As the new year opened, the Maine Department of Agriculture, Conservation and Forestry adopted new rules making it illegal to “import, export, buy, sell or propagate” thirty-three terrestrial plants that are known to be invasive in Maine, likely invasive in Maine, or potentially invasive in Maine. Until now, Maine has been focused primarily on the problem of aquatic invasive plants that can quickly choke lakes and ponds; and many of us felt that the regulation of terrestrial invasives was overdue.
The extent of the problem became clear when the Department of Agriculture, Conservation and Forestry investigated the availability of these plants and found that all but three were being sold at Maine nurseries. To help commercial nurseries adapt, the prohibition on sales is being delayed until January 1, 2018, allowing nurseries to sell existing stock of prohibited plants.
This does not mean that gardeners should rush out to buy these plants while they are still available. Most of these invasive plants were deliberately introduced to Maine by gardeners or landscapers who planted them for their ornamental value, and I’ve found that many gardeners don’t want to admit they are part of the problem. This is partly a result of confusion among gardeners about what constitutes an “invasive” plant. Gardeners often confuse invasives with what are commonly called “garden thugs,” plants that take over areas of the garden and crowd out other plants. Gardeners will defend their prized burning bush (Euonymus alatus), barberry (Berberis thunbergii or Berberis vulgaris), porcelain berry (Ampulopsis glandulosa) or purple loosestrife (Lythrum salicaria) by saying, “But it’s not invasive; I’ve been growing it in my garden for years and it hasn’t spread at all.” But here’s the thing: What makes a plant invasive is not how it behaves in your garden, but how it behaves when it escapes from your garden.
If an invasive plant is not a plant that is rampaging through your garden, what is it? Invasive plants are defined by three criteria (and all three must be met):
- It is a non-native plant. (None of the thirty-three plants on the Maine list are native to Maine.)
- It has naturalized, meaning that it has spread into minimally managed natural landscapes where it was not planted.
- It “causes economic or environmental harm by developing self-sustaining populations that are dominant or disruptive to native species.”
Many of the plants on the Maine list have naturalized in my part of Maine and are visible along roadsides or the banks of rivers and streams. These include purple loosestrife, Norway maple (Acer platanoides), multiflora rose (Rosa multiflora), Asiatic bittersweet (Celastrus orbiculatus), swaths of Morrow’s honeysuckle (Lonicera morrowii), and large monocultures of Japanese knotweed (Fallopia japonica). Two of these, Morrow’s honeysuckle and Asiatic bittersweet, regularly pop up at the edges of the woods around my garden. I try to be vigilant and remove them as soon as I notice them and preferably before they produce berries. It is not coincidental that many of these invasive plants produce colorful berries that birds like to eat. Those berries contain seeds. As they pass through the bird’s digestive system, the seeds are scarified to promote germination; and they are then pooped out (often far from the parent plant) with a nice packet of fertilizer to help them get a good start in life.
How can we gardeners ensure that we are part of the solution, not part of the problem? First, do not plant invasive or potentially invasive plants, even if doing so is not prohibited by law. Be wary of cultivars that are marketed as “sterile” or “non-invasive;” those claims often turn out to be mistaken. Years ago, gardeners were sold cultivars of purple loosestrife which were supposed to be sterile; it turned out none of them were. When I took a class on invasive plants in the fall with Ted Elliman, an invasive plant expert from the New England Wildflower Society, I asked him about cultivars of Japanese barberry that were being sold at local nurseries with claims of sterility. He told me that research on these cultivars in Massachusetts had found all of them to produce viable seed. The new Maine regulations include a process that nurseries can use to petition for a waiver for a sterile cultivar, but the onus of proof is on the nursery asking for the waiver and the standards of evidence are high.
What about plants that have been in your garden for years, planted before they were known to be invasive? The first thing is to never, under any circumstances, share these plants with other gardeners. If someone asks you for a division or a cutting, just explain that, while you agree the plant is attractive, it has turned out to be invasive, which means that it would be irresponsible to propagate it. The best course of action would be to remove invasive plants from your garden. It’s wise to see if there is a fact sheet available with advice on the best way to do so. (Some attempts to pull up plants, for example, can leave roots behind and trigger invigorated growth.) The Maine DACF website includes factsheets on all thirty-three listed plants. Many states and organizations also publish suggestions for native plants that can be substituted for invasives. (The University of Maine Cooperative Extension has a publication on Gardening to Conserve Maine’s Native Landscape that includes recommended plants to use and plants to avoid.) The only way to responsibly keep a beloved invasive plant in your garden is to ensure that it does not escape from your garden. To do this, you will need to educate yourself about how the plant disperses its seeds. If a plant spreads by means of fruits or berries that are eaten by birds or small mammals, the only way to responsibly keep that plant in your garden is to deadhead with 100% success in making sure that no fruits are allowed to form. If the seeds are primarily wind-dispersed, plants will need to be deadheaded as soon as flowers open.
There are many ways that gardeners can act to improve the environment. Not growing invasive plants is surely one of them.