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Invasive Plants

February 2, 2017

imageAs 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):

  1. It is a non-native plant.  (None of the thirty-three plants on the Maine list are native to Maine.)
  2. It has naturalized, meaning that it has spread into minimally managed natural landscapes where it was not planted.
  3. 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.

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13 Comments leave one →
  1. February 2, 2017 4:24 pm

    As you know, I run an invasive plant removal program in Maine. You have done an excellent job of explaining the issues involved. People always tell me that invasive plants are not spreading from their garden. However, the birds that eat the berries don’t remain by the plant or even in the garden at the time they “disperse” the seed. They are usually roosting in the local forest and often it is a conservation property with minimal maintenance due to budget constraints. Once most invasives get established it is a lot of work to get them out. Sometimes I want to give up and the resistance from gardeners doesn’t help. Thanks for spreading the message in such a clear way.

    • February 7, 2017 10:27 pm

      Carolyn, I’ll admit to some sympathy for gardeners who discover that a plant they’ve fallen in love with is a problem. But I also think gardeners have to be educated (and sometimes coerced!) to garden responsibly. I hope the new Maine rules will make your efforts a bit easier. I have two problem plants growing in my garden, Hemerocallis fulva and Spiraea japonica ‘Magic Carpet’. Naturalized populations of both have been documented in Maine, and both have been listed as invasive further south. The old-fashioned tawny daylily is a plant I have loved since childhood, when it was already naturalized throughout New England. When I bought and planted the spirea, it was being marketed as a sterile hybrid of S. japonica and S. bumalda. Turns out it is just a cultivar of S. japonica and has viable seed. I was relieved that neither of these plants made it onto the Maine list, but I know that I can’t just bury my head in the sand about them. I won’t share these plants or acquire more of either, and I am monitoring both. Fortunately, H. fulva really is sterile and spreads vegetatively, so I can keep an eye on how far and how fast it is spreading from my planting. The spirea disperses its seed primarily via water, which means my dry, sandy growing conditions, pretty far from any streams or rivers are a help. Nevertheless, I’m steeling myself to yank these plants out at the first sign of trouble.

  2. February 2, 2017 4:52 pm

    I desperately love goldenrods, but I can’t have them. I love how they can create a full hedge each season, but having seen them in too many natural settings where they don’t belong it’s hard to defend bringing them to the neighbourhood. They belong on the prairie, not in Scandinavia.

    I do allow some potentially invasive species to remain in my garden, though, as they were here when I took ownership of it. I have one border that’s completely full of physalis, so getting rid of them would be a nightmare and they ARE quite attractive in autumn with their little lantern-like husks around the fruits (that are also delicious). But I’d NEVER recommend them to anybody! (And they also seem to be quite out of fashion in gardening; they were probably more of a “thing” in the 70’s which was the heyday of my garden.)

    • February 7, 2017 10:31 pm

      Soren, It’s good that you have resisted your love of goldenrods, since they are invasive in your area. I am lucky to be able to grow several species of Solidago that are native to my area, mostly as woodland edge plants. Like you, I find it harder to deal with potentially invasive plants that were already growing in my garden before I realized they might be a problem. These I don’t share, don’t plant more of, and carefully monitor for any sign of escape.

      • February 27, 2017 8:35 am

        I get some help from being completely surrounded by farmland for hundreds and hundreds of yards all the way around my house; their monoculture makes it more difficult for plants to escape from my garden – though of course birds and high winds CAN carry small seeds much further. (Hence the self-imposed solidago ban.)

        The physalis are an issue in that respect since their seeds are likely to be passed on through bird droppings – but I haven’t seen any of them in the wild on my many walks so perhaps they’re just not strong enough to compete with local plants. (And we have some TOUGH local weeds! Chicory, camomile, chervil are all rampant around here. And I love that they are mainly pretty, edible – and tasty…)

  3. February 3, 2017 1:54 pm

    Hello Jean, I look a look of the list of plants and was stunned to see just how many common garden varieties are listed. You can go to a garden centre in the UK and pick up many of these plants and indeed, some are prized. It just goes to show how plants behave under different conditions, angelic in the garden in one part of the world and devilishly invasive in another. We have problems with invasive plants in water courses, there seems to be more of a focus on (avoiding) those rather than land-based plants.

    • February 7, 2017 10:36 pm

      Sunil, Many of these plants are common garden varieties here, too, which is what makes them so problematic. During my invasive plant class with Ted Elliman, we visited a church parking lot in downtown Boothbay, Maine that seemed to have been planted entirely in invasive plants, probably done by a professional landscaper who was choosing plants that would need little attention and thrive at the edge of a parking area! Until now, Maine has also been focused primarily on the aquatic invasive plants, although ours are more often lake and pond plants than plants of streams and rivers. There’s even a program here for inspecting boats at boat ramps before they are put in the water to make sure that they have been cleaned and are not transporting invasive plants from one lake to another.

  4. debsgarden permalink
    February 5, 2017 8:05 pm

    Great post! I have struggled with invasive plants in my garden since the day we moved here. English ivy, bamboo, Nandina domestic, and ‘the plant that ate the South” – the ubiquitous kudzu, are ever present challenges to control. I am shocked to see that English ivy, bamboo and and Nandina domestic are still sold in local big box stores and nurseries.

    • February 7, 2017 10:39 pm

      Deb, I too have been frustrated by nurseries selling invasive plants, mostly by owners who know there is a demand for the plants and have convinced themselves that the cultivars they are selling aren’t really a problem. This is why I’m so happy that Maine now has a law prohibiting the sale of these plants, which will keep the nursery owners honest and give responsible consumers recourse when they find these plants being sold.

  5. February 6, 2017 1:31 pm

    Jean this is an excellent article for probably just about every state in our country. We have our share of invasive plants in SC. My property was overrun with non-native honeysuckle, Chinese privet, barberry, burning bush and one of the parents of the supposedly sterile Bradford pear. I have eradicated most of it but seedlings continue to sprout despite my best efforts. I have neighbors who think the pears are pretty in the spring and refuse to get rid of the offspring. Education is key, not only educating the public, but also other gardeners, nursery owners and the big box stores. I had a Master Gardener friend who gave me an iris, which turned out to be the bane of my existence. I now investigate everything before planting it!

    • February 7, 2017 10:43 pm

      Kathy, I’m not entirely surprised by your Master Gardener friend. There was remarkably little focus on invasive plants in the curriculum of my Master Gardener course. We had one class on integrated pest management that included brief discussion of invasive plants. But, in another part of the course, the small fruit expert was encouraging people to grow hardy kiwi vines, without any mention that the Massachusetts Audubon Society had listed this as an invasive plant.

  6. February 6, 2017 5:42 pm

    oh yes sigh.
    We had a lot of invasive alien trees removed while this house had tenants. Left gaping holes, two of which still need filling with the autumn rain we hope for. And that ivy on the boundary walls …
    There are hack groups of volunteers who clear invasive aliens in Table Mountain National Park – and I’m hoping that we will get to a second hack one day.

    • February 7, 2017 10:45 pm

      Diana, Here we also have groups of volunteers who take on the removal of invasive plants in some local areas, but it’s a tough job (see Carolyn’s comment above). If you want to remove most invasives here without chemicals just by cutting them back, they usually need to be cut at least three times in a season for at least three consecutive years.

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