Japanese Knotweed
Polygonum cuspidatum / Fallopia japonica
(Also known as Mexican bamboo)
Japanese Knotweed is a perennial herb that emerges in the early spring and can grow up to ten feet tall. Their tickets may be so dense that practically all other surrounding plant species are shaded out. Large colonies frequently exist as monocultures. This reduces the diversity of plant species and this can significantly alter natural habitat. Reproduction from rhizomes (horizontal underground stems) enables the plant to be easily transferred to new locations through flowing water or soil transportation. Once this species is established, it is extremely difficult to remove.
Japanese Knotweed is most commonly found in open, moist locations such as riverbanks, river islands, and disturbed wetlands; alongside road margins, and in areas with disturbed soils. Colonization of natural habitats is caused by disturbances (ice, or high waters of rivers and streams). While Japanese Knotweed is native to eastern Asia, its invasive population extends from Nova Scotia to Newfoundlands south to North Carolina. Here in Maine, it has an established presence in every county except for Piscataquis and Hancock.
Description
Robust, very tall (grows up to 10’), perennial herb growing in dense stands.
Leaves - flat at base and abruptly tapering to a pointed tip. 6’’ long and 3-4’’ wide.
Flowers - small, white, and abundant. In small spikes along the stem. Appear in late summer in Maine (late July or August).
Fruits - small (<½’’) with thin “wings” to enable wind dispersal.
Stem - 1-2’’ in diameter. Round, hollow, with swollen nodes where leaves meet the stem. When dead, brown-red stalks remain throughout the winter. Its hollow stems are the reason for its nickname “Mexican bamboo.”
Native Range : Eastern Asia.
Arrival to the United States : Used as an ornamental and as fodder (livestock food) and erosion control. It has been reported invasive in the states/territories of Arkansas, Connecticut, the District of Columbia, Delaware, Georgia, Indiana, Massachusetts, Maryland, Maine, Michigan, Missouri, North Carolina, New Hampshire, New Jersey, New York, Ohio, Oregon, Pennsylvania, Rhode Island, Tennessee, Virginia, Vermont, Washington, Wisconsin, and West Virginia.
Reproduction: Fragments of living stem or rhizome. Fertile seeds are sometimes produced. All seeds should be treated as potentially viable. Can sprout from any stem node or rhizome fragment.
Habitat: Open uplands, riverbanks, lakeshores, forest edges, distributed areas within the forest. Extremely adaptable and tolerant of dry to seasonally saturated soils. Especially problematic along larger rivers where spring flooding transports live rhizomes downstream.
Impacts of Japanese Knotweed
Structural
If Japanese Knotweed is found adjacent to man made structures (bridges, roads, sidewalks, parking lots, and foundations), the rhizomes can weaken the structural integrity of such structures when expanding in size. Upward pressure is exerted as the rhizomes increase in diameter and size. This can split such man made structures at their weak-points. Japanese Knotweed’s rhizomes can also lead to damages in subsurface drainage, underground conduits, septic systems, and other underground infrastructure as well.
Environmental
Unfortunately, Japanese Knotweed is notorious in its rapid spread and its dense colony formation that outcompete our native plant species. Knotweed is known for blocking out sunlight, releasing allelopathic chemicals from its rhizomes that suppress plant growth and germination of other species, and hogging virtually all nutrients and water from the soil. Plant reproduction through seed germination isn’t common for Japanese Knotweed plants. The low probability of Japanese Knotweed reproduction through seeds (plant sexual reproduction) indicates that the spread of Japanese Knotweed positively correlates with human activity with the spread of its rhizomes.
Economic
While the economic impacts caused by Japanese Knotweed are difficult to fully and completely identify, the economic impacts are attributed to structural damage and failure, safety concern for motorists, flooding damage, and loss of important habitats. According to New Hampshire’s 2016 cost estimates from Rockingham County Conservation District, the average cost in managing Japanese Knotweed using the herbicide glyphosate is approximately $500 per acre for the initial treatment. The second year’s follow up treatment of glyphosate is approximately $300 per acre, and if there’s a third year treatment, the approximate cost is $200 per acre. These costs may very well be similar to costs here in Maine. These figures only refer to the use of herbicide treatment, not the costs of site remediation, removal of vegitative growth, soil stabilization, or revegetation.
Control Methods
The best method in controlling Japanese Knotweed is by preventing its establishment altogether. It should be removed as soon as possible. Once established, it can be eliminated by repeatedly cutting the stalks. Three or more cuttings in a single growing season can offset growth of the rhizomes. An alternative to cutting the Japanese Knotweed repeatedly is to apply glyphosate (a herbicide) to the plant. Digging up the plant’s roots is not suggested because digging the plant can lead to root fragmentation that can repopulate the area with the species.
Mechanical or Manual Control
Mechanical control methods include pulling seedlings, rhizomes, mature plants, and repeated clipping of Japanese Knotweed. Manual control of Japanese knotweed takes special precaution because any live part of the plant (½’’ or larger) may resprout. Any plant part must be disposed of properly and must not enter waterways. Stems and roots must be contained or dried with no contact with soil or they may sprout. DO NOT COMPOST PLANT PARTS. Instead, burn or incinerate the rhizomes
Unfortunately, uprooting Japanese Knotweed will only get rid of a portion of the root system. Pull as much root out as possible. Every time there are new sprouts (look after a week of the initial uprooting and from at least 20 feet from the plant), repeat the uprooting and pull up as much root as possible. Hand cutting and mowing aren’t recommended because of the plant’s ability to spread from plant fragments. This method is highly ineffective and should only be considered if it’s the only option available.
