The Usual Suspects: Five Invasive Plants That Are Taking Over Our Wetlands
Why do we hate invasive species? They’re still plants, right? Invasive plants are the enemy because they overpopulate and outcrowd our native species which causes damage to our natural ecosystems. Here are five of the most common invasive plant species in the upper Midwest – and how to best deal with them.
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1) Reed Canary Grass (Phalaris arundinacea)
Why it’s bad: RCG spreads quickly through rhizomatous roots and forms a dense, impenetrable mat beneath the surface, which often prevents native seed germination. Above ground, it forms large monocultures that crowd out native plants and forms a shade cover that prevents seedlings from establishing.
How to identify: RCG is 2-6 feet tall, with flat, hairless, alternate leaves. The most easily identifiable characteristic is the transparent ligule where the leaf blade meets the stalk. Blue-joint grass, a native species, looks very similar but will not have the ligule.
How to control: Grass-selective herbicide (such as Intensity) has proved to be the most effective method of control. When working in areas with standing water, it is important to use water-safe chemicals. Mowing and burning actually encourage more growth and should be avoided!
Read More: Phalaris arundinacea (Reed Canary Grass): Minnesota Wildflowers
Photo Credit: Ryan Hodnett, CC BY-SA 4.0, via Wikimedia Commons
2) Purple Loosestrife (Lythrum salicaria)
Why it’s bad: Purple Loose-strife grows quickly and in thick patches, which deters wildlife and crowds out native vegetation. Because it forms these thick monocultures, it can trap sediment and fill in the wetland, which drastically impacts the ecosystem service of flood retention that native wetlands typically provide.
How to identify: The adult plant grows 2-6 feet tall, with clusters of purple flowers on long terminal spikes. The leaves are hairy and grow in opposite pairs or whorled down the square-shaped stem.
How to control: Glyphosate-based herbicides like Round-up or Rodeo are the most effective control method for large areas. For very small patches, hand removal can be effective but cannot be performed when the plant is producing seed because this will just spread more seed into the seed bank and cause a new infestation. Another method that has been utilized is the use of insects for biological control. There are a few European beetle species that will consume the entire plant, seeds and all, and a species of weevil that eats the root systems and prevents new infestations. Biological control should only be performed by experts as the effects of introducing non-natives can be potentially devastating.
Read More: Purple Loosestrife | Factsheet | Cooperative Extension | University of Delaware (udel.edu)
Photo Credit: Fungus Guy, CC BY-SA 3.0, via Wikimedia Commons
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3) Canada Thistle (Cirsium arvense)
Why it’s bad: Competes with other species for soil resources and inhibits native seedling establishment through shading. It spreads with an aggressive root system that sends out shoots to create new colonies, as well as seeds dispersal via wind.
How to identify: 1.5-4 feet tall, prickly leaves and stems, purple-pink flowers which turn to fluffy white seed heads in mid-July to August.
How to control: Hand-removal is not effective because C. Thistle spreads by vegetative reproduction and can regenerate from root fragments less than 1 inch in length. Mowing before the plants begin to seed can also be effective, but is most effective when combined with fall and early-spring foliar herbicide application. Canada thistle is a very persistent plant and requires intensive management over several years in order to significantly reduce the cover and spread of colonies.
Read More: https://www.mda.state.mn.us/plants/pestmanagement/weedcontrol/noxiouslist/canadathistle
Photo Credit: Lmmahood, CC BY-SA 3.0, via Wikimedia Commons
4) Smooth Brome (Bromus inermis)
Why it’s bad: Smooth brome forms dense colonies and spreads quickly, outcompeting native grasses. It reproduces vegetatively as well as via seed dispersal.
How to identify: Mature brome grows 2-4’ tall, with alternating, usually hairless, ¼” wide leaves. Leaves have a distinguishing M-shaped crinkle in the middle of the leaf. This constriction is useful in distinguishing smooth brome from reed canary grass.
How to control: Mowing in early spring (to avoid damaging warm season grasses) is known to be effective but requires repetitive mowing to inhibit rhizomes. Burning can reduce spreading but it will not kill the rhizomes. Herbicide application for dense populations is recommended in late April/May, however if brome is intermixed with warm season natives it is best to spray when those native plants are dormant (early spring or fall).
Read More: https://www.dnr.state.mn.us/invasives/terrestrialplants/grasses/smoothbromegrass.html
Photo Credit: Matt Lavin from Bozeman, Montana, USA, CC BY-SA 2.0, via Wikimedia Commons
5) Common Reed (Phragmites australis)
Why it’s bad: Phragmites spreads via seed as well as vegetatively through rhizomes and even fragments of rhizomes. Crowds out natives and create monoculture, alters wetland hydrology and degrades wetland wildlife habitat.
How to identify: Common reed looks very similar to its native counterpart, American reed, but there are distinguishing factors that set them apart. Non-native reed grows 12-15 feet tall, hollow stem, with bluish green leaves 1 to 1.5 inches wide, and roots are typically 3’ in depth. Native reed only grows up to 6.5 feet tall, and the ligules of natives are frayed and shredded while nonnatives are firm and sturdy. Another distinguishing factor is that the native is more yellow-green while non-native is bluish-green. Non-native reeds grow in dense colonies while natives are typically interspersed with other native grasses.
How to control: Adequate control requires using multiple methods. Non-selective herbicide application (such as glyphosate) is the most effective primary method, with burning and mowing as follow-up strategies. Another method that could be useful is hydrology control to induce stress on the plants.
Read More: Common Reed (Phragmites australis) (invasive.org)
Photo Credit: Speifesender, CC BY-SA 3.0, via Wikimedia Commons
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