Dear Kathy,

Thank you for contacting us at the UNH Extension Yard & Garden Infoline.  Regarding your questions on Japanese Knotweed, I would like to share the following information.

Non-Chemical Management Recommendations

Mowing/Cutting can result in the spread of Japanese knotweed under certain conditions. Mowed/cut stems/fragments with nodes/joints have the ability to develop adventitious roots and shoots if they come in contact with moist soils or water. This occurs when clods of mowed/cut knotweed stems accumulate on equipment and eventually drop off. Larger stem pieces usually have sufficient moisture reserves to retain their viability, whereas mowed/chewed up step fragments are less likely to regenerate on their own. Mowing/cutting does nothing to manage or reduce knotweed populations. In fact, these types of impacts typically break dormancy of lateral buds along the rhizomes thus expanding the outer limits of the population. Mowing/cutting should only be done if safety is an issue and the equipment is 3 cleaned before moving off site. If mowing/cutting is required, then foliar herbicide treatments or smothering should be integrated as part of the management effort (see below).

The key to Japanese knotweed's success is its ability to spread vegetatively through its root system. While some populations also reproduce via seed, colonies of knotweed are usually formed from an interconnected, underground system of horizontal roots called "rhizomes." These rhizomes are prone to splitting when disturbed and each fragment is capable of forming a fully functional clone of the parent plant. Fragments can be dispersed along waterways during flooding events or by the movement of soil containing root fragments. Additionally, if stems are cut, both the still-rooted stem and the trimmed portion are capable of regrowing into new plants if in contact with moist soil. Due to these traits, knotweed stands are extremely persistent even after multiple removal attempts.

I realize that you would rather not use a herbicide due to being near water sources so before covering herbicide recommendations, I want to share a few links to some great reliable information about knotweed and its control:

• Best management practices from the NH Department of Agriculture (long document):
• https://mm.nh.gov/files/uploads/agriculture/documents/japanese-knotweed-bmps.pdf
• Japanese knotweed control (short document): https://mm.nh.gov/files/uploads/agriculture/documents/japanese-knotweed.pdf
• Penn State resource on Japanese knotweed: https://extension.psu.edu/japanese-knotweed
  
  Herbicide Recommendations
• This plant thrives on land that is at least seasonally wet.
  If you decide to spray herbicide please be sure to follow the label exactly and wear all recommended personal protective equipment. The current recommendation is any glyphosate-based herbicide, such as Roundup, at the rate specified on the label. Other selective broadleaf herbicides such as triclopyr or dicamba have not shown to be effective in knotweed control. Herbicide is best applied in September/October after flowering and no sooner than 8 weeks after cutting back in June. Cutting in June results in shortened regrowth (2 to 5 feet) and elimination of persistent stems from the previous season. This timing recommendation is important because we want to avoid spraying when beneficial insects are pollinating the flowers. Once flowering is complete the knotweed begins heavy translocation of energy from the leaves and shoots back into the roots in preparation for winter dormancy and this helps bring more of the herbicide into the roots and rhizomes for more effective treatment.
• The primary objective in controlling Japanese knotweed is eliminating the rhizome system. Rhizomes are creeping underground stems that give rise to new shoots and roots. As long as you are willing to invest the effort and follow a few key timing guidelines, it can be successfully controlled.
• There are two phases of knotweed management: initial control and maintenance. 
• The control phase for knotweed takes at least two seasons and consists of either two applications of herbicide or a cutting with a follow up of herbicide. Late season application of herbicide in the control phase is especially effective because this is when the foliage is sending sugars produced through photosynthesis to the roots and rhizomes; systemic herbicides move through the plant with those sugars. After initial control efforts have nearly eliminated the knotweed, you will need to periodically monitor the site and treat any new growth to prevent reinfestation.
  Cutting alone is not an effective suppression approach. However, cutting prior to an herbicide application can be very helpful. Cut in June and wait at least eight weeks after cutting to treat the resprouting plants with herbicide; knotweed regrowth will be much shorter than if it had not been cut, and the rhizomes will be forced to redirect their energy reserves toward resprouting instead of expanding their underground network.
• Typically, knotweed regrows to 2 to 5 feet tall during the eight-week window after cutting, but this waiting period is critical—if you apply herbicide too soon after cutting, the herbicide will not be effectively translocated to the rhizomes. 
• Cutting is also useful when knotweed is growing near water because it is easier to treat the shorter regrowth without inadvertently spraying herbicides into the water during follow-up treatments. Treating intact knotweed towering over your head can be difficult, but cutting may be even more work. As long as you are able to effectively spray all the foliage, cutting is not critical.
• Wait at least eight weeks after cutting before applying herbicide.
• Glyphosate, a nonselective herbicide available as aquatic-labeled products for use in or near water. Glyphosate is effective, has low toxicity to nontarget organisms, has no soil activity, and is relatively inexpensive. The herbicide imazapyr (e.g., Polaris, Habitat) is also effective against knotweed, but it has considerable soil activity and can injure nearby trees through root uptake. Broadleaf herbicides such as triclopyr or 2,4-D provide significant foliar injury but have limited effect on the rhizome system. Mixing glyphosate with other herbicides makes sense if knotweed is not your only target during spray operations. Combinations with triclopyr or imazapyr provide a broader species spectrum and do not reduce activity against knotweed. 
• Glyphosate is widely available for purchase at major online retailers, home improvement centers, farm supply outlets, and specialty pest control stores. Because the patent on glyphosate expired in 2000, it is manufactured by many different brands
• I did find several services that could be hired to remove invasive species however, here at the UNH Extension we are not allowed to recommend one over another.  It should be easy though be doing a simple search for professional services for invasive species in NH.

I hope this information is helpful.  I realize that you would rather not use a herbicide however, it seems that it is the best option for combating Japanese Knotweed at this time.  Having a herbicide that is recommended for use in or near water though is at least promising.    

If you have further questions or concerns please reach out to us again.  I wish you the best in combatting this invasive species.