Background │ Origin │ Habitat │ Introduction and Spread │ Impacts │ Identification │ Prevention and Control │ Occurrences | New York Distribution Map

Background

Pale swallow-wort or dog-strangling vine (Vincetoxicum rossicum; syn. Cynanchum rossicum) and black swallow-wort or black dog-strangling vine (Vincetoxicum nigrum; syn. Cynanchum louiseae) are herbaceous invasive plant species. They were originally placed in the milkweed family, Asclepiadaceae, but in recent years the Asclepiadaceae has been recombined with the periwinkle family, Apocynaceae. Pale and black swallow-wort are two distinct species, but do have a similar growth habit and many similar physical characteristics. Both species are perennial climbing vines that form extensive patches and overgrow and smother native vegetation.

Origin

Black swallow-wort is native to the western Mediterranean region. Pale swallow-wort is native to Ukraine and southeastern European Russia.

Habitat

Black and pale swallow-wort tolerate a variety of soils and environmental conditions. They are typically found in upland habitats and have been observed in rocky shores, agricultural and natural fields, and woodlands.  Swallow-wort favors full sun, but may thrive in semi-shaded areas as well. Pale swallow-wort also can be found in densely shaded sites. These species are disturbance- and drought-tolerant and can quickly spread once established.

Introduction and Spread

Swallow-wort may have been originally brought to North America for horticultural purposes as an ornamental plant. The shoots of swallow-wort start to emerge from the soil in the spring. The plants flower in the early summer and produce seed pods towards late July and early August. A square meter stand of swallowwort is capable of producing 1000-2000 seeds per year, which are then dispersed by wind. Black and pale swallow-wort are capable of self-fertilization and are polyembryonic, meaning each seed may contain multiple seedling producing embryos (Lumer and Yost 1995; St. Denis and Cappuccino 2004). The main method of spread is by seed; however, if the main stem is damaged, the perennating buds on the root crown will activate to produce axillary tillers (Weston et al. 2005).

Impacts

Black and pale swallow-wort are moderately aggressive. The extensive patches of swallow-wort may overgrow native plants and disrupt natural succession. Herbivores and grazers, such as deer, tend to avoid swallow-wort to feed on more palatable plants. Additionally, due to the tangling nature of swallow-wort, large patches can be difficult to walk through and may interfere with forest management and recreation.

Lower arthropod diversity and abundance have been observed on invasive pale swallow-wort when compared to native plants (Ernst and Cappuccino 2005). Bird populations and soil microbial communities may also be indirectly impacted when swallow-wort invades. Pale swallow-wort grows particularly well on limestone-based soils and is invading globally rare alvar habitats in New York (DiTommaso et al. 2005).

Studies have looked at the potential impact of swallow-wort on monarch butterflies (Danaus plexippus). Most monarch butterflies tend not to lay their eggs on the two swallow-wort species, but if they do the larvae cannot survive. Additionally, the butterflies may indirectly be threatened if swallow-wort outcompetes their preferred larval host, common milkweed (Asclepias syriaca) (DiTommaso and Losey 2003).

Identification

Pale and black swallow-wort look very similar when not in flower. Both species have long, oval, opposite leaves (about 3-4 inches long by 2-3 inches wide). The leaves of pale swallow-wort tend to be a slightly lighter shade of green than black swallow-wort. Flowers of both species are small and star-like. Pale swallow-wort has a pink to maroon flower with petals that have a narrow base and no hairs, whereas black swallow-wort’s are a deep purple-black and have petals with a wider base (about half as wide as they are long) and hairy inner petals. Both species produce seed pods that are 1.5″ to 3″ long with wind-distributed seeds inside. Swallow-worts possess a fibrous root system.

Prevention and Control

Mechanical and manual control:

Mechanical and manual control of swallow-wort are difficult due to the presence of a deep, fibrous root system. If you dig up either of these plants, try to get as much of the root crown as possible. For small stands, the plants and seed pods can be collected in plastic bags and destroyed or disposed of properly in a landfill before they release their seed. Collecting seed pods is not a long-term solution and may be impractical for large stands.

