Japanese knotweed is a tall, fast-growing perennial that forms dense stands, crowds out native plants and resprouts after cutting. The Maine Natural Areas Program lists it as widespread and severely invasive.[1] Since January 1, 2018, it has been illegal to import, export, buy, sell or intentionally propagate Japanese knotweed for sale or distribution in Maine.[2]
How to identify Japanese knotweed
- Grows in dense stands up to 10 feet tall
- Hollow, round stems one to two inches across, with swollen nodes where the leaves meet the stem, like bamboo
- Leaves about 6 inches long and 3–4 inches wide, with a flat base and a pointed tip
- Small white flowers in spikes along the stems in late July or August
Giant knotweed grows taller, up to 12 feet, with larger, heart-shaped leaves. The two species hybridize, and the hybrid is invasive too.[1]
Why mowing and digging don’t work
Knotweed can sprout from any stem node or fragment of rhizome,[1] and its rhizomes can be 30 feet long.[3] Small patches of fewer than 20 stems can be cut as often as once a week for several years, and covering a patch with heavy fabric needs maintenance every two weeks for up to 10 years. Large patches generally can’t be controlled by hand.[1]
What works
Avoid foliar treatment while knotweed is flowering, because bees are attracted to it. Special rules apply to herbicide use in or near wetlands and water, so check with the Maine Board of Pesticides Control first.[1]
A multi-year plan
- Year oneIdentify the full extent of the patch, then treat at the right time of the growing season.
- Year twoTreat regrowth. Follow-up is needed in almost all cases.[1]
- Following yearsMonitor and spot-treat any new shoots until the patch stops returning.
Disposing of knotweed
Never compost knotweed. For small infestations, bag the plant material and let it rot, or burn it. For large infestations, move material to a dry spot where it can’t take root, watch for sprouts, then pile, dry and burn it.[4]
Japanese knotweed is one of the invasive plants Tree Medics manages across Southern Maine.