Invasive or Aggressive Plant?

Can a native plant also become invasive? I get asked this question a lot and it is more than a simple yes or no. The short answer is yes, when a native plant takes over your flowerbed and prevents or crowds out other flowers, both native and non-native, from growing.

What is an invasive plant? The National Invasive Species Council adopted the following definition: “a species that is non-native to the ecosystem under consideration and whose introduction causes or is likely to cause economic or environmental harm or hurt human health.”

Aggressive Native Plants

Two candidates for aggressive native plants come to mind immediately. These aren’t causing economic or environmental harm, but can be ‘thugs’!

• Canada Goldenrod (Solidago canadensis)
• Ostrich Fern (Matteuccia struthiopteris)

In my own garden, I have a meadow dominated by Canada Goldenrod which spreads rapidly by rhizomes. Where the Goldenrod grows, it out-competes every other plant in that area. I killed a good portion of the Goldenrod by solarizing the plant with black plastic, so that I could diversify and plant some other native wildflowers that would not have a chance growing near this aggressive spreader.

Goldenrod is such a great late nectar source for native pollinators, that I don’t completely remove it and rather control it to a relatively small area. Leaving it standing all winter also creates habitat for small mammals and overwintering insects.

Sometimes aggressive native plants like Goldenrod are the answer to outcompete truly invasive plants that are imported from other ecosystems, like Japanese Knotweed, aka Godzilla Plant. This import from Asia probably could survive a nuclear bomb and thrive. As a beekeeper, my honeybees flock to this invasive in droves, so they are getting some benefit from the plant.  But even though Knotweed provides nectar, this plant forms thick dense colonies that completely crowd out any other herbaceous species and can even damage concrete foundations and roads! – the ultimate definition of an invasive.

Lesser Celandine

Another invasive import that I have experience with is Lesser Celandine (Ranunculus ficaria).

Lesser Celandine was originally brought to the United States as an ornamental plant like so many other invasives. The spread of this plant is unbelievable. Planted in Cleveland, Ohio, in two flower beds in the 1970’s, it escaped and less than 40 years later, Lesser Celandine had taken over nearly 300 acres of parkland along the Rocky river, leaving little room for native vegetation. It has since spread throughout the U.S. Emerging early in the spring, Lesser Celandine sprouts and forms a thick glossy mat of foliage and golden flowers and rolls over most native ephemerals (disappearing by summer), like Spring Beauties, Trilliums, and Bloodroot, which native pollinators depend on.

Should I Plant Aggressive Natives?

The answer to ‘aggressive’ native plants is to plant them in the right spot. For my Goldenrod, the plants are growing in a meadow where they are hemmed in by a tree line on one side, a mowed lawn on another side, and my efforts in maintaining a wildflower meadow in the front. Milkweed is the dominant plant in my managed wildflower meadow and helps keep the Goldenrod in check. When the Goldenrod blooms in the fall, the meadow is alive with pollinators darting in and out, providing a nectar and pollen source when food is scarce.

Ostrich Fern (Matteuccia struthiopteris) is a good example of another ‘aggressive’ native. But the very nature of this plant, spreading rapidly to cover a shady or part-shady area, makes it valuable as a landscape specimen. I plant it as a ground cover in shady areas where other plants would struggle. Also, it is beautiful, and the spring fiddleheads are edible!  Just be sure to plant it in a large area where it can spread out and do its thing.

The ideal native garden will have a balanced assortment of species, much like the Delaware Botanic Garden. Having at least 70-80% natives with non-natives is a good mix with each plant adapted to keep others in check as well as provide food and shelter for many kinds of insects.

Your guideline for all plantings is to plant the right plant in the right place, and use regionally appropriate, site appropriate plants, plus do your research!