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Plants Behaving Badly: Aggressive vs. Invasive

July 28, 2019

tradescantia in bloomAs terrestrial invasive plants have become a more prominent issue, I’ve noticed that many gardeners are confused about what makes a plant “invasive.” Part of the confusion is that gardeners have long used the word “invasive” to refer to plants that behave badly in their gardens. Gardeners also call them “garden thugs,” these plants who don’t play well with others. Whether you invited them into your garden or they showed up uninvited, they quickly shouldered others aside and hogged more than their fair share of garden real estate. But this is not what scientists mean when they call a plant “invasive.” So, to help clear up the confusion lets refer to these plants that behave badly in the garden as aggressive.

Invasive refers to how a plant behaves when it escapes from your garden. To be defined as invasive, a plant has to meet three criteria:

  1. An invasive plant is a non-native plant. Native plants have coevolved with other species (plants, animals, insects, fungi) in our native ecosystems and have long-standing relationships of interdependence with those other species that provide a system of checks and balances. This means that, although a native plant like spiderwort (Tradescantia virginiana) may behave aggressively in the garden, self-sowing rampantly and spreading itself around where it is not wanted, it will not create a problem in the wild.
  2. An invasive plant has naturalized in the wild. This means that the plant is growing where no human being planted it and that the plants are reproducing on their own and spreading in wild places.
  3. A plant is invasive when the naturalized populations of the plant in the wild are causing harm. This usually means harm to native ecosystems because the plant is out-competing native plants for light, water, nutrients and space, creating ripple effects through all the species in the ecosystem that were dependent on the native plants that have been crowded out.

Some plants that behave badly in the wild also behave badly in your garden (they are both invasive and aggressive). A good example is goutweed or bishop’s weed (Aegopodium podagraria). But many plants that seem perfectly well behaved in your garden (not aggressive) create havoc in the wild when they escape from gardens. This is why it is so important to understand the difference between an aggressive plant and an invasive plant. I’ve often heard gardeners arguing that a plant is not invasive by saying, “I’ve had it in my garden for years and it has never spread,” or “If you just keep it pruned, you can keep it under control.” But these assertions focus on how the plant behaves in your garden, not on the chance that it will escape and behave badly in the wild.

Most documented invasive plants in Maine have escaped from ornamental plantings, usually through having their seeds dispersed far from the parent plant by wind or by animals. It’s not just coincidence that many of our most problematic invasive plants have bright berries that are attractive to birds. The birds eat the berries, the seeds are roughed up and prepared for germination as they pass through the bird’s digestive system, and the seeds are then pooped out far from the parent plant, enclosed in a nice little packet of fertilizer to give them a good start in life. The gardener growing the parent plant usually has no idea that the plant has escaped from their garden.shrub honeysuckle berries

There’s a good reason why states adopt lists of invasive plants; landscapers, homeowners, and gardeners are usually not good judges of how likely it is that a plant will escape from an ornamental planting and behave badly in the wild. (To see the Maine list of invasive plants, click here.)

Bottom line: Aggressive (how a plant behaves in your garden) and invasive (how a plant behaves when it escapes from gardens) are not the same thing, and we can’t predict the likelihood that a plant will escape from our gardens and behave badly in the wild from how it behaves in the garden.

17 Comments leave one →
  1. July 28, 2019 11:34 pm

    Thanks for your post, Jean. It led me to look up plants considered invasive in California, which turned out to a quite a rabbit hole as there are a LOT of lists from a wide variety of sources, both official and unofficial. California has a large number of ecosystems and plants invasive in some areas of the state aren’t considered invasive in others but getting a comprehensive list for specific areas doesn’t appear to be easy. Wikipedia lists many plants I expected to see while omitting at least a few others, without any reference to geographic or cultural parameters. I’ve always found it concerning that even reputable nurseries and garden centers often persist in selling invasive and aggressive plants without any kind of warning.

    • August 5, 2019 8:05 pm

      Kris, One of my frustrations with the Invasive Plant Atlas of the United States (see link in my sidebar) is that it allows you to see where (at the county level) populations of a problem plant have been naturalized in the wild, but it doesn’t allow you to get a list of all the problem plants documented in the wild in a particular place. That would be a really useful thing to be able to do! If you’ve got the time and energy (might be nice task to focus in on when you want to shut out construction chaos), it would be possible to go through your various lists of possible invasive plants for California and see which ones are naturalized in the wild in your county.

