How to Kill Knotweed

Japanese Knotweed (young)

What it is: Knotweed is a perennial that was introduced from Asia many moons ago as an ornamental.  However, it is EXTREMELY INVASIVE.  It takes over entire forests and river banks because of it's ability to spread by root and stem fragments (meaning if you break off a piece and lay on the ground, OR if you happen to dig up a root system of it and throw it in your compost pile,  it will pop up 10 fold!) This is what happens on riverbanks, when the bank gives way and the plants wash down stream and pop up else ware. Its a vicious cycle and becoming a HUGE environmental problem in the U.S and Europe.  IF YOU SEE IT, KILL IT. If it isn't on your property, educate the property owner or report it to your local invasive plant authority. (You can Google your city and "invasive plants" to find out who to contact.)  It is bad news!

Japanese Knotweed (full grown)

Timing: This process works best if you wait until the plant is about to bloom (see picture to the right showing the full grown knotweed). I live in the PNW so this is around mid to end of August. The reason you want to wait until its big and blooming is because after this stage it will shoot all it's energy and nutrients from the leaves and stems down into the root system, which is prime timing for you to start the poison process. See the directions for more details on this. Be sure that there is no chance of rain during the two days following the spraying and injection steps below. 

What you'll need:

• Weed Killer (must be 41% glyphosate) - I used the generic "HDX" brand from Home Depot, much cheaper than "RoundUp" and essentially the same thing. Follow the mixing directions on the bottle when you put it in the sprayer)
• Sprayer (handheld/pump) - I bought the HDX one from Home Depot. Doesn't have to be fancy.
• Injector/Syringe (large) - I just went to local feed store/farm supply and asked for the largest syringe and needles they had, they gave me one that was used for cattle. You can buy official weed injector guns that would probably work better, but I couldn't find any at local stores and didn't want to wait for them to ship if I ordered online. Syringe worked just dandy..just be careful not to stab yourself :)
•   Large Plastic Trash Bags

Directions:

• Right before it starts to bloom (before the little white flowers emerge or are starting to- Usually around mid to end of summer) spray the foliage (leaves) with the weed killer using the sprayer. DRENCH THEM. Spray the tops and undersides of all the leaves. 
• Tip: For even more effectiveness put a splash of dish soap (like Dawn) into your weed killer mix. The soap acts as a super-duper surfactant that strips the plant leaves of their natural protective film. Weed killer already has surfactants in it, but this will supercharge it! 

 
• After the leaves and stems turn brown (see pic to the right), cut them so there is stem sticking out of the ground that is about 5 inches high. Do NOT lay the dead stuff on the ground, put them in garbage bags or plastic container and dispose of them.  (They say if they're brown/dead they can't propagate, but I don't trust that theory. This stuff is relentless.)

• Now you should only have your little stubby stems. Inject the stems in the top (it will be tube-like, like bamboo) with the Weed Killer solution - I used straight solution, no mixing with water.
• You're done!!! Leave the injected stems alone. By winter  they should become mushy stems that will slowly fall apart. You can pick them up and throw them away.
• The following summer be sure to monitor the area for any new sprouts. I only had one little one sprout up and I immediately painted some weed killer on it (I just used a little paint brush with pure weed killer solution, no water.) As long as you kill any new sprouts the root system will starve and likely be gone for good. Experts say it takes 2 seasons to officially kill the whole root system - which is huge. 

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  Result (grass will come back by end of summer)

Additional Information/Resources:  
www.invasive.org