Mowing will not eradicate the plant, but can be utilized to prevent seed formation if timed correctly. If swallow-wort stands are cut too early, the plants can recover and still produce viable seed. It is suggested to mow when immature seed pods are just starting to develop and do not yet contain viable seeds (McKague and Cappuccino 2005). Mowing of swallow-wort after seed development is not recommended as it could aid in seed dispersal.

Chemical Control:

Chemical herbicides, such as glyphosate or triclopyr, may be used for control as foliar sprays or on cut stems (Weston et al. 2005). Recent work in central New York State has shown that applying these herbicides in late summer (i.e. mid- to late-August) to plants that have been mowed or trimmed in early July may provides the best results. The mowing of plants in early July also ensures that no viable seeds are produced prior to application of the herbicides (DiTommaso et al., unpublished). These chemicals work on living plants and need ample leaf surface area to absorb the herbicide to effectively kill the roots. For this reason, it is not recommended to apply them on seedlings soon after emergence. Results may been seen in one to two weeks, but repeated application may be needed. Check with your local extension office about herbicide use and permits needed for chemical application. As always, with any chemical, follow the instructions on the label.

Biological Control:

At this time, there are no approved biological control methods for swallow-wort. Biological controls for swallow-wort species are currently being investigated.

Occurrences

There have been reports of invasive black swallow-wort from Ontario, Canada south to Pennsylvania and from the northern Atlantic coast west to Missouri. Pale swallow-wort is most abundant in the lower Great Lakes Basin and has been observed in New York, Connecticut, Indiana, Massachusetts, Michigan, New Hampshire, New Jersey, and Pennsylvania as well as in parts of Canada.

New York Distribution Map

This map shows confirmed observations (green points) submitted to the NYS Invasive Species Database. Absence of data does not necessarily mean absence of the species at that site, but that it has not been reported there. For more information, please visit iMapInvasives.

History & Distribution | Biology & Habitat | Identification | Ecological Impacts | Human Health Impacts | Control

One of New York’s most striking and dangerous invasive plants, the giant hogweed (Heracleum mantegazzianum) can make a case of poison ivy seem like a mild rash.

History and Introduction

A member of the carrot and parsley family of plants (Apiaceae), giant hogweed is native to the Caucasus region of Eurasia. Because of its unique size and impressive flower head, the plant was originally introduced to Great Britain as an ornamental curiosity in the 19th century. The plant is named after the mythological god Hercules (he of robust size and strength). It was later transported to the United States and Canada as a showpiece in arboreta and Victorian gardens (one of the first North American plantings of giant hogweed was in gardens near Highland Park in the City of Rochester, New York). It was also a favorite of beekeepers because of the size of its flower heads (the amount of food for bees is substantial). A powder made from the dried seeds is also used as a spice in Iranian cooking. Unfortunately, as with so many invasive plants, giant hogweed escaped cultivation and has now become established in a number of areas in New York (see map, below), as well as in Connecticut, the District of Columbia, Illinois, Maine, Maryland, Massachusetts, Michigan, Ohio, Oregon, Pennsylvania, Washington, Wisconsin, and Ontario and Vancouver Island in Canada. Because of its public health hazard potential and, to a lesser extent, to its potential ecological impacts, giant hogweed is on the federal noxious weed list and several state lists of prohibited plant species.

Biology & Habitat

Giant hogweed (Heracleum mantegazzianum) is a member of the carrot or parsley family, Apiaceae (Umbelliferae). Except for its size, the plant can be mistaken for a number of native, noninvasive plants such as cow parsnip (Heracleum lanatum), Angelica (Angelica atropurpurea), and poison hemlock (Conium maculatum). Of these, the plant most likely to be misidentified as giant hogweed is cow parsnip. A fourth, not-so-innocuous, invasive giant-hogweed imposter found throughout North America is wild parsnip (Pastinaca sativa). Information on how to distinguish these giant hogweed wannabees from the real thing can be found later in this profile.