  2. July 29, 2019 1:44 am

    GREAT POST! Like you, I think there is some confusion about what is invasive and what is aggressive. Some folks place plants to close to their neighbors in flower beds only to have them overshadowed… I know because I seem to be one of “those people”. 🙂 Here on the farm, there are several species that have become quite invasive in the hayfield. They are hard to control because spraying would cause issues with native species and the abundance of Red Clover. Thanks for sharing!

    • August 5, 2019 8:18 pm

      LOL, It’s always hard to believe when you’re putting those plants in and they seem miles apart that two years later, they will be elbowing each other for space.
      I can imagine that prairie invasive species would be a big problem in your area. Here in heavily forested Maine, our biggest problems are plants that invade forest ecosystems.

      • August 5, 2019 10:53 pm

        Like the Kudzu in the south. I saw areas in Arkansas and Mississippi where it is consuming forested areas.

  3. July 29, 2019 3:08 am

    Well Jean, I rend to call them, plants which get out of hand in my garden. In Aberdeen the council went to war on the Rhododendron ponticum and won. I understand why they did it but the countryside doesn’t look quite so cheery in June these days.

    • August 5, 2019 8:21 pm

      Alistair, Maine also has some much-loved exotic plants that have been here for so long that many people think they’re natives and that have become or are threatening to become invasive problems. Our parallel to your Rhododendron ponticum is probably Rosa rugosa, which is becoming invasive in our coastal areas.

  4. July 30, 2019 12:53 pm

    Good clarification of an important distinction.

  5. August 1, 2019 1:11 am

    ‘Aggressively invasive’ is how I describe those that are both aggressive and invasive. There are a few here. Some not only crowd out other plants, but also make their conquered area toxic to other species. Blue gum eucalyptus covers the ground with bark that prevents other seedlings from germinating. Many make the fores more combustible than it already is, with is a technique to incinerate competition in a forest fire, and then recover first.

    • August 5, 2019 8:25 pm

      Tony, I think it is easier to get gardeners on board with not growing the aggressively invasive plants that behave badly both in ornamental plantings and in the wild. Our bigger problems are plants like barberry, burning bush and shrub honeysuckles that homeowners love in their ornamental plantings and don’t want to believe are a problem.

      • August 7, 2019 10:39 pm

        Yes. When I lived in town, some of the aggressively invasive plants did not bother me because they had no place to go. In Southern California, we were a bit too close to rural areas to grow things like pampas grass, which would escape into the wild. (It was already there though.) Where I am at now there are so many things that I grew in town that would not be considered here.

  6. August 1, 2019 10:04 am

    Enjoyed your article. I compared your Maine list to the Missouri list for certain species. Those bradford pears!!! Bush honeysuckle as we call it here is the bella variety on your list. These are the two most publicized invasives in Missouri, and seem to be problems in Maine also. Ranunculus is on your list, not ours. But I battled that a few years ago – and won – in my native garden as it was overpowering my natives. My current problem is bush honeysuckle which is invasive in Missouri and in my yard. By the time I manage to get rid of one bush, another has sprung up! Thanks for the article.

    • August 5, 2019 8:38 pm

      Debbie,
      Bradford pears have not yet been detected in the wild in Maine, but they’re on a watch list because they are known to be very invasive. I don’t think they were widely planted as street trees here as they were in the mid-Atlantic states (and maybe in the Midwest, too?). Our big honeysuckle problems are Morrow’s honeysuckle (Lonicera morrowii and Tartarian honeysuckle Lonicera tartarica. Lonicera x bella seems to be a naturally occurring hybrid of these two. If you click on the name of the plant in the Maine list, it will take you to a fact sheet that provides suggestions for how to control the plant, which may be helpful.

  7. August 4, 2019 6:54 pm

    Great post, Jean! My property has been cursed with three invasive plants that were here long before we arrived: bamboo, kudzu, and Boston ivy. There is so much of this stuff in the woods surrounding us that there is no hope of eradicating it, short of something like agent orange. I watch carefully. The Boston Ivy is very close to the woodland garden, but I have drawn a line and said, “Not one inch further.” At least not while I am around. Sometimes, however, I envision our house in the far future, abandoned and crowded by bamboo, with kudzu and ivy fighting for dominance over the roof. Sigh.

    • August 5, 2019 8:40 pm

      Deb, The class I took on invasive plant management this year emphasized that eradication is usually impossible and that we need to focus on control (as you are doing with your “not one inch further” strategy). When I give talks on invasive plants, I emphasize that fighting them is not a “once and done” event, but an ongoing struggle.

  8. August 10, 2019 6:18 pm

    On our invasive alien list are Mediterranean pines and Australian Eucalyptus – both prone to shedding heavy branches in winter storms. But – you know the knee jerk response – it’s not a problem in MY garden. Sigh.

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