Smothering - Covering the Japanese Knotweed with plastics and geotextile fabrics is somewhat successful, but it’s very costly, and it requires pulling, cutting, or herbicide treatment near the edges. Cut the stems to the ground then cover the stems with at least 10 feet extending past the furthest cutting. Weigh down the edges and then monitor for sprouts. Leave the cover in place for two growing seasons.
Another method in smothering Japanese Knotweed following this procedure:
Cutting the Japanese Knotweed stems.
3-4” layer of bark mulch or wood chips to act as a cushion layer.
7-mil black plastic or non-woven geotextile material.
3-4” of bark mulch or wood chips.
Chemical Control
Herbicide applications to Japanese Knotweed must be carefully carried out and planned because the plant is usually found beside water sources. Glyphosate (brand names are Roundup, and for near-water use, Rodeo) is a nonselective herbicide that kills grasses and broad-leaved plants while triclopyr (brand names are Garlon, Pathfinder, and others) is a selective herbicide that kills broad-leaved plants but doesn’t harm grasses.
Foliar Treatment : (This refers to feeding the plant through its leaves rather than its root system). Before using foliar treatments, use stem treatments to reduce the risk of negatively impacting and damaging non-target species. The best time to apply foliar spray to Japanese Knotweed is when the plant is 3-6’ in height. This will require follow-up treatments later in the growing season. Apply a 2% solution of glyphosate or triclopyr and water and wet all of the plant’s foliage. Don’t apply the solution so heavily that the access solution drips from the leaves. Air temperature should be 65 degrees Fahrenheit. The most efficient application of foliar treatment is to apply 1-4 treatments over a two-season period.
To control the Japanese Knotweed population, it’s most necessary to eliminate its rhizome system. Rhizomes are horizontal underground stems that allow for the production of new plant roots and shoots.
There are two phases of knotweed management: initial control and maintenance:
Initial control - the initial control stage of managing Japanese Knotweed plants lasts up to two seasons and consists of either two applications of herbicide or a controlled cutting with a following application of herbicide. (Late season application of herbicide(s) is most effective because this is when the knotweed’s foliage transports sugars produced through the process of photosynthesis to its roots and rhizomes; systemic herbicides move through the plant with these sent sugars). This means the herbicide will reach the knotweed’s roots and rhizomes alongside the sugar, with the roots and rhizomes being its most effective way of reproduction. This is after the flowering season. One main reason for waiting after the flowering season is to avoid the unintentional negative impacts towards pollinators!
As previously established, cutting along is not an effective control method in managing the invasive Japanese Knotweed population. Even so, cutting prior to herbicide application is known to be an effective method. One should cut the knotweed in June (strictly following cutting protocol) and wait at least 8 weeks after the cutting to apply the herbicide. The knotweed regrowth would be much shorter than if it hadn’t been cut due to the rhizomes being forced to redirect their energy towards resprouting instead of expanding their horizontal underground network. Usually, Japanese Knotweed grows 2-5’ tall during the 8-week window between the cutting and herbicide. This waiting period is critical. If one applies herbicide too soon after the managed cutting, the applied herbicide won’t be effectively transported to the knotweed’s rhizomes. Cutting in combination with a singular application of herbicide is more fitting for Japanese Knotweed plants near water sources due to the decreased possibility for accidentally contaminating the water during follow-up herbicide applications.
While utilizing herbicides on Japanese Knotweed doesn’t guarantee a complete success rate and follow up applications of herbicides are required for 3-5 years after the initial application, 100% control can be achieved. The average success rate after the first herbicide application is a strong 85%. This success rate increases after every annual herbicide treatment.
Cultural Control
Cultural control methods are indirect weed control. One example of a cultural control method relating to Japanese Knotweed is preventing the spread of rhizome fragments in soil and excavation machinery. Construction and earth-moving activities are the leading cause for the spread of Japanese Knotweed. Small rhizome fragments can survive long periods of time. The most common human activity that causes the spread of Japanese Knotweed is construction activities in areas where knotweed plants are already established. Some construction activities that cause such spread are routine manage of roadway drainage channels, slope work, or sites-work involving excavation.
Biological control
While biological control methods are currently unavailable for putting into practice, research is currently being conducted to come up with possible biological control methods in managing invasive Japanese Knotweed populations. Currently, researchers are evaluating a leaf-eating insect from Japan called a psyllid. The psyllid, native to Japan, was found on knotweed in Japan and is undergoing host specificity tests through the USDA (United States Department of Agriculture).
Sources
https://www.maine.gov/dacf/mnap/features/invasive_plants/fallopia.htm
https://extension.umaine.edu/publications/2511e/
https://extension.umaine.edu/publications/wp-content/uploads/sites/52/2015/04/2511.pdf
https://www.maine.gov/dacf/php/gotpests/weeds/factsheets/jap-knotweed-usda.pdf
https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb1081649.pdf
https://extension.psu.edu/japanese-knotweed
https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb1081649.pdf
https://www.agriculture.nh.gov/publications-forms/documents/japanese-knotweed-bmps.pdf