You may have noticed NCF or other conservation organizations’ staff out working along Nantucket roadsides or bike path edges. In more typical years, the Nantucket Biodiversity Initiative’s Invasive Plant Species Committee organizes group digs, but with Covid-19, we are all out fighting the good fight separately and with the help of some dedicated solo volunteers. Armed with trash bags and odd-looking circular-handled digging tools with long narrow blades, we often get mistaken for volunteers cleaning up trash. But in fact, we are digging up non-native invasive plants to prevent them from spreading into natural areas such as our globally rare sandplain grasslands and dune fields.
Most often during the summer months, our target is spotted knapweed (Centaurea stoebe), a tall plant with pink to purple flowers, that resembles a thistle. A similar relative, brown knapweed (Centaurea jacea) is much less common and seems more limited to richer moister soils on Nantucket, but spotted knapweed’s pink flowers may be seen in many parts of the island growing on our dry sandy soils. Brown knapweed has green leaves rather than silver lightly fuzzy leaves that are cut into leaflets found on spotted knapweed, and the floral bracts of brown knapweed are not dark at the tip (which gives the base of spotted knapweed flowers a splotched appearance and gives the plant its common name).
Unlike thistles, knapweeds lack sharp spines, but they are by no means weaponless—toxic chemicals produced in their foliage deter browsing animals like deer and livestock, and allelopathic chemicals exuded by the root systems suppress the growth of plants nearby, and even prevent their seeds from sprouting.
It’s this tendency to hinder other plant’s growth, paired with the extremely rapid spread along road corridors adjacent to natural areas—such as Head of the Plains, Smooth Hummocks, Sanford Farm, and Linda Loring Nature Foundation— that has concerned local ecologists. At first seemingly limited to a few disturbed fields and road edges, spotted knapweed started expanding quickly with the construction of bike paths and the corresponding increase in mowed road edges in recent years. Mowing when in flower or just past flowering carries the seeds quickly, carrying the seeds to new areas much faster than in un-mowed areas.
Even more disturbing, spotted knapweed started showing up in random isolated spots along trails and sand roads at sites from the middle of Head of the Plains to Altar Rock, presumably carried by trail edge mowing, on muddy tires, and by dumping of landscaping and garden wastes in our conservation areas. We work hard to keep invasive species from taking a foothold in these protected landscapes to safeguard our diverse native ecosystems and rare plants, along with the wide variety of insects and wildlife they support. Usually the best— and often, the only—way to do that successfully is to check an invasive species before it becomes widespread.
While knapweed’s attractive magenta flowers may provide a welcome source of nectar for late season insects and make it an attractive plant to many beekeepers, if knapweed is allowed to spread, it generally becomes the dominant (or only) species. It uses its chemical arsenal and its large size to crowd out native plants that flower at different times of the season and act as valuable host plants and food sources for a wide variety of species, not just hungry honeybees.
How can you help? Planting and encouraging native late season wildflowers and shrubs such as mountain mint (Pycnanthemum muticum), joe-pye weed (Eutrochium dubium), asters (Symphyotricum spp., Pityopsis falcata, and Ionactis linariifolius), sumac (Rhus copallinum and Rhus glabra), and goldenrods (Solidago and Euthamia spp.) helps support late season pollinators from honeybees to monarch butterflies through the driest and hottest part of the summer. Check out the Landscaping with Native Plants on Nantucket brochure to learn more.
What else? If you find spotted knapweed in your yard, you can dig the plants, bag them, and dispose of them in the Invasive Plant Species bin at the Nantucket Landfill and Recycling Center. Be sure to shake off the soil clinging to the roots when you dig and bag the flowers or seed heads to prevent them from seeding in elsewhere. Don’t place them in your home compost pile. Plants need to go through the high-temperature digester rather than lower temperature landscaping waste compost piles in order to kill the seeds. If you spot a large population of plants near your home or at a natural area that you visit, please get in touch with the Nantucket Biodiversity Initiative Co-chairs of the Invasive Plant Species Committee to let us know. As we have already been successful in lowering knapweed populations in some areas, we can move our efforts to other places in the future.