Knotweed is extremely difficult to control, no matter which method you choose. Successful eradication is likely to take multiple treatments over several successive years. It is best to treat upstream patches first and move down the waterway as knotweed often spreads downstream.
To prevent further spread, please dispose of knotweed appropriately. Always dry out stems before composting and never compost root parts; never allow stems or roots to enter waterways or roads.
View a summary (PDF) of knotweed control methods.
Manual Control Methods
You can use manual control methods if you have easy access to your site and if patches are small and isolated. These methods require a large commitment of time and effort.
- Digging - Each summer, dig up and remove as much root as possible for at least five consecutive years. Remember to call before you dig. Carefully bag and dispose of roots in the garbage or landfill. DO NOT compost the roots or allow them to enter waterways. Stems can be crushed before or after drying. Always dry stems prior to composting, and never allow them to enter waterways. Carefully secure any knotweed material before transporting it to the garbage or a landfill.
- Cutting - Cut stems close to the ground every week between April and August, and then biweekly until the first frost. Carefully dry stems or dispose of them in the garbage or landfill. Stems can be crushed before or after drying. Carefully dry stems prior to composting and never allow them to enter waterways. Carefully secure any knotweed material before transporting to the garbage or landfill.
- Covering - Cover loosely with a heavy-duty, non-woven landscape or weed control fabric for 3-5 years. Cover the entire patch extending out at least 20 feet beyond the outer stems. Secure the covering with heavy rocks or cement blocks. Periodically stamp down plants growing underneath and check the perimeter for new sprouts.
Chemical Control Methods
When using chemical control methods, be sure to read and follow all label instructions carefully. Only a licensed applicator with an aquatic endorsement and an aquatic herbicide application permit can apply herbicide in locations where there is a risk of herbicide entering the water. The Snohomish or King County Noxious Weed Program and the Washington State Department of Agriculture can provide more information about proper use of herbicides.
- Spraying - Spray the entire plant with an herbicide registered for use on knotweed. Apply once annually between August and the first frost. Two chemicals that have proven effective when used correctly are: glyphosate (e.g. Roundup, Rodeo, Aquamaster) and imazapyr (e.g. Arsenal, Polaris, Habitat). It may take several weeks to show results. Wait at least one month before cutting dead stems. Annual reapplication is usually required for 2-3 years. Backpack sprayers are available for loan. Please email Geraldine Saw or call her at the Snohomish County Noxious Weed Control Board 425-388-7548 .
- Stem Injection - Inject a measured amount of concentrated herbicide into each hollow cane using a stem-injection gun or large needle once annually between August and the first frost. Only glyphosate is labeled for this method (e.g. Aquamaster, Aquaneat, and Roundup Pro Concentrate). Stems will regrow too small to inject after the first year, and another method will need to be used. Stem injector guns are available for loan. Please contact Scott Moore at Snohomish County Surface Water Management: 425-388-6462, or email Scott Moore.
- Check your property. If you have knotweed, contact us.
- Avoid spreading knotweed. Be careful when working around knotweed, particularly if you are mowing or doing yard work. Small fragments can spread and form new colonies.
- Encourage natives. Plant suitable native species around your property so that knotweed cannot gain a foothold.
- Tell others about knotweed! Knotweed control is most effective when neighbors take action together.
- Do not let knotweed move into rivers and streams. Stems and roots carried downstream can sprout in new locations.
- Do not compost root parts. Instead, discard with the trash or take to a transfer station for disposal. Be sure stems are fully dried before composting.
- Do not spread contaminated soil. Any soil gathered from within 20 feet of a knotweed patch may contain root fragments, which can grow into new plants. If you are using fill dirt, check to see if there is knotweed nearby or ask your supplier.