Invasive plants

Japanese knotweed seeds  ©Janet AllenJapanese knotweed seeds along our Onondaga Creek walkway, ready to continue their invasion

Invasive plants are a serious and costly threat to the world beyond our yards. Many of these invasives are planted in home landscapes, and indeed are the source of their spread to the wild.

What's invasive? When we first started our habitat garden, I hadn't heard of invasive plants, but over the years, we've come to think of what some people term "invasive" in three categories:

  • overly eager native plants—not truly invasive
  • the real invasives: non-native plants that are ecological catastrophes—below
  • rapidly spreading non-native plants—at the end of this page

Here are just a few of the commonly-planted non-native invasives:

Non-native, invasive plants

Black swallowwort ©Janet AllenBlack swallowwort invading another state park natural area

Ironically, in contrast to the previous two categories of plants, many non-native invasive plants cause few problems in people's yards. They're generally attractive and may even provide food for wildlife. There are reasons for their popularity!

But they're considered "invasive" because of the serious problems they cause in the world beyond our yards. They're the plants that escape from our yards and outcompete native plants in our forests, meadow, and lakes. A famous example is kudzu, but since no one in Central New York plants kudzu, it's easier for us to judge that to be an invasive plant. What we do plant—however familiar and comfortable they feel to us—can be just as bad.

Black swallowwort seedpod ©Janet AllenThe similarity between this black swallowwort seedpod and our native milkweed's seedpods makes it easy to see that they're related.

While I was walking around Green Lake one day (part of Green Lakes State Park) I noticed this black swallowwort and remarked that it was a shame that it has invaded such a beautiful area. My companion replied, "It's okay as long as it stays out here." For a moment, I didn't understand what she meant, and then I understood. People are used to thinking that it is our yards that are at risk, and anything that happens "out there" in nature is okay. We need to change how we think about this. Our yards are endangering natural areas, not the other way around.

Why "invasive"?

Like most suburban homeowners, we used to have many of these invasive plants. Why not? As young homeowners, when we asked people at garden centers for suggestions or looked at landscaping books, those were the plants recommended. They're generally readily available and quite affordable, too.

At first, it seemed odd that these could be considered invasive. We could see some of our native plants, like jewelweed, going hog-wild in our yard. At the same time, our supposedly invasive plants were just calmly sitting there. How could this be? We learned that much of the invasiveness of invasive plants is invisible to the homeowner. Most of the invasion occurs in natural areas, not in our own yards.

How do they spread? One major way this can happen is through bird droppings. Birds eat the berries, fly to natural areas, as they tend to do, then "plant" the seeds in their droppings, complete with its own little packet of "fertilizer." Other ways they spread are by wind or water. At any rate, these invaders can be seen in our natural areas, and the source is primarily our yards.

Our original invasives Some of the invasive plants we had (but no longer!) in our yard were burning bush (Euonymous alata), buddleia (also known as butterfly bush) (Buddleia davidii), Japanese barberry (Berberis thunbergii), vinca (Vinca minor), English ivy (Hedera helix), Bradford pear (Pyrus calleryana), Japanese honeysuckle (Lonicera japonica), (L. fragrantissima), (L. maackii), (L. morrowii), (L. standishii), (L. tatarica), (L. xylosteum), and (L. x bella)), Norway maple (Acer platanoides), and probably others.

One of the first things we did when we started habitat gardening was to remove these invaders. Except for an occasional seedling that still occasionally pops up, it was easy to eradicate the invasive trees and shrubs—once they were gone, they were gone. Vines such as vinca (Vinca major) and (V. minor) (sometimes called periwinkle), and English ivy (Hedera helix) call for continual effort, but I think we're winning the battle.

Enthusiastic non-native plants

Zebrina  ©Janet Allen
Zebrina: finally conquered?

These are non-native plants that multiply with abandon. We used to have some of these, but as we've gradually replaced non-native plants with native plants over the years, we have fewer and fewer of these.

Technically, like the category above, these probably aren't officially "invasive" either since as far as I know they don't spread into the wild, disrupting native ecosystems (although there's sometimes a time lag of even decades until plants become a problem).

The biggest problem that I see with these plants—at the moment anyway—is that they take up space that would be better used by natives. (And I do like to be cautious when it comes to the possibility of endangering neighboring ecosystems.)

An example in our yard is zebrina, a type of mallow with a pretty little striped flower. After much work and vigilance, I think we've pretty much eradicated it, but each year a few plants still spring up. I can't believe I ever actually paid money for one of these plants! The grower must have laughed to get $6 for a plant that was probably volunteering everywhere in his yard. The space formerly taken up by this plant is now being more productively used by native plants.