Giant hogweed is a perennial herb with tuberous root stalks. It survives from one growing season to another by forming perennating buds (surviving from season to season) and enduring a period of dormancy during the winter. The plant develops numerous white flowers that form a flat-topped, umbrella-shaped head up to two and a half feet across, resembling “Queen Anne’s Lace on steroids.” Flowers form from late-spring through mid-summer. Numerous (up to 100,000), half inch long, winged, flattened oval seeds form in late-summer. These seeds, originally green, turn brown as they dry and can be spread by animals, surface runoff of rain, or on the wind, establishing new colonies. Seeds can remain viable in the soil for up to 10 years. The plant’s stems die in the fall and remain standing through the winter, topped with the huge, brown dead flower heads.

Giant hogweed’s thick hollow stems are generally one to three inches in diameter but can reach four inches. Also impressive are the plant’s lobed, deeply incised compound leaves, which can reach up to five feet in width. The plant may grow to 15 to 20 feet in height.

Identification

As mentioned earlier, there are several plants in New York and the Northeast that can be mistaken for giant hogweed. Key features for distinguishing these plants from giant hogweed are explained below. Click the identification tables to enlarge.

Giant hogweed may grow to 15 to 20 feet in height. Stems are 1 to 3 inches in diameter, but may reach 4 inches. Stems are marked with dark purplish blotches and raised nodules. Leaf stalks are spotted, hollow, and covered with sturdy bristles (most prominent at the base of the stalk). Stems are also covered with hairs but not as prominently as the leaf stalks. Leaves are compound, lobed, and deeply incised; can reach up to 5 feet in width. Numerous white flowers form a flat-topped, umbrella-shaped head up to two and a half feet across. [See ID table 1]

Native Cow parsnip, while resembling giant hogweed, grows to only five to eight feet tall. The deeply ridged stems can be green or slightly purple, do not exhibit the dark purplish blotches and raised nodules of hogweed, and only reach one to two inches in diameter, contrasted with hogweed stems which can reach three to four inches in diameter. Where giant hogweed has coarse bristly hairs on its stems and stalks, cow parsnip is covered with finer hairs that give the plant a fuzzy appearance. Both sides of the leaves exhibit these hairs but they are predominantly on the underside of the leaves. In contrast to hogweed’s two- to two-and-a-half-foot flower heads, cow parsnip flower clusters are less than a foot across. The size difference carries over into leaf size with hogweed’s five foot, deeply incised leaves replaced by leaves that are less incised and only two- to two-and-a-half-foot across.

Native purple-stemmed Angelica is more easily differentiated from giant hogweed by its smooth, waxy green to purple stems (no bristles, no nodules), and its softball-sized clusters of greenish-white or white flowers, seldom reaching a foot across. As with cow parsnip, Angelica is much shorter than giant hogweed, usually no more than eight feet tall. Angelica leaves are comprised of many small leaflets and seldom reach more than two feet across.

Poison hemlock, a non-native biennial, is also shorter than giant hogweed, growing to only four to nine feet in height. While the stem has some purple blotches, it is waxy and the entire plant (stems, stalks, leaves) is smooth and hairless. The leaves are dramatically different from those of hogweed, being fernlike and a bright, almost glossy, green. All branches have small flat-topped clusters of small white flowers. Another distinguishing characteristic is poison hemlock’s unpleasant mouse-like odor. The entire plant is toxic, and the volatile alkaloids can even be toxic when inhaled. [See ID Table 2]

Wild parsnip, like giant hogweed, is of special concern because it, too, can cause phytophotodermititis, only not usually as severe as that of giant hogweed. This plant can be found extensively throughout NY’s Southern Tier, in the region east of Lake Ontario, some Central and Western New York counties, parts of the Catskills and counties east of the Hudson River. Unlike the perennial giant hogweed, wild parsnip is biennial, producing a rosette of leaves close to the ground in its first year and a single flower stalk with a flat-topped umbel with clusters of yellow flowers in its second year. The plant reproduces by means of the seeds of these flowers; it does not re-grow from its root as does giant hogweed. Wild parsnip is much smaller than giant hogweed, seldom exceeding 5 feet in height. Wild parsnip stems are yellowish-green with vertical grooves running their length. Wild parsnip has compound pinnate leaves with 5 to 15 toothed and variably lobed yellowish-green leaves.

