Have been digging out the roots of a bed of japanese knotweed in an area of our yard about 20 feet by 10 feet for the past 6 weeks. Although I think we are making progress, new shoots keep cropping up. Please advise the best way to eradicate. Not opposed to weedkiller as it is an isolated back area of our yard and not used by anyone. Many thanks!!!
As you've discovered, Japanese knotweed is one of the more difficult weeds to eradicate. Roundup (glyphosate) has good activity on it, but because of the extensive root system may take more than one application. If you want to do a foliar spray, you should allow regrowth to reach 2-3 ft before spraying. This will allow the glyphosate to be translocated to the roots. Another option is a cut surface treatment. Rather than spray the foliage, cut the stems off at the soil surface and then paint a 25-50% concentration of glyphosate on the stem. The other option would be to continued removing new growth by hand, the key is to stick with it and remove the foliage before it reaches 18-24" because you don't want to allow the plant to replenish the root system. The herbicide should allow you reduce the time needed to completely kill the root, but it won't be a one-time solution.