It is that time of year when weeds take over gardens and landscapes, absorbing moisture and feeding bugs. But what makes a weed a weed, and how does it relate to introduced and invasive species? Merriam-Webster defines a weed as “a plant that is not valued where it is growing and is usually of vigorous growth, especially one that tends to overgrow or choke out more desirable plants”. Introduced species are organisms that did not evolve in the given location, but were brought by humans; they may or may not spread intensively and become invasive. The USDA defines invasive species as meeting two criteria: “1) non-native (or alien) to the ecosystem under consideration and 2) whose introduction causes or is likely to cause economic or environmental harm or harm to human health.” As we can see from the subtle differences in these definitions, not all introduced species are invasive, but all invasive species are introduced. Colloquially, we consider many invasive species to be weeds.

A range hoop engulfed by russian thistle.

I had a couple of conversations recently with two other Bird Conservancy Wildlife Habitat Biologists (WHBs) along with my Aunt about the plants we associate with the places we were raised. We all had visceral memories of certain plants that define our childhood homes and our connections with the environment. For Olivia Laws (WHB out of Sterling, CO) and I, originally from Northern California, Eucalyptus trees, which are native to Australia, swayed in the wind, emitted a distinctive odor, and shaded hiking trails. For Ashley Mertz (WHB out of Greeley, CO), tumble weeds (Russian thistle, kochia, tumble mustard, all native to Eurasia) rolled over the landscape of her Kansas home. My Aunt, having grown up in Colorado, has a fondness for the silvery leaves and subtle scent of Russian olive trees. Without a trained eye, these plants are part of the nature around us and have become representative of certain places, but now as biologists looking back, we see how these species can be detrimental to the ecosystems they invaded. In our current positions, we fight against Russian olives and tumble weeds as they outcompete native plants which are essential to the wildlife we’re trying to conserve.

Cheatgrass being sprayed via helicopter to encourage the growth and reestablishment of native annual sunflowers and bluestem grass.

Invasive species are a huge management issue when it comes to conservation. In the grasslands, we manage invasive annual grasses, which outcompete native species, and encroachment of native trees and shrubs turning grasslands into forest. In the wetlands, Russian olives and reed canary grass suck up water that is already a precious resource in arid climates. In forests, mullein and Canada thistle invade ecosystems after disturbance. Left unmanaged, these species can take over a landscape, pushing out native biodiversity. To address this issue, there are a variety of strategies we can try. For example, the timing and intensity of grazing can be managed to target certain species. Woody plants can be cut down. Prescribed fire can be a tool for invasive species management. Biological control, such as introducing a natural enemy of the target species, can effectively remove unwanted species. Finally, chemicals can be used to kill plants. There are pros and cons to each of these methods. The landscape, magnitude of the problem, ecosystem, human needs, landowners’ goals and interests, timeline, cost, and plant biology must be considered when coming up with an invasive species management plan.

Biologist admire native grasses planted at a ranch in southwest Kansas.

Sometimes, however, it is nice to have a reminder that invasive plants are not exclusively harmful to the system. This was one takeaway from a visit I made with partners to Mark Sexson’s ranch in southwestern Kansas. Sexson has been planting cropland and degraded rangeland with native grasses. Throughout years of experimentation on his farm, Sexson has seen firsthand the power of “weeds” to help cover soil, minimize erosion, protect young plants, and provide food for wildlife. He reflects that the “first year of using weeds as a [soil] cover was a big assault on my paradigm of believing weeds were a big problem in grass establishment.” He’s not alone in this thinking. Weeds are typically seen as a detriment to the ecological and economical value of an area. However, we can think creatively about how our goals can be compatible with the presence of invasive species and in some situations maybe working with them can help us restore healthy ecosystems. Despite his success keeping invasive species on the landscape while establishing native grasses, Sexson acknowledges that in certain scenarios, the weeds do need to be managed to reduce competition. Sexson concludes that “weed control is a complicated issue and must be decided based on current conditions.” This is a great problem for a WHB to tackle!

Anna Gaw is a Wildlife Habitat Biologist based in Fort Morgan, Colorado.