Ecological Impacts

Colonies of giant hogweed can become quite dense owing to the plant’s prolific seed production and rapid growth rate. Such dense stands crowd out slower growing plants, the thick hogweed canopy displacing native plants that need direct sunlight to grow. The decreased abundance of beneficial native plants can reduce the utility of the area for wildlife habitat. When riparian plants are displaced, stream bank erosion can increase and streambeds can become covered with silt.

Human Health Impacts

Giant hogweed is one of a very few North American invasive plants that can cause human health impacts as well as ecological damage, causing a significant reaction when humans come in direct contact with the plant. Spread of this plant in urban and suburban areas is viewed as an incipient public health hazard. [Wild parsnip can result in almost as severe reactions.]

Soon after humans bruise the leaves or stems of the more common poison ivy, poison oak, and poison sumac, an allergic reaction to the plants’ poisonous oil (akin to carbolic acid) causes significant skin irritation, itching, rashes and open sores. In the case of giant hogweed, however, simply simply brushing against the plant’s leaves or stems does not cause the skin inflammation. For giant hogweed to affect a person, sap from a broken stem or crushed leaf, root, flower or seed must come into contact with moist skin (perspiration will suffice) with the skin then being exposed to sunlight. Irritation is not immediate, but will usually appear within one to three days after exposure. This form of skin irritation (dermatitis) is called “phytophotodermatitis”. The plant’s clear, watery sap contains a glucoside called furanocoumarin that is a psoralen. Psoralens sensitize the skin to ultraviolet radiation and can result in severe burns, blistering, painful sores, and purplish or blackened scars. These phototoxic effects are the result of the binding of the psoralens to nuclear DNA under the influence of ultraviolet irradiation, and the subsequent death of affected cells.

The first signs of giant hogweed-caused photodermatitis are when the skin turns red and starts itching. Within 24 hours, burn-like lesions form, followed by large, fluid filled blisters within 48 hours. The initial irritation usually will subside within a few days, but affected areas may remain hypersensitive to ultraviolet light for many years and re-eruptions of lesions and blisters may occur. On rare occasions, particularly in very sensitive individuals, the burns and blisters may be bad enough to require hospitalization. A side effect of exposure to the psoralens is the production of excessive amounts of melanin in the skin, resulting in residual brown blotches called hyper-pigmentation; scars and brown to black blotches may last for several years. The worst risk of exposure to giant hogweed is to one’s eyes – getting even minute amounts of the sap in the eyes can result in temporary or even permanent blindness. Medical help should be sought immediately; by the time symptoms of burning and hypersensitivity to sunlight are apparent, the damage could already be irreversible.

The only known antidote to contact with the sap is to immediately wash skin thoroughly with soap and water, removing the sap and hopefully preventing any reaction with subsequent exposure to sunlight. Once the irritation begins, medical advice should be sought. Treatment with prescription topical steroids early on may reduce the severity of a person’s reaction. It will also be important to cover the burns and blisters with light sterile dressings to prevent infection. Long-term, use of sunblock in subsequent years may be required to prevent sensitization by sunlight again.

People most at risk include landscape technicians and yard maintenance laborers who may come in contact with the sap when cutting the plant down or using line trimmers to control new growth. Children breaking off the long, bamboo-like stems to use as play swords are also at great risk. However, sometimes direct contact with the plant is not necessary for a reaction. Farmers have been known to develop symptoms when they touch cows who have gotten the sap on their skin while grazing (cows, themselves, seem impervious to the sap). The best prevention measure is to wear long sleeves and long-legged pants when contact with the plant is a possibility.

Control

If it weren’t for giant hogweed’s public health impacts, the plant most likely would not be worth the effort of controlling it. Although it does have ecologic impacts, they are not as severe as many other wetland invasive plants. However, the health impacts can be severe and the plant has found itself on the federal noxious weed list and several state lists, as well. It is a particular target of parks and transportation/highway departments’ invasive plant eradication efforts. Such eradication programs can incorporate a combination of physical removal and chemical control. If undertaken properly, such programs can be done without harm to humans or damage to the environment. Recently some landowners have been known to refuse permission to allow highway departments to chemically treat giant hogweed thickets. It is believed that this is usually a case of lack of knowledge on the landowner’s part.

