Japanese knotweed can grow up to 8 inches a day and choke out native species. (COURTESY PHOTO)
Japanese knotweed can grow up to 8 inches a day and choke out native species. (COURTESY PHOTO)
By Paula Brown, special to The Conway Daily Sun
CONWAY — It is finally spring, and we are all looking forward to seeing our gardens and yards come to life again with the new growth of our plants.
But there is an unwanted pest invading our yards.
Japanese knotweed is an aggressive and highly invasive herbaceous to somewhat woody perennial originating from eastern Asia. It is similar to two closely related invasive species found in New Hampshire: giant knotweed and Bohemia knotweed.
The Japanese variety was first brought to the United States in the late 1800s for ornamental and horticultural purposes. It became popular in the nursery trade and has been planted in landscapes throughout North America. It was also planted for erosion control and as a forage crop. It has lovely white flowers that bloom in late summer. People may have them growing in their yard, thinking that they are an attractive plant.
But this plant can grow up to 8 inches a day and has a root system that can go 10 feet deep and 40 feet wide. It forms dense colonies and spreads by lateral root systems that can regenerate from pieces only a half-inch long.
That’s why moving soil with living or viable root fragments is a violation of the New Hampshire Invasive Species Law.
Japanese knotweed outgrows and competes with native species of plants, causing a reduction in plant diversity. It is found in almost every single state, at this time.
These plants can be moved through construction, earth moving, mowing and cutting. They do not reproduce greatly by seed, and are reproduced much more by movement by humans.
The root system can damage structures such as bridges, foundations and culverts, along with underground structures such as septic systems. The colonies can choke rivers and lead to increased risk of flooding.
To identify Japanese knotweed on your property, or to notify the town, look for:
• Fleshy red-tinged shoots when breaking through the ground.
• Large, heart or spade-shaped green leaves.
• Leaves arranged in an alternate pattern along the stem.
• A hollow stem, like bamboo.
• Dense clumps that can be several meters deep.
• Clusters of cream flowers towards mid-August that attract bees.
• Plants that die back between September and November, leaving brown stems.
Doug Cygan, the invasive species coordinator in the N.H. Division of Plant Industry, works independently and with cooperating municipalities, other state agencies and groups to combat the spread of this and other invasive pests.
Cygan is encouraged to see more and more towns taking invasive plants seriously.
Towns that have undertaken control programs include Jackson.
Ken Kimball, a member of the Jackson Conservation Commission, said the town of Jackson hired a professional forester to update their town's Prospect Farm Forest Management Plan, which recommended closing its stump dump there due to imported soils introducing knotweed, and eradicating this infestation.
This location is of serious concern because it could spread both into the abutting White Mountain National Forest Land and downstream along the designated National Wild and Scenic Wildcat River.
A General Management Plan for Prospect Farm was voted in at the March town meeting that incorporates this recommendation.
The highway department has used a professional herbicide applicator to treat knotweed in roadside drainage ditches and at Prospect Farm.
At infested sites, future spot herbicide treatments may still be needed to eliminate this invasive species.
Jackson also has an information page on Japanese knotweed as a link from the Conservation Commission page for the general public to make use of: jackson-nh.org/conservation-commission/pages/japanese-knotweed.
Conway also has a Japanese knotweed eradication program in effect. Andrew Smith, director of public works, said that the town has been working to eradicate the invasive plant for 5-6 years.
Starting with several sites, the town is now controlling 17 locations. These areas are on town land and are mainly around bridge abutments and along the roadside.
The town did hire a professional applicator to spray the herbicide, but the Parks Department was trained through a UNH program and is now certified to spray.
The town also must file a permit with the state for each site that it sprays. The department used to mow the knotweed when maintaining the roadsides, but does not do that anymore as it actually helps to spread the plant. Now it mows around it and comes back to work on suppression.
Usually, the department will spray an area, then keep an eye on it for the next year and reapply spray if there are signs of continued growth in the area, which is not unusual.
So what should homeowners do if they have a Japanese knotweed problem on their property?
There are two ways of managing this plant; herbicide use or smothering.
Herbicide treatments are often not successful the first time and need to be reapplied, often for up to 3-5 years. When construction is being done, the treatment should be applied prior to starting any construction that may inadvertently spread the rhizomes of the plant.
It is recommended that the plant be cut back in June, then allowed to grow again and herbicide applied in the fall after the plant has finished flowering. The recommended herbicide is Glyphosate.
If smothering is going to be used to control the plants, the plant should be allowed to grow in the spring, then cut to the ground the first week of June. The plants should be allowed to dry in the sun, and the ground should be covered with a heavy tarp for five years, after which the area can be replanted. This is a time-consuming method but has been used successfully in sensitive areas in New Hampshire.
Hand pulling or digging should only be used if the plant colony is quite small with less than a handful of stalks. The stalks should be cut to the ground and the roots dug up, on a continual basis until totally eradicated. The plants should be allowed to dry out and should never be composted.
Cygan gives well-received presentations to 25-35 groups each year for municipalities, conservation commissions, UNH Cooperative Extension pesticide training programs, garden clubs and more.
Interested groups can contact him at doug.cygan@agr.nh.gov or call (603) 271-3488.
For more detailed information on eradication, the New Hampshire Department of Agriculture, Markets and Food, has several brochures prepared by Cygan, on how to eradicate Japanese knotweed. The first is more in depth for areas of construction and includes multiple photos of the plant for identification purposes. Go to tinyurl.com/5f228uyt.
And the second one is geared toward the homeowner eradicating plants around the property. Go to tinyurl.com/2s4a3vd5.
Paula Brown lives in Center Conway.
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