Perfect garden plants don’t exist, but if they did, they would spread rapidly, fill the desired space and then stop spreading. As a gardener, I spend half my time coaxing plants to grow and the other half ripping out or cutting back plants that have overgrown their allotted space or arrived uninvited.
When a plant gets planted in a new place, there’s a risk of that plant out-competing native plants to the detriment of the local ecosystem. A widely known example is Purple Loosestrife (Lythrum salicaria.) From a gardener’s perspective, it’s a lovely plant, but it has spread into wetlands in North America choking out native plants and animals.
Trying to establish a clear vocabulary, here’s my understanding of how to categorize plants based on how much of my garden they might take over.
A vigorous plant grows well. A vigorous plant MIGHT smother a neighboring plant, especially if that plant is delicate.
In my garden, Helleborus orientalis is vigorous. It grows well with minimal effort, gets bigger and fuller every year and spreads slowly.
This is vigorous early spring growth and flowers on a plant grown from seed and transplanted multiple times over several moves.
An aggressive plant grows well enough to threaten the plants around it. An aggressive plant WILL smother its neighbors unless they are equally aggressive. My previous garden included Bee Balm (Monarda didyma) and False Sunflower (Heliopsis scabra ‘Summer Sun’) planted together. I stood back while they fought it out in terrible dry clay soil in full sun.
Artemesia ‘Limelight’ (Artemesia orientalis ‘Limelight’) is an aggressive grower. I spent an hour this week ripping out every bit I could find in one of my flower beds because it was smothering other less vigorous plants. I have no delusion that I’ve killed it; I’m just trying to give the poppies, lilies, and other plants a little breathing room. I try to plant aggressive plants in tough spots where they can take care of themselves and spread without endangering anyone around them.
I see this used two ways. First, a weedy plant doesn’t look nice. Second a weedy plant grows where the gardener wants something else to grow. It’s at least an aggressive grower because it has spread into a place I don’t want it. It may or may not be an invasive grower. Dandelions (Taraxacum sp.), Japanese Stilt Grass (Microstegium vimineum) and Garlic Mustard (Alliaria petiolata) are weedy in my garden Not all three are invasive however. I tolerate some weedy plants and get rid of others. Many people consider violets weeds, but I love them and encourage them in my yard.
Although it’s often used more loosely, I consider plants invasive only if they are are threat to the LOCAL ecosystem. Invasive plants spread very aggressively, and can easily escape one garden and spread a very long way to native areas where they out-compete native plants. They may be attractive, but they’re still ruthless invaders.
I thought for years that I had a lovely patch of Marsh Marigolds (Caltha palustris), but they were actually Lesser Celandine (Ranunculus ficaria) which is an invasive plant. (I’ve learned to tell them apart because Marsh Marigolds have only five petals.)
The two plants doing best in my woods (Japanese Stilt Grass (Microstegium vimineum) and Garlic Mustard (Alliaria petiolata)) are invasive in my state and the surrounding states. To find this out, I checked the invasive plant lists for Maryland, Virginia and Pennsylvania. Resolved to make a dent in the population in my woods, I spent several hours today pulling up Garlic Mustard in the back yard. I might have pulled up 1/4 of it. I think my children are going to pull weeds for Mother’s Day..
On the other hand, I don’t automatically stop growing a plant just because I hear it’s invasive. One of my favorite water garden plants, Water Hyacinth (Eichhornia crassipes) is a horribly invasive plant in warm climates like Florida, but is killed by cold temperatures here and not considered invasive in Maryland. My water gardens are also totally separate from any natural waterway so the water plants can’t spread to local lakes or rivers. This is a case where growing an “invasive” plant isn’t risky in my opinion.
” Legally, a noxious weed is any plant designated by a Federal, State or county government as injurious to public health, agriculture, recreation, wildlife or property. (Sheley, Petroff, and Borman,1999)” As an example, weeds that kill livestock, block waterways, or injure crops might be considered noxious. Many states have noxious weed lists in addition to invasive plant lists. Thistles are on the noxious weed list for Maryland. I pull them up, not because they’re “noxious” but because they’re prickly and aggressive. On the other hand, I’ve grown Papaver somniferum which is on the noxious weed list in some states but not in Maryland.
So, I plant carefully, considering how far plants will spread and paying attention to what is invasive where I live. Safe and happy gardening to all!