Japanese Knotweed has become an invasive menace in Allamakee County over the last 15+ years.  This bamboo look-alike is found along many streams in the county and spreads rapidly.  The biggest issue with the plant is that it is difficult to kill.  If a knotweed plant is cut, the pieces have the potential to grow into new plants if they contact soil.  Knotweed also spreads through its roots.  If the roots are disturbed, new plants can also grow. 

Non-chemical control of knotweed is usually not as effective as chemical control.  Small populations can be suppressed by mowing at least 4 times a year.  Make sure to use a mower with a bag or rake and bag the material.  If you mow it but don’t remove the cut material, you will just be spreading the plants.  Multiple years would be required to have much impact on the knotweed.  Cutting prior to herbicide application can be much more effective.  Cut knotweed in June and wait at least 8 weeks to apply herbicide to the resprouting plants.

Prescribed burning as a stand-alone practice is not very effective in managing knotweed.  Spring burns can kill seedlings and set-back established plants, however they will likely resprout quickly.  Fire may be most beneficial in fields with native grasses and forbs (such as some CRP practices) to help the natives better compete against the knotweed.     

For small areas, a tarp can be spread over the area to block out sunlight and slow the spread of knotweed.  Plants must be covered for at least 2 years to this method to be effective. 

Late season application of herbicides can be most effective because the plants are sending sugars to the roots, carrying herbicide with sugar.  However, this will still require at least 2 years of application to kill the knotweed and will require continued monitoring.  Herbicide applied too early may injure the above ground plant but not have an effect on the robust root system. 

According to Penn State Extension, glyphosate is often recommended for chemical control of knotweed as it can be found in aquatic-labeled products, has low toxicity to nontarget organisms, and has no soil activity.  It should be broadcasted as a foliar application.  Due to its non-selective nature, overspray or drift can damage nearby desirable plants.  Imazapyr may also be effective at managing knotweed but has the potential to injure nearby trees through root uptake and remains in the soil for several months.  Triclopyr and 2,4-D can damage or kill the top-growth but has little impact on the root system.   The University of Wisconsin Extension (UWEX) also indicates that glyphosate can be used on cut stumps while the plant is actively growing.   Chemicals such as glyphosate and triclopyr can also be injected directly into the stem between the ground and the top of the second basal joint.  This method is fairly effective, but very labor intensive. 

If small areas of Japanese Knotweed are found on a property, start treating it as soon as you can to minimize spread, especially in riparian areas. 

Additional information can be found below: