It’s that time of year again: the weather’s hot, the ice cream stands are open, and King County’s invasive knotweeds (Polygonum spp.) are in full bloom.

Don’t worry too much about your knotweed going to seed: King County’s most common knotweed, bohemian knotweed (Polygonum x bohemicum), is almost always a male clone that doesn’t produce seeds, and the species that do aren’t very viable, anyway. However, for many knotweed control methods—digging, covering, cutting, injecting, spraying—now’s a great time to get out there and do it!

(For more information on knotweed identification, visit our webpage on the species.)

Digging, covering, and cutting are best for small, isolated patches (50 stems or less):

Digging: it’s not too late to start now! Use a shovel to uproot your knotweed plants, getting as much of the roots as possible. Established plants’ roots can be 7-10 feet deep, and their rhizomes are often very large, woody, and hard to dig up. Check the site for new sprouts every week for the rest of the growing season (usually into October) and pull up any you find. Be sure to look in a 20-foot radius around your site. Carefully dispose of all roots and rhizomes in the garbage so that they don’t resprout. Plan to spend at least 3 years on this treatment, and 7-10 years for well-established infestations.

For more information on digging, visit our Invasive Knotweeds: Best Management Practices page.

Covering with a heavy duty geotextile fabric of black plastic is another option for small, isolated patches, especially those on open, undisturbed terrain. It’s best to start this method at the beginning of the year or after cutting the plants down several times during the growing season, to reduce knotweed’s usual rapid growth. After cutting the stems back to ground level, lay down a geotextile fabric or heavy duty black plastic over the plant bases and at least 7 feet around them. Weigh down the fabric with rocks or cement blocks, leaving the fabric loose so that new plants can’t break through it. Check the site for holes and new growth at the edges, and stomp down all growth every 2-4 weeks during the growing season. Plan to leave the covering in place for 5 years or more (7-10 years for well-established infestations).

For more information on covering, visit our Invasive Knotweeds: Best Management Practices page.

Cutting is most effective when you start earlier in the growing season (around April). Cut knotweed stems to ground level at least twice a month through August, and then once a month until the first frost. Try to prevent the plants from growing taller than 6 inches. As usual, throw away roots and pile up stems in a place where they can dry out. Throw dry stems into your backyard compost or yard waste bin. Plan to spend at least 3-5 years on this method, and 7-10 years for well-established infestations.

For more information on cutting, visit our Invasive Knotweeds: Best Management Practices page.

Injecting and spraying are your best options for a larger knotweed infestation. Both methods involve the use of herbicides to control your site.

If you’re using herbicides on your knotweed, don’t cut the plant. For herbicide applications, the later in the season, the better. Try to catch the plant when it’s reached maturity and is starting to focus on storage for the winter. That way, you can maximize the amount of herbicide the plant absorbs into its roots. If you cut the plant, it’ll just focus on growing all over again.

Injecting: now through the end of September is a prime time to inject your knotweed with a stem injection gun. After that, the canes become woody and are more difficult to properly inject. Stem injection is highly effective, with often 95% control in your first year, but it’s labor intensive: you’ll have to inject every cane in the stand between either the first and second or second and third nodes from the ground. If you’re interested in this method, you can borrow a knotweed stem injector from our office in downtown Seattle. For more information, see our How to Borrow a Knotweed Stem-Injector page.

For more information on stem injection and the herbicides involved, visit our Invasive Knotweeds: Best Management Practices page.

For spraying, you’re best off working later in the season, after the knotweed plants have finished flowering. That way you can avoid interfering with any bees or other beneficial insects that might use the plants. Alternatively, you can spray during a time of day when flower visits are at a minimum. When spraying, don’t worry too much about getting the tops of the leaves; any green part of the plant (e.g. undersides of leaves, stems) will absorb herbicide.

For more information on knotweed spraying and the herbicides involved, visit our Invasive Knotweeds: Best Management Practices page.

As always, feel free to contact us at 206-477-WEED (206-477-9333) or noxious.weeds@kingcounty.gov with any questions. Good luck!