What is a weed?

As I’ve written before, the word “weed” is nothing more than an anthropogenic label.  But nearly everyone (with the exception of ecologists and other well-read people) uses the word “weed” so freely as if it’s just another thing under the sun.  They don’t give much thought as to what a weed truly is, nor how they function ecologically in this world.  While the definition of a weed is generally subjective (commonly defined as “a plant growing where it’s not wanted”), those in the fields of ecology have brought forth distinct definitions that are much more objective than subjective.

Ecologically speaking, the only plants most ecologists consider weeds are the ones that are highly aggressive and form large monocultures or invade an ecosystem in such a way as to change the fire ecology in a negative manner.  Of the first type, examples would be Smooth Brome (Bromus inermis), Common Bermuda (Cynodon dactylon), and St. Augustine (Stenotaphrum secundatum) – the common and boring lawn grasses.  Of the latter, Cheatgrass (Bromus tectorum).  A good definition for the aforementioned plants and other common lawn weeds, would be as written in Shinners and Mahler’s Illustrated Flora of North Central Texas, by George M. Diggs, Jr., Barney L. Lipscomb, and Robert J. O’Kennon, “a plant with the genetic endowment to inhabit and thrive in places of continual disturbance, most especially in areas that are repeatedly affected by the activities of humankind.”  Lawns are easily invaded by weedy species, simply because of their need for constant disturbance in the form of mowing, weedeating, and pesticides in order to maintain that low-diversity, high-input state.  Common Dandelion and Henbit were probably more easily introduced than they might have been otherwise, thanks to the shortsighted concept of the European-influenced lawn.

Ecologist Stephen Packard writes, weeds “in the ecosystem often functions much like a scab on a mild wound that you or I might suffer. Weeds are an ecosystem’s response to degradation or disturbance. The scab helps the wound heal. Weeds help quality ecosystems heal. They prevent erosion and start a succession process that is likely to end with the conservative plant species that were there before the (mild and temporary) wound.”
Please follow the link above to read Packard’s post – it is excellent and provides some very forward thinking that outsmarts the antiquated mentality of, “it’s a weed; spray it!”

An example I like to use is the Annual Sunflower (Helianthus annuus) dominating recently exposed bare soil turned over by plowing.  Farmers hate and spray H. annuus for fear of competition with their corn.  Conventional cattle ranchers hate it because their cattle won’t eat it.  There’s nothing bad about Annual Sunflower, it’s just doing what it has evolved to do over the last ca. 5,000+ years.  And it is also one of the few crop species that evolved in North America – Corn is another.
More H. annuus history can be found here.

Annual Sunflower on SR Hill.  Brewster Co., TX
Annual Sunflower on SR Hill. Brewster Co., TX

These opportunistic, pioneer, or occupier species are in the ecosystem for a reason.  They serve as a bookmark or placemark for the later-coming long-lived plants that work on their root systems while the “weeds” enjoy their time in the sun and flower and make seed.  Most of the occupier species tend to be annuals and biennials, but that doesn’t mean those plants need to be eliminated in any manner.  “Spraying weeds” doesn’t really help anything over the long term; it’s a short term clean-up that’s not even necessary.  The longer lived plants will eventually take over the occupiers.  You just got to have patience – Mother Nature moves at her own pace.

The federal government classifies invasive species as follows, “[…] a species that is non-native (or alien) to the ecosystem under consideration and whose introduction causes or is likely to cause economic or environmental harm or harm to human health.” (Executive Order 13112)
There are many non-native plants (as well as animals) that have moved into various North American ecosystems but have not caused any economic or environmental harm or have negatively affect our health in any way.  Some examples would be Common Mullein (Verbascum thaspus), a non-native plant that has become naturalized in many areas, Nebraska especially, although Colorado and Hawaii have listed it as a noxious weed.  In the central Platte River valley, Mullein apprears in areas of disturbed soil, overgrazed rangelands, and in native plant communities that are temporarily weakened by environmental pressures such as drought and in forby areas that experienced frost heaving.  But it can’t out-compete the natives in a well-managed and healthy prairie.  A non-native animal species that was introduced to North America is the Ring-necked Pheasant (Phasianus colchius).  Brought over from Asia around 1860, it has since established successful breeding populations, mostly in the Great Plains and northern areas of the US.  Just because a non-native plant is noxious in one ecosystem does not mean it may be noxious in all others.  Use this website for determining whether or not your state lists a plant as a noxious weed.

Consider the Tumbleweed, or Russian Thistle (Salsola tragus), it is a native of Russia but has long been inhabiting many ecosystems across the West to the point that it is an iconic plant of old western movies. It was brought over from Russia to the Dakotas in the 1860s and has spread westward ever since. In many overgrazed areas in the semi-arid and arid rangelands of the West, it is the only plant that grows on bare soil. It may be an ugly plant but at least it is holding the soil with its roots and protecting it form the aerial bombardment of precipitation with its foliage.

Native ecosystems are now much more globally susceptible to many invasive threats, but we can no longer draw hard lines in the ecological sand with our battle swords.  After so long, some non-natives blend into ecosystems and we never know they are present, until we fret needlessly.

Enjoy a plant for what it is, not what you think it is or isn’t, especially the weeds – they are guardians of the soil!

Resources:
Invasive Plant Atlas of the United States
Alien Plant Invaders of Natural Areas

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