By Jonny Lupsha, Wondrium Staff Writer
Herbicide-resistant weeds like Palmer amaranth pose an increasing threat to farms. Repeating the same weed treatment can cause unwanted plants to evolve resistances to them. How do they compare to other weeds?
Palmer amaranth is one of the most aggressive weeds affecting agriculture. It can grow three inches a day and spit more seeds out in its dying moment after being sprayed with herbicides. Even increasingly strong weed-killing chemicals are no match for so-called “superweeds” like Palmer amaranth, which are slowly overtaking American farms. Building a better herbicide may not be an option, as some herbicides are already so strong they drift to other fields and damage crops and trees.
So where do weeds come from and which are the worst offenders in the United States? In her video series The Science of Gardening, Dr. Linda Chalker-Scott, Extension Specialist in Urban Horticulture at Washington State University, said there are plenty of origin stories for weeds.
Weed Origins
Gardeners often define a weed as an unwanted plant, while Dr. Chalker-Scott called it “a plant out of place.” But where did they come from?
“Europe and Asia and Africa tend to be the more common places that our weeds have come from, and many of them were brought here as food,” she said. “Ornamentals have always been a source of particularly problematic weeds, and they’ve come from plant explorers through history who find interesting things and have brought them back here, or on the other hand, have come here and taken our plants elsewhere where some of them have become weedy.”
Ornamental weeds are largely propagated by the horticulture industry, often by people unaware of how problematic certain plants are and how weedy they can become. Additionally, some weeds were brought to the United States to be functional plants.
“In other words, they’re brought to help with controlling erosion, to make hedgerows, or windbreaks,” Dr. Chalker-Scott said. “And then finally, a lot of them came in accidentally—they were packed in ballast and packing materials as seeds, and they just got here and they got a foothold and they took off.”
A Weed by Any Other Name
One very invasive weed in the United States is Hedera helix, commonly known as English ivy. Hedera helix is native to Europe. Dr. Chalker-Scott said it was initially brought here by immigrants because it’s a beautiful plant with emotional appeal. She mentioned the classic buildings with ivy-colored walls as a very alluring aesthetic.
However, English ivy has many problems. It creates a large monocultural ground, so nothing else can grow alongside it except trees that can reach up over its canopy. However, when English ivy reaches maturity, it climbs those trees and damages them due to its weight, leading to limb breakage or, during an ice storm, the death of the entire tree. It also makes a great home for rats.
Other vining plants are problematic as well.
“[Kudzu] is a variety of different kinds of Pueraria species—it’s not just one species,” Dr. Chalker-Scott said. “It was introduced in the late 1800s as an ornamental vine, and then it was used during Dust Bowl years to stop soil erosion, so it was used a couple times both because it’s pretty and because it has a function. Unfortunately, it also really got out of control.”
Kudzu presents similar problems to English ivy—it’s damaging to the health and survival of trees and shrubs and anything growing under it—but it grows even faster.
English ivy and kudzu aren’t as aggressive as Palmer amaranth, but all of them are troublesome weeds threatening other plant species wherever they grow.