Giant hogweed is very difficult to eradicate. Although the stems, stalks, leaves and flowers can be killed with a number of common selective herbicides, such as 2,4-D (the third most-often used herbicide in North America), dicamba (a benzoic acid herbicide), TBA (terbuthylazine) and MCPA, these herbicides are not effective at killing the plant’s tuberous perennial roots. Another common, selective broadleaf herbicide, triclopyr (a common brand name is Brush-B-Gone®), is also effective, particularly when applied directly to the entire surface of leaves and stems during periods of active growth; numerous applications may be needed to kill the root stalk. Early application (during the bud stage and the period of active plant growth) of glyphosate (commonly sold under the trade names Rodeo® and Roundup®) is most effective. Care should be taken when using any herbicides to control giant hogweed; particular care should be taken when using glyphosate as it is nonselective and will kill both the hogweed and desirable plants such as grass. Before using any herbicide, check with your state environmental regulatory agency to find out which products are licensed for homeowner use in your state.

For those hesitant to utilize herbicides, giant hogweed can be managed using various “cultural” methods. Unfortunately, owing to the plant’s persistence and spread by blowing seeds, such control can take many seasons worth of effort to achieve 100% control. Individual plants can be dug out, removing the entire rootstalk, a difficult process, particularly in patches where the plant has spread by root growth. Mowing, cutting and use of line trimmers can be used to remove a standing crop and starve the rootstalk. Unfortunately, unless performed numerous times during a season, mowing only serves to stimulate budding on the rootstalk. All of these methods should be done with extreme care and only while wearing protective clothing and eye protection. Skin contact with soiled clothing should also be avoided. Biocontrol by grazing cows and pigs (which are apparently not affected by the plant’s sap) may also help to manage but not eliminate the plant. Care should be taken not to get sap on uncovered skin when touching livestock after the animals contact crushed or bruised hogweed.

Control of wild parsnip is less difficult than controlling giant hogweed because as a biennial, wild parsnip reproduces only from seed, not from its rootstock. This plant can be controlled by cutting the stem from the root below ground level with a shovel, spade or machete before the seed head matures.

If you find giant hogweed in New York State, you are encouraged to call New York State’s Department of Environmental Conservation (DEC) Giant Hogweed Hotline:

845-256-3111

Reports

2014 Giant Hogweed Annual Report

The DEC Forest Health giant hogweed crews and partner agencies (Oswego SWCD and four PRISMs (Partnerships for Regional Invasive Species Management) — APIPP, CRISP, Lower Hudson, and SLELO) had an extremely productive field season which resulted in the following achievements:

• 1,495 (93%) of the 1,613 active sites located throughout 47 counties in NY State were visited
• 501 sites previously infested (28% of all sites), no longer had any giant hogweed plants
  • 239 of these sites have been monitored for 3 years with no plants found and we now consider them eradicated
• 516 sites had only 1-19 plants (34% of active sites). The sites are getting smaller in size
• 556 sites were treated with root-cutting- 22,255 plants controlled
• 551 sites were treated with herbicide- 397,000 plants sprayed
• 316 sites were treated with flower/seed head removal- 7,677 flower/seed heads removed
• The hotline, based out of the New Paltz DEC office, received 2,491 calls and emails

2017 Giant Hogweed Annual Report

View the 2017 Report

• 2,253 confirmed sites in 49 counties (no sites discovered in any new counties)
• 1,755 of the confirmed sites are in the monitor or treatment stage
• 123 sites newly designated as eradicated for a total of 498 eradicated sites (no plants for 3 consecutive years)
• 1,804 sites (80%) have 0-99 plants
• 140 new sites identified
• 1,872 sites visited
• 1,233 sites controlled – approximately 668,000 plants controlled
• 1,106 phone calls and e-mails responded to by GH Information Line staff
• 205,857 visits to DEC’s GH webpages

Printable Giant Hogweed Factsheet

Printable Giant Hogweed Identification Table

Habitats | Origin and Introduction | Identification | Impacts | Prevention and Control | New York Distribution Map

Several species of honeysuckle found in NY are characterized as invasive, including: Morrow’s honeysuckle (Lonicera morrowii), Tatarian honeysuckle (Lonicera tatarica), Amur honeysuckle (Lonicera maackii), and Japanese honeysuckle (Lonicera japonica). To the non-botanist, native and invasive non-native honeysuckles appear very similar. One way to distinguish between native and invasive honeysuckles is by looking at the stems – native honeysuckles have solid stems while invasive honeysuckles have hollow stems.

