Noxious Weeds - Knotweed
Japanese, Bohemian and Giant knotweeds (Polygonum cuspidatum, Polygonum x bohemicum and Polygonum sachalinense, respectively) are on Colorado's noxious weed "List A," which requires these weeds to be removed and kept from propagating on all property. The three knotweeds are similar in appearance and habitat. Native to eastern Asia, knotweeds were introduced to North America by the late 19th century as ornamentals and landscape screens.
Knotweeds invade rapidly and are able to grow under a wide variety of conditions. They easily out-compete local native plants, reduce biodiversity and habitat for fish and wildlife, and are difficult to eradicate or keep under control. Knotweeds find weak spots in pavement, drains, and buildings and can cause breaches in these structures.
Waterways are especially vulnerable, as knotweed proliferation can completely obstruct water flow. Knotweed also increases bank erosion because its aboveground structures die every year, leaving bare soil in spite of its extensive root system.
Knotweeds are often compared to bamboo, with stems (canes) that are hollow, straight, unbranched, and swollen at the nodes. Once they emerge from root crowns in April, knotweeds grow quickly, reaching their full height of 4-to-12 feet by mid-June.
Their leaves alternate along the stem, are spade- or heart-shaped, and are pointed at the tip. Sprays of small white flowers on four-inch spikes appear in July and August. The three knotweeds can be distinguished by their height and the size, shape, and texture of their leaves. See photos of knotweeds in the right column.
Herbicide-free control of any of the knotweeds will have the most success on infestations less than 1000 square feet, using a combination of the methods described below. Any knotweed control program will require a minimum of three-to-five years of commitment because the root system is extensive and regrowth is rapid.
- Cut or mow shoots as close to the ground as possible. Repeated over time, this will deplete root reserves. Cutting tends to produce more numerous small shoots, which should also be cut. Try to pull out the roots by grabbing numerous shoots simultaneously. Repeat cutting every two-to-four weeks until the first frost. Try to keep knotweed shoots from growing more than six inches, as the aboveground tissue is photosynthetic and replenishes belowground reserves.
- Digging works best for small, isolated patches. Dig up as much root as possible and watch for sprouts within a 20-foot radius.
- Cover with heavy-duty black plastic or geotextile (non-woven or felted) fabric after cutting, digging and pulling. Extend fabric at least seven feet beyond the edge of the area. Weigh the fabric down with large rocks or blocks, allowing some slack so new shoots don't punch through. Every two-to-four weeks, stomp down regrowth under the cover and cut, dig, or pull out any shoots around the edges. Keep the area covered until there is no more growth. Keep in mind that this will kill all plants under the fabric and require replanting after the treatment period.
- Disposal - A root fragment that is less than ½-inch or any node or joint from a knotweed plant can produce a new plant. For this reason, it is imperative to collect all cuttings from a knotweed control project and not allow any cuttings to go into a waterway. After mechanical removal, rake the area. Contain all weed material in heavy plastic bags and send it to the landfill. Do not compost! Remember that Colorado law requires proper disposal that prevents further spread of this or any other "List A" weed.
- Grazing by goats or livestock can control knotweed about as well as mowing. If the infested area is converted to pasture grasses, grazing could keep knotweeds in check for the long term. Be aware that goats will eat desirable vegetation, as well.
- Continuously monitor your property. New infestations should be evident by May or June.
- Plant native replacements in cut or dug-out areas. Conifers may offer the most promise, as knotweed success might be less under dense forest cover. Native grasses will also offer desirable alternatives.
* Please do not compost noxious weeds as this will spread the weeds to other locations.
Plant native species in areas where knotweed has been growing. Be sparing with fertilizer, which can give weeds an advantage over native species. Some native replacements are:
- Colorado Four o'clock (Mirabilis multiflora);
- Sulphur-flower Buckwheat (Eriogonum umbellatum);
- Kinnikinnick (Arctostaphylos uva-ursi);
- Creeping Barberry (Mahonia repens);
- Four-nerve Daisy (Tetraneuris acaulis);
- Tufted Evening Primrose (Oenothera caespitosa)
To provide feedback on the the proposed City Manager's rule, contact Jennifer Riley at 303-441-1877.
For a full list of noxious weeds contact the Colorado Department of Agriculture at 303-239-4100.
Information courtesy of the Colorado Department of Agriculture, USDA Forest Service Fire Effects Information System, Oregon State University Extension Service, and King County Department of Natural Resources and Parks. All photos are courtesy of the Colorado Department of Agriculture.