Noxious weed control tips for September

Tansy ragwort (Senecio jacobaea)
You might still be able to catch some late-flowering tansy ragwort before the seeds get loose. Also, areas that were mowed or impacted by cinnabar caterpillars are likely to have plants flowering this month, especially after we get some rain. Read more about this  plant in our July Weed of the Month article.

Spotted knapweed (Centaurea stoebe)
Spotted knapweed is another noxious weed that usually recovers from being mowed and flowers again in the fall on shorter plants. Some spotted knapweed plants also just flower later, so make sure to recheck spotted knapweed areas in the fall and pull up any stragglers before they go to seed. Read more about this plant in our June Weed of the Month article.

Spotted knapweed flowering again in the fall. Photo by Maria Winkler.
Spotted knapweed flowering again in the fall. Photo by Maria Winkler.

Purple loosestrife (Lythrum salicaria)
Purple loosestrife is still flowering in some places in September. Since each plant can produce millions of seeds, it’s worth the effort to control as many plants as possible. Also, purple loosestrife seeds are slow to develop, so even on plants with no blooms left, there may still be time to cut and bag the seed heads before the seeds disperse.

Phragmites or common reed (Phragmites australis)
Phragmites is in full flower and this is definitely the time to get it controlled. Large, dense sites need to be sprayed with an aquatic herbicide to control the plants, but it might be possible to dig up small, isolated plants.

Spanish broom (Spartium junceum)
Unlike its cousin Scotch broom, you may still see some blooms on Spanish broom, a less common but equally invasive European broom species on Washington’s Class A noxious weed list. Avoid the temptation to cut the plants down; they will simply regrow the following year. Instead borrow a weed puller from us and remove the whole plant.

Butterfly bush (Buddleja davidii)
Butterfly bush seeds are just getting started, so now is the time to cut off faded blooms and prune back overgrown stems (or just remove the whole plant and replace it with a better behaved shrub). Remember the seeds are tiny and wind-borne so the sooner they are removed the better.

Garlic mustard (Alliaria petiolata)
Although young garlic mustard rosettes are not easy to find, if you look around in the areas where plants were controlled in the spring and you may spot them, especially if you look before leaf litter covers them up. Garlic mustard rosettes grow throughout the fall and winter, building up roots so they can bolt and flower early next spring. Use a digging tool to make sure to get all the roots out. Read about how King County Parks acted quickly last September to stop an infestation at Tolt McDonald Park.

Knotweed (Polygonum species)
September is a great time to control knotweed. The plants are moving sugars down to their roots for next year, so chemical control is highly effective this month. Also, since the plants are done flowering, bees will be gone as well. Knotweed can be sprayed until the leaves fall off and you can also use the stem-injection method through September, but you may have to go higher on the stem to avoid splitting it with the needle. If you are using manual control methods, you can cut re-growth down one last time or, if the soil is loose and the patch is small, carefully remove any surviving roots and discard in the garbage. Read more about this plant in our post last month about controlling knotweed.

For more information about noxious weeds to watch for visit our website or contact us for more information.


  1. Does anyone know of a contractor that will remove large areas of purple loosestrife in a wetland that is difficult to access??

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