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Say ‘no’ to knotweed

September 8, 2009

Japanese knotweed is listed as one of the world's 100 worst invasive species and is becoming rampant in Adirondack hamlets and villages. Roots creep up to lengths of 65 feet; broad-shaped leaves form a canopy that towers 10 feet above the ground; and dense stands of hollow, reddish-green stems create impenetrable thickets. Cascades of white flowers distinguish knotweed in late summer, the primary trait that made it popular as an ornamental planting in gardens when it was first introduced from Asia in the late 1800s. By the 1930s, however, horticulture magazines began posting information on how to get rid of the plant.

This perennial is difficult to control because it has extremely vigorous rhizomes (roots) that form a deep, dense mat. Plants grow early in the year, outcompeting natives for sunlight, space and nutrients. Its roots plow through soils, causing erosion, which is particularly of concern if infestations are located along stream banks. Sedimentation can degrade aquatic insect and fish spawning habitats. In addition, plants will resprout from the tiniest fragments; along streams, plant fragments may fall into the water and start new infestations downstream. Some of the worst infestations span the distance of several river miles.

Japanese knotweed grows in a variety of habitats including stream banks and wetlands and along roadsides, forest edges and other disturbed areas. It prefers full sunlight but can tolerate moderate shade and is tolerant of high temperatures, dry soil and salt. It is extremely intolerant of frost though, and after the first frost, it turns brown and dies back for the season.

Article Photos

Emerald ash borer
(Photo provided)

Management can take many years but with persistence, infestations can be knocked back. Digging out plants may limit some infestations but it can also fragment the rhizomes and aggravate growth. The most effective treatment is repeated cutting followed by treating the cut ends with herbicide. This procedure will slowly deplete the plant of energy. Before beginning any herbicide treatment, always read and follow labels. Then, it is critical to dispose of the cut plants so as not to infect your property or the property of others. Bag up all plant material and dispose of at the transfer station, or take plants to a burn pile. Do not compost.

Community led efforts are underway in Inlet, Blue Mountain Lake and Indian Lake that involve village staff, local and state departments of transportation, resource managers and landowners. They are mapping where infestations are, contacting landowners for permission to control the plants, and developing a program to treat knotweed stands. To become involved in a community-based effort, or for identification and management assistance, call the Adirondack Park Invasive Plant Program at 518-576-2082.

'Eye on Invasives' is a biweekly column that spotlights a top invader when it is easiest to identify. The Adirondack Park Invasive Plant Program is a partnership program housed at the Adirondack Chapter of The Nature Conservancy in Keene Valley, NY. Find out more about this award-winning program online at

Fact Box

EAB: What to look for:

Last week I described signs and symptoms of an Asian longhorned beetle infestation. Another forest pest on the move is emerald ash borer (EAB). More than 40 million ash trees have died or are dying in the U.S. from EAB, and ash comprise 8 percent of trees in state forests.

Here are its signs and symptoms:

1. Bright, metallic green insect, 1/2 inch long and 1/6 inches wide with a flattened back

2. Ash dieback in the top 1/3 of the canopy, progressing until the tree is bare

3. Sprouts growing from the roots and trunk (epicormic shoots) and larger leaves than normal

4. Vertical fissures in the bark

5. Serpentine larval feeding galleries under the bark, packed with frass (sawdust)

6. Small D-shaped exit holes (1/8 inch diameter)

7. Increased woodpecker damage

Note that there are native ash borers and EAB look-alikes. Native ash borers include banded ash clearwing, ash/lilac borer, redheaded ash borer, banded ash borer and Eastern ash bark beetle. Native look-alikes include six-spotted tiger beetle, caterpillar hunter, Japanese beetle, bronze birch borer and two-lined chestnut borer. More information about these look-alikes can be found online at

EAB does not affect mountain ash (Sorbus spp.), only true ash species such as black, white and green (Fraxinus spp.) There are other problems that cause die-back and death on ash trees, so check for the additional signs of EAB listed above. Report sightings or suspect trees to the Adirondack Park Invasive Plant Program or your local Cornell Cooperative Extension office.



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