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Pest SolutionsPESTS + WEEDSUSING PESTICIDES OR HERBICIDES?
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Knotweed
Fallopia sachalinensis , F. x bohemica , F. japonica & Persicaria wallichii
Updated Apr 26, 2024
Make a Positive Identification
- There are several related, invasive knotweed species: Giant, hybrid, Itadori (or Japanese) and Himalayan.
- They have escaped from landscapes and are spreading into new areas. Management is the same for all.
- Knotweed plants live many years (perennial). They are shrub-like, broad-leaf plants that grow to 4–15 feet tall each year.
- Knotweed has large oval-to-heart-shaped leaves. Their stems are long, hollow, and segmented like bamboo.
- To identify knotweed in the winter, look for clusters of tall, reddish stems with bamboo-like segments.
- They spread quickly with roots, stem pieces and seeds. Knotweed dominates an area when left unmanaged.
Species: Knotweed (invasive)
David J. Moorhead, University of Georgia, Bugwood.org
The leaves grow opposite each other along the red stems. They can grow as long as 15 inches and 11 inches in width.
Species: Knotweed (invasive)
Barbara Tokarska-Guzik, University of Silesia, Bugwood.org
Knotweed leaves vary in size as shown in the photo.
Species: Knotweed (invasive)
Jan Samanek, Phytosanitary Administration, Bugwood.org
Stems are long, hollow, and segmented like bamboo. They grow from 4–15 feet tall and are often arched.
Species: Knotweed (invasive)
John Cardina, The Ohio State University, Bugwood.org
Knotweed spreads far beyond shoot growth with underground root structures (rhizomes). Stems and root pieces broken or cut away from rooted plants can make new plants.
Species: Knotweed (invasive)
Barbara Tokarska-Guzik, University of Silesia, Bugwood.org
In the fall and winter, knotweed stems die and remain standing. New growth emerges from the soil in early spring.
Compare Knotweed (invasive) Species’ Leaves
Species: Giant, Hybrid, Japanese, and Himalayan knotweed
National Biodiversity Data Centre
The photo shows a comparison of Giant, Hybrid, Japanese, and Himalayan knotweed leaves next to centimeter scale. Divide the number of centimeters in the chart by 2.54 to convert to inches.
LOOK-ALIKES: OTHER KNOTWEED SPECIES AND POKEWEED
Species: Weedy “Knotweeds”
Lynn Sosnoskie, University of Georgia, Bugwood.org
There are many other plants with the common name “knotweed,” including the Polygonum shown in the photo. These weedy plants are common and sometimes abundant.
Different risks or methods
Weedy knotweed species are not as harmful as the knotweeds (invasive) detailed on this page. Control them in garden, landscape, and orchard areas as needed.
Species: Pokeweed
Weston Miller, Oregon State University
Pokeweed (Phytolacca americana) is also an invasive plant. Like knotweeds (invasive), pokeweed has hollow, red stems. The leaves are large with a similar shape.
Knotweed flowers are smaller and more abundant. The flowers grow upward along the stem. Pokeweed has drooping flowers, by comparison. Pokeweed forms berries. Knotweeds don’t form berries.
Take action
Pokeweed is an invasive plant species that spreads via birds that eat its berries. Take action to control pokeweed on property you manage.
Knotweed Benefits
- Knotweed flowers attract pollinators, especially bees. Beekeepers make knotweed honey products.
- Japanese knotweed and related species are edible. The young stems in the early spring are harvested and cooked like asparagus.
Only eat wild edible plants you correctly identify. Thoroughly wash before eating. Only eat wild knotweed from your own property. Don’t eat knotweed from plants that have been sprayed with chemicals or growing on roadsides. Consider whether the soil where the plant is growing has been contaminated.
Knotweed Risks
- Knotweed spreads quickly and is difficult to control.
- It suppresses and displaces native plants. Knotweed dominates an area when left unmanaged.
- It increases erosion on streambanks, riverbanks, and embankments.
- It can damage pavement and septic systems.
Risk Card
Does it cause harm?
Adults & Children
None
Property
High
Dominates an area when left unmanaged
Pets
None
Annoyance
High
Difficult to control
Environment
High
Rapidly spreading to new areas and taking over
Action Highly Recommended
Take action to report and control this invasive weed
Take Action
If you have knotweed on property you manage, report it. Take action to control it right away.
Do I need to take action?
