Manual and Mechanical Control
Stem cutting, mowing and digging can be effective on small newly established patches (50 stems or less). Methods must be repeated at least three times during the growing season and continued for more than three years for successful control (Duncan, 2013) (King County BMP, 2015)
Cutting and Mowing
Cutting will not likely eradicate but will reduce the impact and deprive the rhizomes of stored energy. (King County BMP, 2015). Cutting is labor intensive but effective. Cut stems as close to the ground as possible at least three times during the growing season, for at least 3 to 5 consecutive years. This will significantly reduce rhizome reserves. For best results, cut stems between May and September, making sure the last cutting occurs before plants begin to lose their leaves with the onset of winter. King County recommends a more thorough cutting regime of cutting twice a month between April and August and then once a month until the first hard frost (Parkinson & Mangold, 2017; King County BMP, 2015).
Mowing can be effective if repeated for several years. Mower height should be as close to the ground as possible, and mowing should be repeated when plants reach a height of 6 inches. Continue mowing throughout growing season until a killing frost occurs. (Parkinson & Mangold, 2017)
Rake and pile up the cut stems where they will dry out. Dried stems can be crushed, composted or burned. Do not leave plant fragments on moist soil or where they can be transported into water, they will sprout nodes and re-infest the area. (King County BMP, 2015)
Combination of cutting/mowing followed by herbicide application can be effective. Repeated cuttings tend to produce numerous small stems that may make future treatment with stem injection more difficult, foliar spray is recommended for this method (King County BMP, 2015). Cut knotweed stems to the ground, single time or several times over the growing season, spray regrowth 6 weeks after the last cuttings during July through October. This method reduces the amount of herbicide use and is more labor efficient (King County BMP, 2015).
Digging
Digging can be effective if done consistently on new small patches when plants are young and the soil is moist. Dig up as much root as possible in August if soil remains moist, if the soil is dry and compact dig in early summer after rainfall. The patch should be treated twice every month after first digging, to remove new sprouts as they emerge from June to first hard frost. Repeat for at least three consecutive years (King County BMP, 2015; Parkinson & Mangold, 2017).
Biological Control
Surveys for existing natural enemies in North America found no native herbivores with any potential for control against knotweed (Reardon, Japanese Knotweed Biological Control).
Insects and Pathogens
Three insects and one pathogen have been a focus for potential biological control agents for release in North America: Aphalaris itadori (a sap sucking psyllid), Gallerucida bifasciata (a leaf feeding chrysomelid beetle), Ostrinia ovalipennis, (a leaf and stem feeding moth), Mycosphaerella sp., (a leaf spot pathogen). (Reardon, Japanese Knotweed Biological Control). In 2003-2005, release and control sites were identified in Washington State and data collected concerning knotweed growth patterns. In 2003, the technical advisory group (TAG) recommended release of Aphalaris itadori in the US (Reardon, Japanese Knotweed Biological Control).
Grazing
Grazing has been observed to reduce the establishment and growth of knotweed where grazing pressure is high. Young shoots are palatable to sheep, goats, cattle and horses. Grazing will not kill the plants, but repeated grazing can weaken them. (Parkinson & Mangold, 2017)
Chemical Control
The most effective herbicide for controlling knotweed is imazapyr, followed by glyphosate (King County BMP, 2015). Herbicides with the active ingredients imazapyr, glyphosate, and aminopyralid have shown to be variably effective in controlling or suppressing knotweed (King County BMP, 2015). Triclopyr and aminopyralid will provide short-term control but generally will not kill the plants. 2-4 D is not effective on knotweed (King County BMP, 2015). This list is not all encompassing and will likely change over time. When treating knotweed use a systemic herbicide that translocates from leaves and stems into the roots. Always read and follow the label, and do not apply herbicide near waterways or wetlands without a National Pollutant Discharge Elimination System (NPDES) permit and WSDA applicators license with an aquatic endorsement. Contact your local county noxious weed coordinator or local WSU extension office if you have any questions regarding the use of herbicide.
Foliar Treatment (Backpack Sprayers, Handheld Sprayers)
Easiest and fastest method, but risk of drift onto desirable vegetation or dripping of herbicide off leaves into water and soil (King County BMP, 2015). Apply herbicide to green foliage, from July to October before the first hard frost (when leaves begin to discolor and fall off). Fall application may be the most effective because plants will translocate more herbicides to rhizomes rather than above ground growth (Parkinson & Mangold, 2017; King County BMP, 2015).
Avoid spraying knotweed when bees and other pollinators are present on flowers whenever feasible (King County BMP, 2015. If knotweed patch is in an upland area, mow or cut knotweed in early summer to prevent or delay flowering during treatment window. Before spraying allow plants to regrow for 6 weeks or when regrowth is at least 3 feet tall (King County BMP, 2015).
Spraying the same patch twice once in the spring/summer and once in the fall sets plants back so they can be sprayed at the appropriate growth stage at the easiest height, but increases overall herbicide use (King County BMP, 2015).
Imazapyr (Foliar Treatment)
- 1% solution with 1% surfactant
- Apply midsummer until first hard frost
- Imazapyr has residual soil activity and may impact roots of non-target plants.
