Battling Japanese Knotweed

The lot my house sits on is a nice size and when we moved in a couple years ago, I had plans of doing lots of landscaping. Primarily, I wanted to plant several patches of native wildflowers on some steeper slopes and around some of the permitter. The first year, I smothered a couple sections with heavy black plastic in preparation for hand casting wildflower seeds during the following spring. The permitter of the lot had too many trees and shrubs for me to consider using more black plastic. Therefore, I started mowing and weed whacking some of this area to clear a lot of the brush; however, it wasn’t long until I realized what I was dealing with.

Quickly, and I mean really quickly, some of the brush started coming back at a rate that seemed like a foot a day. At this point, I started doing some research and learned that I was dealing with Japanese Knotweed (Fallopia Japonica). Probably one of the worse and most aggressive invasive species around. It thrives in disturbed soil, riparian areas, and along wooded lots…all of which applied in my situation. This amounted to approximately 400 feet of border, up to 20 feet deep, around my lot infected with this invasive species, not to mention a couple standalone areas.

I want to be a good steward of the planet and at least rid my property of Japanese Knotweed without the use of chemicals. So, I will tell you some of the techniques I have used to battle Japanese Knotweed. I will warn you, that this is labor of hate (love for native plants in the future though) and an effort you will have to dedicate at least 3 years, but probably more.

Japanese Knotweed has massive rhizomes that form underground and which can spread great distances at an alarming rate. These rhizomes are very strong and require a long game plan to destroy. In order to avoid using chemicals, your plan needs to be to starve this plant of energy and weaken its foundation. In order to do this, you first need to cut these giant bamboo like shoots down. This can possibly be done by trampling them with a board or tractor depending on the terrain and obstacles; however, your best option is a commerical weed whacker (or weed trimmer), such as the Echo SRM-225. You will want to combine the weed whacker with heavy duty string that is not round, but has multiple edges, such as the Echo Cross-Fire 0.095 Nylon Trimmer Line. This string is very tough, but the stalks of mature Japanese Knotweed will put up a really good fight. Your first goal is to whack all of your Japanese Knotweed down and to never let it get that big every again.

Once you have knocked all of the Japanese Knotweed down, it is suggested by some to rake up the debris and burn it. I do not fully believe this is necessary; however, it may be advised to complete the next possible step. These stalks, if left to dry, will become woody and splinter. Therefore, if you want to smother the ground with heavy black plastic, they will likely pierce the plastic. So, go ahead take the time and burn as much as you can (do not send to the dump or compost, as one small piece could potentially start a new plant).

At this point, I would advise trying different approaches to see what works best for you, but here is what I did. I had a large section of Japanese Knotweed in an open drainage area, which was perhaps 75 by 20 feet. I smothered this section with the black plastic for two seasons. This section was on the one end of my infected border and I figured I would work my way into the middle. The middle section and the other end were completely in and on the border of a wooded area. Do not kill any of the shrubs and trees in these areas! They are your friends and will help shade out Japanese Knotweed. Japanese Knotweed is so aggressive that no new plants will survive and emerge from the canopy of Japanese Knotweed, which is up to 12 feet high, without your assistance. Therefore, leave whatever else is growing and established alone, if you can.

Under your black plastic, Japanese Knotweed will keep growing and try to poke through your plastic. From time to time, you will see the plastic lifting or ballooning up, so you will need to trample it back down. You will likely get some holes here and there over time and you should try to cover them up with stones, logs, or more plastic before the Japanese Knotweed starts coming back from the brink. After two seasons, I lifted the plastic and found that most of the Japanese Knotweed had died, except around the permitter, where it will seek sunlight, and a couple places where too many holes were located and I had neglected to patch them. Regardless, what was left was a fraction of the original and greatly weakened. At this point, when you lift the plastic off you will need to quickly establish some other types of plant. I have chosen a combination of grass and native wildflower seeds.

Once your new seedlings come up, you will need to make sure they get well established. You will likely need to hand pick, weed whack, or mow sections where the Japanese Knotweed is still growing. You should do this chore once a week if you can ideally, but every two weeks will work, but do not let it go more than a month. All along, I have applied these techniques to the areas in the middle and other end section that were not smothered by black plastic. These efforts will seem fruitless, but you have to have the long road in mind. Each time you do this, you are weakening the rhizomes underneath the ground. At some point, you will notice that the rhizomes are dried up and easily broken by hand or kicking. At the least you will find them much easier to dig up and to pull the root runners out of the ground. I suggest getting a GroundHog Cultivator and a Mattock to help you dig the roots and rhizomes out of the ground.

After two and a half seasons, I still have Japanese Knotweed, but it has been greatly diminished. A couple sections have been eradicated, but most of the infected area remains; however, it is thinned out and very weakened. At this point, I tried smothering some standalone areas with free mulch and straw. I have been very pleased with the results of utilizing the free mulch. I poured it on about 6 inches deep and now even fewer shoots are popping up, but the best part now is that they are even easier to pull out because they are so leggy from working their way through the mulch. At this point, I am hoping to receive a lot more free mulch to smother the middle section, but this section is already fairly weakened due to being shaded out for the better part of spring, summer and fall.

I am fairly confident that I will have have exterminated all of the Japanese Knotweed on my property in the next year or two. The tides of war have changed. The Knotweed has been greatly weakened and now other plants are filling in and helping to accelerate the process. At this point, it is just a matter of time and dedication to mechanically removing and smothering the remaining stands of Knotweed until I win this battle. I’m hoping in three years time I will have a beautiful, established border of native wildflowers to look at.

Tools and Techniques Discussed:

• Echo SRM-225 with Echo Cross-Fire 0.095 Nylon Trimmer Line
• Heavy Duty Black Plastic, plus trampling down
• Leverage existing trees and shrubs to help shade out Japanese Knotweed
• Hand Picking
• Mowing
• Digging Up Rhizomes
• GroundHog Cultivator
• Mattock
• Free Mulch
• Straw