Yes. Remove individual plants and small patches of knotweed before they become a bigger problem. Both small and large patches require several years of attention to control.
What if I do nothing?
Knotweed spreads quickly and is difficult to control. It takes over large areas quickly.
NEED HELP?
Consider a licensed pest control company. Learn How to Hire a Pest Control Company.
Your local Extension Specialist in Oregon and other states can suggest other methods.
Solutions for Knotweeds
Early Detection & Rapid Response
- The best knotweed management strategy is to find small patches and control them before they become a bigger problem.
- It is critical to control the underground rhizomes, not just the top growth.
Physical Removal of Plants & Non-Chemical Methods
- Digging out individual plants and small clump plants may provide control. Follow-up control may be needed.
- Targeted grazing is a good way to knock down knotweed foliage. Follow up with other methods to get rid of it.
Herbicides (Weed Killers)
Herbicides effectively control knotweed when used according to the label directions.
Monitoring & Follow-Up
- Monitor the area for regrowth. Look for knotweed shoots in the early spring. Shoots can appear 10 feet or more from the clump. Take action as needed.
- If this plant is allowed to establish, stands are very difficult to control. It requires annual management to keep it from spreading.
- Knotweed spreads via roots and sometimes stem pieces. Don't move contaminated yard debris, compost, and soil. Dispose of it in the trash.
- Root pieces can break off from stands along creeks and rivers. These can start new clumps downstream.
Jump To
| Method | Does it work? | Is it safe? | Recommendation |
|---|---|---|---|
A Physically Remove |
Effective |
Low risk |
Recommended |
B Targeted Grazing |
Somewhat effective |
Low risk |
Recommended |
C Control with Herbicides |
Effective |
Moderate risk |
Use if Necessary |
|
D If Using Herbicides, Protect Yourself & Minimize Risks
|
|||
|
E Prevent Knotweed
|
|||
A
Physically Remove
Non-Chemical Method
Weston Miller, Oregon State University
Physically Remove
Recommended
With small patches of 50 stems or less, dig out individual plants and sift the soil. The best time is August. Repeat as needed for at least three years.
Digging will not work with larger patches. Roots can extend 7-10 feet down and up to 20 feet away from the patch.
| Does it work? | Effective
|
| How much effort? | High effort Remove all of the stems and root pieces. Dispose of the plants in the trash. Do not compost this invasive plant. |
| What's the risk? | Low risk |
| Possible risk of exposure or harm from chemicals | NONE |
Hand digging may be practical for single knotweed plants or small patches. Follow-up control will be needed.
Ohio State Weed Lab , The Ohio State University, Bugwood.org
Weston Miller, Oregon State University
Knotweed Removal Tips
- Following root crown removal, you must return to the site to kill knotweed plants that sprout after digging.
- Mowing is not an effective control method. It creates stem fragments that may root and create new plants.
- Use of heavy equipment to remove knotweed should be done by professionals trained to remove this invasive plant. Plant fragments may be transported to new locations if equipment is not properly cleaned.
Soil Disturbance & Erosion
- Minimize soil disturbance as much as possible when removing knotweed.
- Regrade the soil after digging knotweed roots. Apply mulch (when appropriate).
- Take steps to prevent erosion as needed.
- Replant the area to shade knotweed seedlings.
B
Targeted Grazing
Non-Chemical Method
Scott Bauer, USDA Agricultural Research Service, Bugwood.org
Targeted Grazing
Recommended
Goats are used by land managers to control invasive knotweed.
| Does it work? | Somewhat effective
|
| How much effort? | Moderate effort
|
| What's the risk? | Low risk |
| Possible risk of exposure or harm from chemicals | NONE |
Grazing is an effective way to reduce knotweed canopy. It won't kill the plants. The root crowns and rhizomes will need to be located and dug out. Regrowth may be treated with herbicides.
Grazing Knotweed Tips
- After livestock eat the plants, knotweed will grow new canes.
- Goats will eat desirable vegetation as well as the targeted weed.
- If you don’t own livestock, you’ll need to find and contract a service that performs vegetation management with animals.
- Contact professional pest control companies with experience grazing goats to control knotweed.
- Ask your Soil and Water Conservation District (Oregon) for referrals.
