A knotweed patch along the bikepath at Sanford farm in 2017. Photo: Libby Buck.

Got Knotweed?

There’s a good chance knotweed is one of those plants you’re battling in your yard…and if not, it’s practically guaranteed there’s some in your neighborhood. It’s one of the “Least Wanted” plant species for island homeowners and conservation organizations. Japanese knotweed (Reynoutria japonica), is often lumped together with hybrid knotweed (Reynoutria x bohemica) and giant knotweed (Reynoutria sachalinense), since they are all non-native and invasive. Originally, knotweed was imported to North America by the Victorians, who thought that it could be a useful ornamental (it looks a bit like bamboo with broad leaves). The Victorians were all about planting large and dramatic new plants from exotic locales in their gardens! In many cases that proved to be a super-sized mistake. Plants removed from their native range can expand without checks and balances in a new habitat, and that is just what knotweed has done with a vengeance.

Knotweed stems are hollow and jointed like bamboo but have large shovel shaped leaves. Photo: K.A. Omand

In just a few seasons a small plant–or even just a piece of knotweed root–can expand to engulf large areas of your yard or favorite nature area. And the roots can damage pavement and foundations, creating and expanding cracks. Meanwhile in pond shores and along roadsides, advancing knotweed “hedges” smother native wildflowers and shrubs, bringing down habitat value for wildlife and insects. What’s a gardener (or a conservation area manager) to do???

Our best attempts to eradicate knotweed have been thwarted by the tenacity and ability of this plant to rapidly take over new areas. Chemical methods like traditional herbicides, manual methods like repeated hand cutting or mowing, grubbing out roots, and smothering with tarps have all been tested as ways to get rid of (or at least contain) this exuberant plant that regrows to 6-15 feet tall each growing season. So far there has been no magic bullet to get knotweed under control.

What are Biocontrols?

Researchers have long been looking into biocontrols–insects or diseases that damage and reduce cover of invasive species. These biocontrol organisms are found in an invasive species’ native range, where they co-evolved for thousands of years, but they are lacking in a new habitat where the plant has been introduced. The theory is, that by reuniting these insects with invasive species, their impacts on native systems can be reduced. Other desirable native plants and the insects and wildlife that depend on them will be able to flourish once again. Some of these biocontrol programs have been highly successful in reducing weeds.

How are Biocontrols Evaluated?

The process for vetting these potential biocontrol insects and pathogens is a rigorous and lengthy one, and candidates must be approved for release in each country where people want to use them. This careful study under controlled conditions developed a few early mistakes. It turns out that selecting the right potential biocontrols is a true matchmaking exercise, to get the right insect or disease for the job, that will work in the new habitat—but not have harmful effects on native plants or other organisms. If choosing the right biocontrol is matchmaking, the testing and permitting processes are like an extensive background check by the CIA. Click here to learn about permitting by USDA.

Exciting New Research

Jeremy Anderson, a Postdoc Research Associate Professor at the University of Massachusetts Amherst, has been working on one such project to assess an insect species for biocontrol of knotweed. He has been working with scientists around the country to test release an insect called the Japanese knotweed psyllid (Aphalara itadori) that has been approved by the USDA. After due diligence in testing whether the insect could feed and reproduce on all related native species–it didn’t, so it met this challenge and was permitted for release–Jeremy has been working on trying to establish populations of the insect in Massachusetts, while other researchers have been doing the same in other parts of the country, to evaluate areas with different climates and other environmental factors.

Jeremy will be presenting online hosted by the UMass Boston Nantucket Field Station on May 24th at 4:00pm (register here to join live), to present the background on biocontrols and on his research, and discuss his hopes for studying the release of this biocontrol on Nantucket. The Nantucket Conservation Foundation and The Nantucket Islands Land Bank are very interested in being a part of this research going forward, as it seems that Nantucket is a good climate and daylength match for this biocontrol insect species, unlike many hotter mainland sites.

Biocontrol Success Stories

Success stories for releases of biocontrol insects include the black-margined loosestrife beetle (Galerucella calmariensis) and the golden loosestrife beetle (G. pusilla). These vegetation eating insects have been released into areas around the country where purple loosestrife (Lythrum salicaria) has become a terrible weed, crowding out the vital native plants of wetlands and shores. In Washington state, at several sites where the beetles have successfully built up their numbers, they have reduced loosestrife plant cover by 90%. Researchers looking into knotweed are looking for the same kind of successful biocontrol options for the various knotweed species.

The Future of Knotweed on Nantucket

Biocontrol has great potential for helping us curb the spread of knotweed and other invasive plants here on Nantucket, especially when herbicides and manual removal are not appropriate. Has Nantucket knotweed met its match in the psyllid beetle? If you’re interested in learning more, please tune in for this talk! (Can’t make it that night? The talk will be recorded for later viewing, and the link will be made available as soon as possible after the presentation here.)