Knotweed Treatment

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Control efforts pertaining to Japanese Knotweed can vary depending on its location and the season. Listed below are suggested control methods.

Please note that when applying herbicide, abide by the herbicide label and consult the Oregon Department of Agriculture or your state’s Agriculture Department for licensing and regulations.

Herbicide (Foliar application)

Probably the most cost effective method of control is a foliar application method, especially when dealing with very dense large stands of knotweed. Although cost effective, foliar applications take repetitive maintenance and several years of treatment (maybe more depending on size and location of knotweed). Foliar applications should come accompanied with the use of a surfactant. Using a foliar application for knotweed is best done in the fall, when the plant translocates its sugars down to its root. A suggested surfactant is R-11 Tm at 1%. If NOAA or any other federal agencies fund one, there may be restrictions of what kind of surfactant to use. A NOAA approved surfactant is Li-700 TM also used at 1%. Below is a comparison between Riparian and Terrestrial knotweed control.

Riparian Areas – Take precaution when using herbicides along creeks and streams and use only glyphosate or imazapyr based herbicides that are approved for aquatic use. An imazapyr-based product is safe to use around water according to the EPA is Habitat TM. Two glyphosate-based herbicides that are safe to use around water are Rodeo TM and Aquamaster TM are products approved for use in riparian areas. The suggested rate of 3%-5% of glyphosate product or 1% imazapyr product with 1% of surfactant has shown good results over time. Please keep in mind that foliar control of knotweed can take several years of treatment.

Click here for more information on imazapyr or Glyphosate.

Terrestrial Areas – The big difference when dealing with knotweed away from creek and stream riparian areas is the type of herbicide applied. The most effective herbicide to use on knotweed in road side ditches or other areas NOT near creeks and streams is Garlon 3A TM. The suggested rate of 3%-5% with a 1% surfactant has shown better results than Glyphosate based herbicides. Please abide by the herbicide label and state licensing and regulations.

Herbicide (Stem-Injection)

A fairly new treatment approach for Japanese Knotweed, the stem injection method was pioneered by Clark County Weed Management in Washington State and has proven to be very effective. PLEASE NOTE: this method of control is approved for Oregon and Washington only, please consult your local Agriculture Department for more information. Also note that Rodeo TM and Aquamaster TM are the only herbicide products that carry a label for stem-injection. The suggested dose per knotweed stem is 5ml of undiluted herbicide. The Nature Conservancy and Metro conducted a dose response studying examining different dose levels to determine the lowest lethal dose. The experiment set up 60 knotweed patches, 30 patches treated in summer and 30 patches treated in the fall. Dosage levels were 1.5ml, 3ml, 5ml, and a combination of 5ml and foliar spray of smaller stems. After collecting only one year’s worth of data, there is no clear distinction between any of the doses. To learn more about the injection study please refer to The Nature Conservancy’ web site at http://tncweeds.ucdavis.edu/.

While stem-injection proves to be the most effective in killing Japanese Knotweed, it is costly in both labor and herbicide. In heavily infested areas, please abide by the herbicide label as 8 quarts per acre can easily be exceeded using 5ml. per stem in dense, large stands of knotweed.

Injecting a Knotweed stem with a
glyphosate-based herbicide.

Cutting and Digging

If you are uncomfortable with using chemical treatments on your property, a manual approach may be more appropriate. Cutting and digging knotweed can be an effective method of control, IF THE KNOTWEED IS DISPOSED OF PROPERLY. Root or stem fragments must be dried out and killed before being throwaway, or else they can start new plants. Cutting knotweed stimulates the plant’s growth and cuttings can form new plants if kept in moist soil or thrown in river or creek drainages. The primary propagation of knotweed is through the extensive root system; pieces of the rhizome or stem get broken off and sent downstream to form new plants. Therefore, while digging the root system can effectively remove its main sink and kill the plant, it can also promote its spread if not disposed of properly.

Cut stems taking root downstream.

Why is it so difficult to control knotweed?

In addition to its rapid growth and ability to survive flood, knotweed has an extensive underground root network that is difficult to kill. Often with knotweed control the aboveground mass will be killed but the plant continues to come back year after year because the belowground portion is still alive. Successful treatmentl programs target both aboveground stems and belowground root/rhizome systems in order to kill the entire plant.