C
Control with Herbicides
Chemical Method: Use with caution
Chameleonseye, iStock
Control with Herbicides
Use if Necessary
| Does it work? | Effective
|
| How much effort? | Moderate effort
|
| What's the risk? | Moderate risk
|
| Possible risk of exposure or harm from chemicals | Using herbicides includes some amount of risk. The lowest risk comes with using alternative methods. You may be exposed to an herbicide if you:
Follow directions closely to reduce risk. |
Some herbicides are available for home use. A few are listed below. These active ingredients, used individually or in a mixture, are suggested chemical treatments to control invasive knotweed. Look for these chemical names in the “Active Ingredients” section of product labels.
- Dicamba
- Glyphosate
- Triclopyr
- Imazapyr
Suggested pesticides are from the Pesticide Information Center Online (PICOL) Database. They are permitted for HOME use in Oregon. Read the label for application directions and effectiveness information.
ONLY professional pesticide applicators can use Restricted-Use Pesticides (RUP) that may be more effective. If you need further support, contact a licensed pesticide applicator.
Herbicide Application Tips
- Proper timing of herbicide sprays is important for them to be effective against invasive knotweeds.
- The most effective time to control knotweed ranges from mid-summer into early fall. This will vary with the product and the weather.
- Check for application timing on the label.
- Good coverage of foliage is critical for success.
- These ingredients will damage most plants and grasses. Don”t let the spray contact plants you want to keep.
- Knotweed may take several months to die after an herbicide application.
- Monitor the area for regrowth and re-treat every year, if needed.
- Using herbicides comes with real risks. ALWAYS read the entire label front to back. Review instructions even for brands you know.
Minimize the potential impact of herbicides to bees and other pollinators. Treat knotweed plants before or after they flower. If plants are flowering when you need to treat them, apply in the morning or evening when bees are less active. Avoid spraying pollinators directly. For more information, see OSU’s How to Reduce Bee Poisoning from Pesticides
Herbicide Application Requirements for Aquatic Areas
- Herbicides applied over or near a water body must be registered for aquatic use.
- Treating knotweed near aquatic areas requires specialized skills. This ensures the herbicide is applied effectively. It also protects waterways.
- Aquatic-use products are rarely sold at plant nurseries or garden centers. They are available through specialty pesticide dealers.
- Consult a licensed pesticide applicator or your local university extension agent before purchasing or using an aquatic herbicide product.
Consider hiring a licensed pesticide applicator to manage this invasive knotweed in aquatic areas.
If Using Herbicides, Protect Yourself & Minimize Risks
Chemical Method: Use with Caution
BrianLasenby, iStock
Why is it important to read herbicide labels?
- They have detailed information on how to use the product correctly and legally.
- They contain information on potential hazards of the product.
- They provide instructions you should follow for poisonings and spills.
- Following label instructions helps you to minimize the risks and maximize the benefits.
Key Herbicide Safety Tips
- Read the entire label front to back.
- Follow the instructions.
- Review the instructions even for brands you know.
- Only apply the product where the label says it may be applied.
- Be precise in your application. More is not better.
The Label is the Law
ALWAYS read the label before using herbicide products. The label is a legal document that provides information on how to safely use the herbicide. This helps avoid harm to human health and the environment. Using an herbicide in off-label ways is illegal. It can result in legal enforcement actions.
READ THE LABEL & Follow Instructions
It has instructions to protect you and the environment.
- Labels are different for every product and they often change over time.
- Use a magnifying glass if needed.
- Pay attention to CAUTION, WARNING, and DANGER statements.
- Pay attention to the PRECAUTIONARY STATEMENTS.
- The law states you must read and follow herbicide instructions.
Protect Yourself
Eye, skin & lung irritants
- Wear the right protective gear. This often includes chemical-resistant gloves, safety glasses, a long-sleeve shirt, pants, socks, and shoes.
- Mix outdoors or in a well-ventilated area.
- Wash hands after mixing or applying, and before eating or smoking.
- Take a shower immediately after handling herbicides.
- Wash clothes worn while mixing or applying separately from other laundry.
Protect Children & Pets
Children and pets are at risk if they eat or touch the plants before it dries.
- Keep them away during and after applying herbicides (read label for how long).
- Remove toys and pet dishes from yard before applying.
- Don’t track herbicide products into your home on shoes or clothes.
Protect Plants You Want to Keep
- Glyphosate and similar herbicide ingredients damage both grass and broadleaf plants.
- Minimize spraying of foliage, stems, exposed roots, or the trunks of desirable shrubs or trees to avoid harm.