Habitats

All four species are successful invaders of a similar range of habitats, including: abandoned fields; pastures; early successional, open canopy, and planted forests; along the edge of woodlots; floodplains; highway, railway and utility rights-of-way; open disturbed areas; vacant lots; edges of lawns; and, gardens. L. japonica can also be found in agricultural fields. All four grow best in full sun; L. japonica is the most shade-tolerant of the four, with L. tatarica and L. maackii being semi-shade tolerant.

Origin and Introduction

L. tatarica is native to Central Asia and Southern Russia and is believed to have been introduced into North America for ornamental purposes as early as the 1750s. L. japonica, – a native of China, Japan and Korea – was introduced for horticultural purposes in 1806 on Long Island; it was widely distributed as a garden plant through the early-1900s when it was finally recognized as a weed. L. maackii, also native to China, Japan and Korea, was introduced as seeds to arboreta throughout the U.S. in the late-1800s to determine whether the plant would grow in North America. This species of honeysuckle was utilized as a soil stabilization and wildlife planning until the mid-1980s and is still available for sale on-line. L. morrowii, a native of Japan, was imported to Massachusetts in the 1860s and was later released as an ornamental. All four species have escaped cultivation and are easily spread by birds.

Identification

L. morrowii, L. tatarica, and L. maackii), are perennial shrubs; L. japonica is a perennial woody vine (although its leaves can remain green throughout mild winters). The shrub forms range from 6 to 15 feet in height, while vines can reach 30 feet in length. The egg-shaped leaves range from 1 to 3 inches in length and are arranged oppositely along stems. Invasive honeysuckles begin flowering from May to June and bear small (less than 1 inch long), very fragrant tubular flowers ranging from creamy white through various shades of pink to crimson. L. morrowii and L. tatarica produce ¼ inch red berries from mid-summer through early-fall; L. maackii’s dark-red berries don’t ripen until late-fall; L. japonica produces dark-purple or black berries in the fall. Stems of all four are hollow.

Impacts

All three species can form very dense populations that can outcompete and suppress the growth of native plant species. These dense stands suppress the growth of other native species. L. maackii leafs out very early in spring, giving it a competitive advantage over native plants. L. japonica leaves are semi-evergreen allowing the plant to grow longer into the winter and giving it a competitive advantage over native vegetation. It shades out understory growth preventing the success of native understory plants and tree seedlings. Its vigorous vine growth covers native trees; the weight of the vine growth can bring down weak trees. By decreasing light availability to the understory, these invasive honeysuckles can alter habitats by depleting soil moisture and nutrients. The invasive honeysuckle berries do not contain the amount of fat and nutrients present in native honeysuckle berries; eating large amounts of the less nutritious invasive berries rather than native berries can have negative impacts on migrating.

Prevention and Control

Because these plants spread rapidly via birds eating seeds, control should be started in late-summer or early-fall before seeds are ready to be dispersed. In early stages of invasion, or in cases where populations are at low levels, hand removal of honeysuckle seedlings or young plants is a viable option when repeated annually. Systemic herbicides can be utilized in cases of heavy infestation. Specific state rules should be followed and the appropriate (low environment impact, legally labeled for control of these plants) herbicides should be used. For invasive honeysuckles growing in open habitats, prescribed burning may be an effective control alternative.

New York Distribution Map

This map shows confirmed observations (green points) submitted to the NYS Invasive Species Database. Absence of data does not necessarily mean absence of the species at that site, but that it has not been reported there. For more information, please visit iMapInvasives.

