JAPANESE KNOTWEED: Reynoutria japonica
If you think you may have Japanese knotweed, it can be very discouraging. Luckily, scientific research has been done on the most effective methods of control. Don’t despair!
“Why do you say “control?” I want to GET RID OF this plant!”
“Control” is a term used for invasive species that can’t be completely eliminated. This helps us know what to expect, which is that over time, even though you will see less and less knotweed, you should expect to see some sprouts, indefinitely. While that can be discouraging, it is important to know that this is normal and not a sign of poor management.
THE SCIENCE
The study most Agricultural extension offices and Departments of Natural Resources are referencing is this one:
Jones, D., Bruce, G., Fowler, M.S. et al. Optimising physiochemical control of invasive Japanese knotweed. Biol Invasions 20, 2091–2105 (2018). https://doi.org/10.1007/s10530-018-1684-5
https://link.springer.com/article/10.1007/s10530-018-1684-5
The most successful and economical treatment found in the study was a 2% glyphosate solution, applied to the Japanese Knotweed leaves in the late summer/fall, around the time of flowering, but before the first frost.
TIME OF APPLICATION WAS FOUND TO BE IMPORTANT
The application time that they found most effective was after flowering and before frost, or generally in September-October. After the plant flowers, it starts taking nutrients and sugars down from the leaves, into the roots so it can store the energy for winter. If sprayed during this time, especially the leaves, the herbicide will flow down into the roots of the plant where it kills cells in the roots. When the scientists applied the herbicide at this time, they found much less growth the following year compared to other methods. They also found that spraying twice a year wasn’t more effective than spraying once in the fall.
Glyphosate takes time to circulate throughout the plant, so applying several weeks before the first frost would give the herbicide more time to move into to the roots.
When the first frost comes, the leaves freeze and die. If herbicide is applied after this time, the leaves will not circulate the herbicide to the roots.
OTHER METHODS THEY TRIED IN THE JONES STUDY
STEM INJECTION
They also tested “stem injection” where they injected glyphosate into the knotweed canes, and they also tried cutting the canes and injecting the herbicide. This worked but was not as effective as spraying, and caused them to use twice the amount of herbicide, and cutting and injecting was a lot more time and effort.
COVERING
Covering the plant was almost completely ineffective in the long term, although there was a reduction in growth in the short term.
DIGGING IT OUT WITH LARGE EQUIPMENT
Excavation to 2.5 meters (8 feet) deep, plus spraying the ground with herbicide was also effective, but not more effective than spraying in the fall. Excavation costs are very expensive and seemed to have no additional benefit. Plus the plant needs to be disposed of carefully, to not start a new infestation somewhere else!
Gentle Disclaimer
Every form of invasive species control has downsides. The only reliable information we have is what is recommended by scientists, departments of natural resources, and state extension offices. Citations are at the bottom of the page, and I encourage everyone to learn as much as they can about knotweed and herbicide before making this very personal decision. Lisa Likes Plants LLC is not endorsing any specific herbicide, or specific treatment. Everyone’s needs and property are different.
A PROCESS
Here is a process many have gone through to treat knotweed
STEP 1 MAKE SURE IT’S KNOTWEED
STEP 2 DON’T CUT OR DIG
STEP 3 HAVE THE KNOTWEED TREATED DURING THE RECOMMENDED TIME
STEP 5 CONTINUE TO MONITOR AND TREAT IN FUTURE YEARS
STEP 1
Make sure you ACTUALLY have Japanese knotweed, Reytounuria japonica. Look at close up photos of JKW online. Take SEVERAL, CLOSE-UP photos of your plant’s leaves and stalks, as you may need to consult a botanist or other professional to identify the knotweed. There are botany groups and invasive plant groups on Facebook, Reddit, and other social media that can help you identify it for free, but always remember the limits of social media information. Morton Arboretum has a Plant Clinic where you can send HIGH QUALITY photos through email for identification. Please do not send a far away picture that is blurry, the botanist needs to see the leaves and stems. You can also use a plant identification app, but these vary in their reliability. You can also bring in pieces of the plant to your extension office, but call first. Always get another opinion and do not rely on apps for identification in this case. You can also bring in pieces of the plant to your extension office, but call first.
