Pests & Diseases

Understanding Invasive Plants

Invasive purple oaxalis

The simple fact is this: Garden centers and nurseries can sell plants that end up liking your garden too well – and it’s up to you to keep these invasive thugs out of the garden. While most of these weed hooligans are exotic non-natives, there are also a couple of native plants whose habits might not be compatible with your garden.

In Dale Batchlor’s excellent article, “Getting to Know Your Weeds,” in Triangle Gardener, she mentioned that Arum italicum had become a weed in her garden. I could have embraced her because I, too, was attracted to its winter foliage, only to find it had become a horrid scourge in my garden.

Plants Out of Control

Years ago, I planted Oxalis triangularis, a handsome purple-leafed shamrock that I thought would be a good accent plant. Its purple leaves contrasted beautifully with my other green-leafed plants – and it produced innocent white flowers that I thought were charming. They remained charming for exactly one growing season.

After that, I had a growing surplus of purple oxalis, a surplus that I couldn’t get rid of. A surplus that grew on anything that stood in its way. A surplus that was so firmly rooted that I couldn’t pull it out. Now, I cut it back at every chance I get in an effort to exhaust it. We have even used glyphosate on it, all to no avail.

On the recommendation of a good nursery, I bought some pots of river oats, Chasmanthium latifolium. Now, this Missouri native has delectable seed heads, seed heads that enchant. Alas, these seed heads spewed forth gallons of seeds, all of which managed to germinate in my garden. To compound the problem, all the baby plants were so firmly rooted in that they had to be dug – not pulled – out.

This leads me to broaden my definition of a noxious weed: A weed is, among other things, a seedy, deep-rooted plant.

And now I shall commence with my yearly hellebore complaint. I have no problem with H. foetidus as it’s very easy to pull out the seedlings. However, H. x hybridus is another matter entirely. Have you ever had a neighbor who grandly announces that they have an abundance of hellebores for the taking? And then they’re surprised when no one takes them up on this generous offer?

This hellebore is a profuse seeder and soon you will have a sea of hellebores, not necessarily a charming garden scene. Like the river oats, it’s difficult to pull the babies out of the ground so the only solution is to cut off all the pregnant flowers before they can give birth. And what good are flowers you cannot enjoy?

The plants I fear the most are those that are heavy seeders and send out deep roots. I will happily pull out pokeweed or chickweed but I curse those multiplying plants that insist on clinging to the soil.

You might treasure your hellebores, welcoming them as a groundcover. You might be willing to put up with the purple shamrock – perhaps it even behaves in your soil. However, the longer you garden, the more likely it is that you will come across plants that drive you crazy. It makes sense to determine which plants you dislike, and more importantly, to determine what qualities cause this aversion.

When I first started to garden, I assumed that all the plants for sale at nurseries and garden centers could be safely planted. And perhaps they can but this doesn’t mean they’re safe in your particular garden. Once, thinking a groundcover would work for me in a shady area, I planted Euphorbia amygdaloides var. robbiae that I had picked up from a reputable nursery.

To say that I regretted planting it is an understatement. It spread while I was looking at it. This is a euphorbia that would have happily scaled Mt. Everest. It spread via seeds and stolons. This was a euphorbia that was euphoric about life. For two years I waged a constant battle against it until I was finally able to pull out the last remnant.

Now some of these plants I have outlined might perform beautifully for you. They might be contained in different soil. The moral of this story is not that you shouldn’t buy them. The moral of the story is that you cannot assume a plant that is for sale will work in your garden. The answer is to do your research first. In my mind these plants are weeds but remember this:
One man’s weed can be another gardener’s treasure.

After joining the Durham County Extension Master Gardeners in 2003, Kit Flynn now has emeritus status. She writes gardening articles for the Durham County Extension Master Gardener newsletter, an online magazine “Senior Correspondent,” and “The Absentee Gardeners” column for “The Blowing Rocket” with Lise Jenkins.

Copy link