Modern folk tales abound in Nova Scotia about the indestructible plant that grows like a weed all over our shorelines, dominating other plants with its giant bamboo-like stalks. I’m talking about Japanese knotweed, which is a problematic invasive plant across the province and beyond. Japanese knotweed was once a prized garden plant, which is how it arrived on our shorelines. Over time, it became less popular because of its invasive quality. There is also some evidence that Japanese knotweed has hybridized with giant knotweed in Nova Scotia, making it an even more formidable foe. Knotweed is a pioneer plant that thrives in disturbed areas. It loves water and roots, spreading along shorelines and wet areas via floating root clumps. The main concern with knotweed is that is creates a monoculture by out-competing all other plants. This monoculture worsens shoreline erosion, decreasing biodiversity and creating an unsightly brown view throughout much of the year. Read more about Japanese knotweed in Nova Scotia here.
Many a gardener has tried to tackle this plant, only to find it re-sprouts seemingly larger than before. Some have resorted to pesticides, which only further deteriorate our shoreline ecosystems. At Earthshine Gardens, we have been experimenting with organic knotweed eradication and have found our method to be extremely effective; having created diverse and beautiful shoreline gardens in what only a few years ago were knotweed monoculture jungles. Our method is simple and achievable, using the following steps:
1. Cut all the stalks and leaves off of the knotweed, prior to flowering if possible. The non-flowering aerial plant parts can be used to build biomass in the area and will not re-sprout if there are no roots attached. The green plant parts can be laid down in the area to decompose, providing valuable nitrogen to build the soil as a foundation for other plants.
2. Dig or cut out as many of the root clumps (called rhizomes) as you can. This is the main way that the plant reproduces. These are very hard and will take some muscle to uproot.
3. Bag all rhizomes and seed heads in black garbage bags for disposal, to ensure they don’t sprout elsewhere.
4. Lay a thick tarp over the entire knotweed area and weight it down well. The tarp will deprive the rhizomes of light and water, which will cause them to come closer to the surface over time so you can dig them out.
5. Lift the tarp once per month for 1-2 years to remove any green sprouts and dig out rhizomes, using the same process as above. Over time, you will see less rhizomes, and eventually none at all. Walking on the tarp once per week or so will ensure shoots break and the tarp is not damaged.
6. Create a sheet mulch over the entire area using thick cardboard topped by brush materials, compost, soil, woodchips etc. This will allow new soil to be created for plants to establish.
7. Plant hardy perennials or shrubs over the area. Ensure that you use a diversity of different plants that are strong competitors but allow diversity. Add 2-3 inches of straw or bark mulch over top of the area.
8. Keep an eye out for knotweed re-sprouting and pull it as soon as possible. If possible, work with other natural plants that are establishing in your new garden. This will allow a diverse, low-maintenance ecosystem to develop in your new garden.
I recommend folks try this method and let us know how it goes. If you need help, we’re here for you. Just call Earthshine Gardens, the South Shore’s holistic ecological land care team.