prepared by the Delaware River Invasive Plant Partnership




KNOW HOW IT SPREADS Japanese and giant knotweed both spread primarily vegetatively by the growth and fragmentation of rhizomes. Once established, knotweed spreads aggressively through an extension of the rhizomes. Lateral expansion rates of 6 to 8 feet per year are not uncommon. Very small fragments of rhizome (as small as 1-2 inches) and fresh stem material are able to produce viable shoots and roots. Stems in water or on damp soil may produce viable plants within 6 days. Knotweed also grows from seeds, which are produced in large numbers (when male and female plants are present) and are dispersed by wind and water.

DON'T DIG! Do not try to dig up the rhizomes. It is nearly impossible to be sure you've gotten all fragments (see # 1 above). Rhizomes can also be very large ~ as much as 60 feet in length! The digging required to completely remove all fragments of rhizome would cause such disturbance and potential erosion that it would only encourage sprouting of remaining fragments or growth of new invaders.


REPLANT WITH NATIVE OR NON-INVASIVE PLANTS It is important to rapidly establish native or non-invasive plants to control knotweed and stabilize the soil. Knotweed seedlings do not compete well with other vegetation. Consult your neighbors, local nurseries, garden clubs, or the internet for suggested plantings. Always check first for the potential invasiveness of a plant to be sure you don't give yourself more problems!

CUT REPEATEDLY Repeated cutting of the stems reduces vigor and can eventually reduce the root reserves in some cases, particularly with small, isolated populations. Cutting is effective at any time during the growing season, but only when done repeatedly as it will result in new shoot emergence. Cutting greatly reduces the reserves in below-ground rhizomes. At least three cuts are needed in one growing season to offset rhizome production. Repeated cuttings must be continued until knotweed stops resprouting and must be checked regularly for any regrowth. Cutting is most effective when used in conjunction with either shading or herbicide (see below).

It is very important to bag and properly dispose of all cut stem fragments (see # 1 above)!! If cut stems cannot be burnt, recommendations include carefully sealing them in plastic bags or spreading and drying them in the sun, away from water or exposed soil. (Remember even moist stalks in the center of a pile can resprout.)

  COVER & SHADE IT Some success has been noted with cutting combined with shading. After cutting near the soil surface, stands can be covered with sheets of plastic. There is some early evidence that clear plastic has greater results due to the sun baking the rhizomes. Be very careful to secure plastic, particularly near streams and rivers, so that it cannot be washed or blown away. Please consult handout from Delaware River Foundation for more information on their use of plastic sheeting.

   DECIDE ON HERBICIDE? Controversial? Yes! Effective? Yes! We do not currently recommend the use of herbicide along the banks of the Upper Delaware River due to potential risks to native fish or rare and endangered freshwater mussel populations. If you decide to use herbicide you must read the labels carefully and take great caution when working near water (for example, painting or injecting cut stalks instead of spraying would be more accurate). Currently the most effective means for controlling large stands of knotweed is a combination of cutting and spraying with an herbicide containing glyphosate. There are several commercially available products including Roundup TM (for dry, upland sites), Rodeo TM (for in or near wet areas), and other competing products. Glyphosate is a non-selective, soil binding herbicide that will kill all vegetation, so it is important not to let spray contact any native or desirable vegetation. Follow all instructions on product label or consult a licensed herbicide applicator. Your local Cooperative Extension Office can provide more information on the use of herbicides and we strongly suggest you learn as much as possible. Cut stalks in early June to draw reserves out of rhizome. In August or when stalks reach approximately 4 ft. tall, spray to coat leaves but not so heavily that herbicide drips off the leaves. The timing of cutting and herbicide application is very important. You will need to regularly check for any regrowth or reinfestation (from new fragments or seeds) and spot treat.

  AVOID EXPERIMENTING WITH HOUSEHOLD SOLUTIONS Always take caution when using any harmful substance -- not only herbicides can damage soil and kill native vegetation and wildlife. Seemingly harmless applications of vinegar, bleach, or other household products can be even more damaging to the environment than herbicide. Please research and learn as much as possible before trying any new treatments.

    SUPPORT RESEARCH FOR NATURAL CONTROLS There are currently studies being conducted on biological controls (fungal agents or herbivorous insects that control the plant in its native habitat). Biological controls are generally studied for several years before release to predict what, if any, other plant species might be at risk from the control agent. Even after approval and release, biocontrols generally take 5 or more years to show a noticeable effect and will not eradicate the weed, simply reduce it to more manageable levels. Although biocontrol has been practiced throughout the world for many years, it is a relatively new concept to many people. Read more about this fascinating topic at
, and follow their links.

     KEEP LEARNING & TEACHING It's important to continue to search for and share information regarding the control of knotweed. New studies are being published all the time, and much like human health, the more we learn, the more treatment options we have. It is often combinations of recommended control methods that work best. Your property has its own, unique qualities that either enhance or detract from the growth of knotweed. Experiment and learn what works best for you, then share with your neighbors and educate the uninformed about this troublesome plant and how it is harming the environment. There are many fact sheets and websites where individuals can learn more.

     AVOID PLANTING IT Knotweed is still sold and exchanged for ornamental use. If you live in an area where knotweed is a problem, avoid introducing any varieties that may cross with the invasive variety to create new troublesome hybrids. Know knotweed's many names: Japanese Knotweed ( Polygonum cuspidatum, Fallopia japonica or Reynoutria japonica ), Giant knotweed ( Polygonum sachalinensis ), Japanese or Mexican bamboo, fleece flower, fallopia, lace plant, and Sally/donkey/gypsy/wild rhubarb.


This tip sheet was prepared by DRIPP (the Delaware River Invasive Plant Partnership) for a workshop in the Upper Delaware River region of NY and PA. It is intended to be accompanied by other detailed handouts. Please contact Joanne Steinhart at 570-643-7922, ext 12 or for more info.



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