- Follow the label to avoid damaging the roots of trees and shrubs.
Don’t Spray into Water
- It’s illegal to apply herbicides in a stream or slow moving/wetland pool.
- You need a product registered for aquatic areas. This includes waterways, ditches, drains, and other places where water collects.
Avoid Wet, Windy, or Hot Weather
Use during favorable weather for best results.
- Don’t spray when it’s raining or when rain is expected in the next 24 hours.
- Wind causes spray to drift that can get on you and desired plants.
- Herbicides may be less effective in hot weather if the target plants are moisture-stressed.
- Some herbicides can turn into a vapor in hot weather and damage nearby plants.
Protect Pollinators
- Apply in the early morning or evening when bees are less active.
- Kill weeds before they flower. Avoid spraying flowering plants.
- Do not spray on bees or insects.
Storage & Disposal
- Store in a secure area away from children.
- Don’t put unused herbicide products in the trash.
- Never pour down any drain or waterway.
- Take unused herbicides to a hazardous waste facility.
Call 1-800-CLEANUP (1-800-253-2687) to find out where to dispose of herbicides.
For the Portland metro region in Oregon, contact Metro’s Recycling Information. Call 503-234-3000, email or visit Metro’s website
More about:
About Using Pesticides on School Grounds in Oregon
If using pesticides on school grounds, there are special rules in Oregon. See School Integrated Pest Management (Oregon Department of Agriculture).
NEED HELP?
The National Pesticide Information Center (NPIC) can answer questions about pest control chemicals.
1-800-858-7378 or npic@ace.orst.edu
Consider using a licensed pest or weed control company. Learn How to Hire a Pest Control Company.
Your local Extension Specialist in Oregon and other states can suggest other methods.
Prevent Knotweed
Leslie J. Mehrhoff, University of Connecticut, Bugwood.org
Ongoing Monitoring
- Look for shoots up to thirty feet from the center of the patch.
- Knotweed stems grow easily through mulch or debris.
- Plan control actions in winter or early spring and flower removal in summer. Continue to monitor the area each year.
Weston Miller, Oregon State University
Remove Dirt from Shoes and Equipment
- After working or traveling in an area with knotweed, clean your boots and tools.
- Use a wire brush to remove all soil that may contain seeds.
- If you drive into a knotweed stand, clean your vehicle.
Weston Miller, Oregon State University
Replant the Affected Area
- After knotweed is removed, the desirable plants that were buried underneath knotweed stems often regrow.
- Replanting is needed when knotweed removal significantly damages a site and few or no desirable plants remain.
- Replanting stabilizes the soil surface and shades knotweed seedlings.
- Plant with a variety of native species.
- Check plantings yearly for new knotweed plants.
Carmen Hauser, iStock
Replant Larger Areas with Technical Support
- Knotweed is difficult to eliminate from an infested area. Replanting a previously infested area requires planning and effort.
- Create a multi-year revegetation plan. Plans include site preparation and planting details, plant care, and follow-up control for knotweed and other weeds.
- Plan for at least 2-3 years of monitoring and maintenance.
- Your local Extension specialist, soil and water conservation district, or a professional revegetation specialist can suggest strategies for your area.
Invasive Species Alert
- Invasives are non-native species that spread aggressively and alter the environment.
- Controlling knotweed is difficult and costly.
- Please do your part to control it on property you manage. It can spread beyond your property and have an adverse impact on your neighbors.
If you think you’ve found knotweed in the grey areas of this map, please report it to the Oregon Invasive Species Hotline at: 1-866-INVADER (1-888-468-2337) or use their online reporting form
Invasive species data @ 2022, iMapInvasives (NatureServe)
View Larger Map >
The map shows the distribution of Itadori knotweed (Fallopia japonica) (also known as Japanese knotweed) in Oregon. If you find knotweed in a new area (orange shows already reported cases), please report it
Content provided by editor Weston Miller and writers Jessica Green and J. Jeremiah Mann. Pesticide safety information edited by Kaci Buhl.
Peer reviewed by OSU Department of Horticulture.
Knotweed References
Invasive Knotweed Identification and Control
King County, Washington
Invasive Knotweeds
King County Noxious Weed Control Program
Knotweeds
4-County Cooperative Weed Management Agency
A Guide to Riparian Tree and Shrub Planting
OSU Extension Service