Glyphosate (Foliar Treatment)
- 2-8% solution with 1% surfactant
- Apply from midsummer until first hard frost
Stem Injection
This method is highly effective and selective, but very time, labor and herbicide intensive compared to foliar application. Keep track of herbicide amount used, as it is easy to over exceed maximum usage rates, as a general rule you can only inject 2,375 canes per acre (King County BMP, 2015). Use the JK Injection Tool or another small hand-operated injection device to deliver 3 to 5 ml of concentrated glyphosate into each knotweed stem that is at least 0.5 inches in diameter. Inject the herbicide between the second and third node of the cane. Mark canes with spray paint or grease pen to keep track of what stems were injected (King County BMP, 2015).
Cut Stem Method
Practical in large restoration sites. This method uses less herbicide than stem injection and large areas can be cut and treated. In riparian areas it is risky to leave cut stems where they could be transported by the stream or contact moist soil and re-root (Davenport R., 2006). Stems less than 3 cm in diameter should not use this method. (Davenport R., 2006).
Cut/mow all knotweed stems greater than 3 cm in diameter. Use a 1:1 to 1:5 (glyphosate:water) dilution rate. Apply herbicide directly to the cut well above the second node, effective if the dilution rate supplies 2 ml of herbicide (Davenport R., 2006). The lower rate is effective provided the stem is large enough to hold sufficient herbicide mixture. (Davenport R., 2006).
Manual Bending of Stems
Bending of growing knotweed stems, then applying a foliar herbicide to the regrowth. Reduces height and vigor of plants while they are rapidly growing and produces a shorter stem that is treated later in the summer/fall when knotweed is more susceptible to herbicide. (Davenport R., 2006). Spray regrowth with a 5% glyphosate solution. (Davenport R., 2006). Bending will create smaller diameter regrowth stems that cannot be inject (Davenport R., 2006).
Covering
Recommended for very small infestations, 50 stems or less. (Parkinson & Mangold, 2017). Cut stems down to ground level, single or several times before covering to exhaust the root reserves. Cover with heavy duty geo-textile fabric or black plastic. Weight down cover with heavy rocks or cement blocks. Extend cover at least 7 feet beyond the outside stems. Leave in place for at least five growing seasons. (King County BMP, 2015). Monitor and stomp down any re-growth under covering material (King County BMP, 2015).
Revegetation
Often an overlooked component in noxious weed control projects, replanting knotweed control sites is a crucial step in the long term control and restoration of knotweed invaded ecosystems. In highly disturbed riparian areas, especially along large streams, active reintroduction of native plant species may help promote recovery and prevent exotic plant reinvasions through increased direct competition or propagule pressure (Claeson & Bisson, 2013). Planting larger native plants appropriate to floodplain riparian settings can provide shade, potentially weakening shade intolerant exotic seedlings while supporting shade tolerant native species. (Claeson & Bisson, 2013). Claeson and Bisson, found that overstory cover was positively associated with greater levels of native species and cover, and native tree cover was negatively correlated with initial knotweed stem count; while knotweed was able to become denser in areas where overhead tree cover was already low.
Both native and exotic plants colonize recently treated knotweed sites. Without successful native vegetation recovery, these ecosystems are prone to reinvasion by the same exotic or secondary exotic species (Claeson & Bisson, 2013). Eradicating knotweed removes a dominant competitor, thus providing resource and niche opportunities that favor the recruitment of rapidly-growing species and fluvial transport of propagules aids colonization by new species in recently disturbed riparian habitats. (Claeson & Bisson, 2013). Revegetation alone is unlikely to be an effective control method. Other methods should be taken to first control knotweed (combination of digging/grubbing, mowing, covering, biological control, and herbicides). (Parkinson & Mangold, 2017).
Once a patch appears to be eradicated, revegetation is strongly recommended to suppress reinvasion of knotweed or another opportunistic invasive species. (Parkinson & Mangold, 2017). Generally, sites should not be planted the first year after treatment because knotweed will often survive the first treatment and pose problems for establishing native species (Davenport R., 2006). If knotweed was treated with imazapyr, the herbicide may still be active in the soil and potentially cause damage to the planting 1-2 years after treatment. After control, replant bare areas with appropriate native vegetation or cover with mulch while desirable vegetation becomes established. Do not leave large areas of bare soil, or other opportunistic weeds will establish. (King County BMP, 2015).
Species recommended for revegetation (Species should be selected based on site conditions).
- Conifer Trees – Sitka spruce, western redcedar, grand fir, Douglas fir, western hemlock, and shore pine.
- Deciduous Hardwoods – Willow (Salix spp.), black cottonwood, red alder, big leaf maple, Oregon ash, western crabapple.
- Shrubs – Pacific ninebark, black twinberry, Nootka rose, red-osier dogwood, douglas spirea.
- Groundcover – Slough sedge, Western sword fern, dull-Oregon grape.
Tree protectors placed over installed plants can protect them from future herbicide treatment and animal damage. Post planting maintenance and weed management can damage native recruits, consider protecting natural recruitment plants, in addition to installed species, as they have proved to be an important component for the recovery of native plant communities (Davenport R., 2006). Choose native plant species that will ultimately create a diverse, competitive native community that is similar in species composition to nearby reference sites and can be maintained over a number of years until shade is established (Davenport R., 2006).