By Abby Bezrutczyk, LIISMA Field Project & Outreach Coordinator

Last Saturday, I had the opportunity to meet a dozen community-gardeners to talk about their local invasive species staples: multiflora rose, honeysuckle, english ivy, and wisteria, to name a few. However, none of these worthy invaders compared to the elephant in the garden: Knotweed (Reynoutria spp).

There, in the southern corner stood the nuisance on everyone’s mind. Shoots sprung up from the foundation of the neighboring building. Thick mats of old stems, resembling bamboo to the untrained eye, obscured any soil. And this garden is not alone – many Long Island residents email us asking what to do about this problem plant.

Anyone dealing with knotweed on their (or their neighbors) property will be ready to share war stories of how this pernicious weed skirted control attempts. Originally from Asia, this plant grows readily in disturbed areas and along streambanks, sending up shoots that can break through concrete, stalks that (in some varieties) can shadow over a tall person, and leaves that shade out competitors. What’s more – given the right conditions, knotweed can regenerate itself from small pieces of its rhizome and stem. Ecological and residential damage ensues. 

As you may know, LIISMA ranks species according to their impact and distribution in the region. Because knotweed is so widespread and difficult to control, we consider it a Tier 4 species – which means we advocate for its local control. That means our focus is on managing it with partners in areas that we’re trying to protect from invasive species, such as in proximity to rare plant communities or intact ecosystems (see ISPZ’s), rather than on personal properties or roadsides. Furthermore, knotweeds are prohibited to possess, transport, import, sell, purchase and introduce in the state of NY (Part 575 Regulations).

Know Your Knotweed

Before our team visit to the Cornell Cooperative Extension (Suffolk County) weed garden early this season, I had only been peripherally aware that there was more than one type of Knotweed. Andy Senesac of CCE patiently explained the differences, using the many examples of common garden weeds and invasive plants that he has on hand for this exact educational purpose (Aside: the weed garden is worth a visit, and you can read a great article that Andy wrote about knotweed here)

There’s Japanese knotweed (Reynoutria japonica var. japonica): the one with 4-7 inch leaves and a squarer leaf base that grows up to 8 feet tall, with conspicuous pistils. Then there’s its larger counterpart: Giant knotweed (R. sachalinensis), the one with 6-12 inch leaves with heart-shaped leaf base, up to 12 feet tall, with drooping branches and flowers. And let’s not forget their hybrid with intermediate characteristics of both: Bohemian knotweed (R. x bohemica) with intermediate length leaves, growing up to 16 feet tall, with upright flowers and conspicuous stamen (see this guide for more details). Each of these varieties is cause for concern, and each is difficult to manage.

At the community garden presentation, we ended our tour at the knotweed stand – Bohemian knotweed, that is. Speckled across the lawn were young shoots that suffered repeated mowing, but their siblings (or perhaps the additional arms of the same organism) towered proudly along the fence. All gazed and wondered: How do you even begin tackling this?

Plan and Assess

First: Assess the situation. 

• Is this a brand-new introduction to your property, or is it a stand that’s gotten out of hand? Is there a huge stand of knotweed on a nearby property that will continue to “feed” your stand, no matter the action you take on your property? Is your property on or near a wetland? Assessing and framing your situation will help you set realistic goals.

Second: Be aware that this will, almost certainly, be a multi-year project.

• Your first inclination might be to cut it away, but research has shown that simply cutting or mowing is rarely effective to eradicate knotweed long-term (Jones et al. 2020). In fact, it may hasten its spread (as knotweed can sprout from improperly disposed fragments) or even trigger the growth of new buds. Instead of running to the clipping shears, take some time to plan for different stages of management– mowing or cutting may be one carefully-used tool among many in your plan (Martin et al. 2020). 
• We’ll tell it like it is: depending on the degree of infestation, eradication may not be possible. However, with the right tools and resources, you can make great strides in suppressing it and planting other natives in its place (Skinner et al. 2012), with fewer shoots to manage in successive years. 

Third: Plan for disposal 

• Before you begin any management, make a plan for proper disposal. Should you choose to mow knotweed, use a bagged mower to pick up all plant parts. Remember that knotweed can regenerate from small stem or rhizome fragments. Do not let any plant parts enter and spread along waterways.
• Do not compost knotweed. Instead, prepare black plastic garbage bags for any living tissue that is managed. Leave these bags in the sun for 3 weeks, so that the material inside is “solarized” and effectively killed. 
• Clean all your gear before and after management to avoid spreading knotweed, or other invasive species.

LIISMA Recommends These Best Management Practice (BMP) Guides

While we’re working on updating our existing best management practices for the LIISMA region, here are some guides that detail some of the best ways to manage knotweed.

1. From our colleagues at the Western NY PRISM: This guide quickly summarizes the primary knotweed control options.
2. From Penn State Extension: This guide provides a management calendar accompanying their recommendations, useful for you or a hired contractor.
3. From Michigan Department of Natural Resources: This guide details planning and control options for Japanese Knotweed, including descriptions on herbicide use.