Irene, Knotweed, and the White River Partnership

invasives on the White River
Chad Hammer (UNH) and Mary Russ (White River Partnership) discuss invasive plants moving into a 2012 tree planting project site along the White River in Royalton, Vermont.


Several years ago, I met Mary Russ, Executive Director of the White River Partnership, while working on the “Northern Forest Watershed Incentives Project” funded by a USDA Conservation Innovation Grant (2009-2012). Mary and her organization were exploring innovative ways to fund conservation work in the watershed – including “landscape auctions” and the use of voluntary carbon offset sales to fund practices like tree planting along the rivers and streams.

This work came to mind again this year during the one-two punch of hurricanes Harvey and Irma as I was beginning to plan field work with my new graduate student Chad Hammer. Chad is studying the impacts of invasive plants on riparian forests (those areas of forest along stream and river banks). One of the questions he is interested in is how invasive plants move into new areas and become established. We know that many invasive plant species are successful at colonizing recently disturbed areas – which make the banks of rivers and streams in New England likely places for establishment following typical ice scouring and flooding events. This is where I made the connection to the hurricane activity as I thought about what all that water moving around must be doing to the forests along rivers and streams throughout the impacted areas in the south – and wondering when we were going to get our turn. Then I realized that we have had our turn recently and have some real-world examples to study right here in New England!

The White River Partnership has been planting trees in the central Vermont watershed for 20 years -some 46,000 trees have been planted by staff and scores of volunteers and school groups. In August of 2011, Tropical Storm Irene gave the non-profit a lot more work to do as record flooding wreaked havoc on the communities throughout the White River watershed. All this disturbance in the watershed surely must have led to new opportunities for invasive plants to move around. I called Mary to check in on their restoration efforts and see what kind of insight she had on invasive plant colonization after the flood waters subsided. It turns out Mary and her husband Greg (and White River Partnership Project Manager) have a great deal of experience battling invasive plants in the watershed, Japanese knotweed in particular is a pesky and persistent invader. They have learned a lot over the years about what works best for restoring riparian forest – including the use of fast growing early successional tree species like willows, birches, aspen, and native shrubs to better get a foothold before the non-natives can sneak in. They have collected some monitoring data and anecdotes over the years, but do not have a formal assessment of what is working best and why. That’s where we come in. With funding from the New Hampshire Agricultural Experiment Station, Chad and I will be studying the restoration plantings since Irene to get a better understanding of the species mixes, proximity to non-native colonies, and other factors that can lead to successful restoration and exclusion of non-native invasive plants like Japanese knotweed, Japanese honeysuckle, and garlic mustard. We will also be investigating the water quality impacts of these non-native invasions. One tributary (Broad Brook) we visited this week seemed to have at least half the stream bank covered in knotweed. Are these non-native colonies able to provide the same kind of bank stabilization and capturing of fine sediments moving over the ground as native plant communities? That is something we’ll have to figure out.

This work will complement Chad’s work in northern New Hampshire (more on that later). Our hope is to develop some guidelines and recommendations for riparian restoration work that is invasive-resistant. With more heavy precipitation events likely to occur in our watersheds in a changing climate, we need to understand how best to restore the function of these streamside forest communities and give them a boost over the invasive plants that are likely to benefit from a warmer world.

knotweed in Broad Brook
High density (and tall!) Japanese knotweed extending more than 30 feet from the banks of the Broad Brook in Sharon, Vermont.