plants that colonized the areas where Pleistocene glaciers advanced and retreated, leaving bare, unvegetated ground. It is said that most of our gardening and farming is “holding back natural plant succession.” Thomas Rainer and Claudia West point out, “All gardening is an attempt to halt ecological succession and freeze it at a point that pleases us aesthetically,” and agriculture specialist Preston Sullivan says “weeds are evidence of nature struggling to bring about ecological succession.” Creating lawns favors weeds for the same reasons. If we stop mowing the lawn, what comes up are pioneer species, as the lawn moves along through the process of succession.

The plants pictured throughout this article are just a few from the weedier end of the spectrum — from those that are a nuisance to those we accept for their ecosystem services, such as providing habitat for wildlife or because they are edible. These plants are all listed as “weeds” in Virginia Tech’s Weed Identification website.