Distribution     |     Identification     |     Impacts     |     Prevention & Control   |  New York Distribution Map

Background

Japanese knotweed (Polygonum cuspidatum Sieb. & Zucc.), a member of the buckwheat family, was introduced into the U.S. from Eastern Asia (Japan, China, Korea) as an ornamental on estates in the late-1800s. It has also been used as an erosion control plant. By the mid-1890s, it was reported near Philadelphia, PA, Schenectady, NY, and in New Jersey. Although once sold through seed and plant catalogs, by the late-1930s knotweed was already being viewed as a problematic pest. The plant, which can grow from three to 15 feet tall, has bamboo-like stems and is sometimes called Japanese bamboo. As with many invasive plants, knotweed thrives in disturbed areas and once established can spread rapidly, creating monoculture stands that threaten native plant communities. Japanese knotweed can tolerate deep shade, high temperatures, high soil salinity and drought. It is commonly found along streams and rivers, in low-lying areas, disturbed areas such as rights-of-way, and around old home and farmsteads.

Distribution

Japanese knotweed can be found in the U.S. in 42 states coast to coast except the arid Southwest, several of the deep South Gulf states and the highest of the Rocky Mountains. It is also in eight Canadian Provinces.

Identification

The plant is an upright, shrubby, herbaceous, woody-appearing perennial reaching heights of 10 to 15 feet (3 to 4.5 m). The stems are smooth, stout, and hollow. Where the leaves attach to the stem, the stem is swollen with a membranous sheath surrounding the joints. The leaves are broadly ovate (broad and rounded at the base and tapering toward the end), 3 to 6 inches (8 to 15 cm) long by 2 to 4 inches (5 to 10 cm) wide, alternating on stem, broadly oval to somewhat triangular or heart-shaped, pointed at the tip. The plant’s shoots come up from a network of spreading rhizomes. These horizontal roots can reach lengths of 65 feet (20 m) or more. Japanese knotweed has branched sprays of small greenish-white flowers from August to September. Although they have complementary male and female organs, those organs are vestigial and the flowers function unisexually. The plant develops small winged fruits Seeds: triangular, shiny, very small, about 1/10 inch (2.5 mm) long. Japanese knotweed spreads primarily by seed (transported by wind, water, animals, humans, or as a soil contaminant), stem fragments, and by shoots sprouting from its system of rhizomes.

Impacts

Japanese knotweed spreads rapidly, forming dense thickets that crowd and shade out native vegetation. This reduces species diversity, alters natural ecosystems, and negatively impacts wildlife habitat. The ground under knotweed thickets tends to have very little other growth. This bare soil is very susceptible to erosion, posing a particular threat to riparian areas. Once established, populations of Japanese knotweed are extremely persistent and hard to eradicate.

Prevention & Control

As with most invasive plants, Japanese knotweed can quickly inhabit disturbed ecosystems. One of the best ways to prevent its colonization is to ensure that disturbed habitats are rehabilitated with native vegetation before knotweed can invade.

Mechanical control

Single young plants can be pulled by hand depending on soil conditions and root development. If all of the root system isn’t removed, re-sprouting can occur. For small initial populations beyond single plants, or in environmentally sensitive areas where herbicides cannot be used, grubbing with a pulaski or similar tool to remove all of the roots after cutting back the standing vegetation can be an effective control measure. All parts of the removed plants should be bagged and disposed of in a secure location. It is best if knotweed not be mown or cut with weed trimmers as the pieces of the plant can easily get moved around and re-sprout, spreading, rather than controlling the plant.

Chemical control

Chemical controls for Japanese knotweed include application of glyphosate and triclopyr herbicides to freshly cut stems or to foliage. Before using chemicals to control any invasive plant, check with your local environmental or natural resources management agency or Cooperative Extension office to find out what chemicals are legal for use on knotweed in your area.

Catskill Regional Invasive Species Partnership (CRISP) Japanese knotweed video

New York Distribution Map

This map shows confirmed observations (green points) submitted to the NYS Invasive Species Database. Absence of data does not necessarily mean absence of the species at that site, but that it has not been reported there. For more information, please visit iMapInvasives.