If you are in the UK, identification of knotweed may have legal consequences if the knotweed on a new property was not disclosed. You may want to pursue a formal identification and get something in writing. Take as many photos as you can.
STEP 2
Do not cut, burn, mow, or attempt to dig the plant out.
DIGGING
The roots of the japanese knotweed are where most of its mass is. The canes on top are like the tip of an iceberg. The beast lies beneath, underground. The roots often go down 2 meters or 8 feet, and digging is very challenging due to the dense, woody root system. It is like trying to dig out an oak tree with a shovel. Even with excavating equipment, the study had not shown excavation to be much more effective than spraying.
MOWING AND CUTTING
Cutting knotweed canes is like clipping its toenails. It only “refreshes” the plant. If it is cut repeatedly, like with regular mowing with a lawnmower, there is a concern that the plant will try to spread out and move away from the “threat,” pushing new shoots out farther and farther looking for a safe place to grow.
To make matters worse, mowed pieces get bagged and sent to municipal compost, where they can start a new infestation on another property. OR SEVERAL OTHER PROPERTIES! There is a huge problem with Japanese Knotweed contaminated soil. Some homeowners have had soil brought in for new landscaping, and suddenly have 25 new sprouts of knotweed in their garden from the new soil! Mowing contributes to this systemic problem.
STEP 3
Responsibly treating knotweed according to the recommendations
⭐⭐⭐HIRE PROFESSIONALS⭐⭐⭐
Hire professional, licensed herbicide applicators.
They are experienced, and will likely do a more thorough job than someone inexperienced.
They have access to better equipment, and knowledge of which spray nozzle tips and settings are best to reduce the amount of herbicide needed.
They are used to mixing concentrations to the correct dilution, and are less likely to mix it wrong.
They are less likely to have a spill that can cause damage to non-target plants. If they make a mistake, and they spray the neighbor’s prized rosebush, they have insurance and will be liable instead of you.
They have access to proper PPE
DO-IT-YOURSELF
If you decide to DIY it is very important you ask yourself these questions:
Can I read and understand the label?
Can I confidently do the math required to mix a concentrated solution down to the correct dilution?
Do I have access and know how to use Personal Protective Equipment necessary to safely apply chemicals (gloves, long sleeves, eye protection etc.)
Am I responsible enough to reschedule the application if it is a windy or rainy day, so I do not harm other plants or trees in the area? Am I willing to change my plan based on the weather? Am I willing to accept the consequences of overspray or other damage if there is accidental spillage?
Do I have a safe place to store the leftover herbicide where it is away from children and pets?
When I am done using the herbicide, how do I plan to dispose of it? What is my plan for disposal?
Am I ABSOLUTELY POSITIVELY 100% certain of the property line and boundaries? (Legal action can be taken against you for spraying on neighboring properties.)
Is the knotweed near a water source, which requires a different, aquatic-safe herbicide? Not taking this into consideration can result in fines and other harms.
This is not an exhaustive list, but a starting point, to help landowners understand the responsibility of invasive species management. You can restore your land and protect your home by responsibly treating invasive species. It takes some knowledge and patience, but anecdotally, many people have seen reductions of 80-90% in their first year.
STEP 4
Monitor the area for knotweed the following year and every year after that. Repeat steps as necessary.
THIS PAGE IS FOR EDUCATIONAL PURPOSES ONLY
Lisa Likes Plants LLC does not endorse any brand or type of herbicide, and makes no claims of effectiveness in knotweed treatments. This page is for educational purposes only. We do not sell herbicide or treat knotweed. We do not recommend specific equipment. The intent is to provide a starting point for following the instructions on knotweed control from many Departments of Natural Resources and State Extension Offices, guided by the research of Jones et al (2018). The primary aim of making this page is to serve the community, as thousands and thousands of people are desperate for answers about Japanese knotweed, and I wanted to create some easy to read, simple information in one place. This is not a How-To, but to Point-To the information laid out by those who have the scientific information about invasive species control.
-Lisa
person who likes plants
Jones, D., Bruce, G., Fowler, M.S. et al. Optimising physiochemical control of invasive Japanese knotweed. Biol Invasions 20, 2091–2105 (2018). https://doi.org/10.1007/s10530-018-1684-5
https://www.fs.usda.gov/Internet/FSE_DOCUMENTS/fseprd529922.pdf
