Hi Sandra, 

Thank you for reaching out to the UVM Extension helpline with your question. Japanese knotweed is very hard to control because it has an aggressive underground rhizome system, and your are correct - mowing, pulling, or digging usually do not remove those rhizomes and can even spread fragments to new areas as cut or pulled plants can reroot. 

By your photo, it looks like you may be ahead of the game a bit here. If you have a small area it may be possible to eradicate it without using herbicides which is always the first approach.

Here are two principles to remember when dealing with invasives:

1. Control small patches quickly to stop the spread before they become well established.

2. When dealing with larger areas, control the perimeter first to avoid further spread and work toward the center.

Try Frequent Cutting: Snap or cut stalks flush to the ground once a week during the growing season.

Or The Interruption Strategy: Allow the stalk to sprout, form leaves, and briefly begin photosynthesis to replace root energy. Then, cut the plant down. Doing this repeatedly forces the plant to burn its stored energy underground.

Remember Safe Disposal: Do not compost knotweed stalks. Burn them or let them dry out completely on an elevated wooden pallet or a paved driveway so the fragments cannot touch the soil and reroot. You can also place them in a black plastic bag and let it "bake" in the sun to kill the plants. Don't weed whack, till or mow them either, this can cause them to spread to other areas.

If your affected area is large or mechanical options don't work, you may have to try a combined approach which can include mechanical efforts with a timed herbicide approach. 

Spraying is most effective in late summer or early fall (August to September) when the plant is preparing for winter and actively pulling sugars (and the herbicide) down into its roots. (Avoid spraying when flowers are present to protect foraging bees) If you would like assistance with creating a herbicide plan I can connect you with our UVM Plant Pathologist. 

Here are a couple links that I think you may find useful.

 Japanese Knotweed

Managing Invasive Plants

Best of luck and please reach back out if you have any other questions!