BY MARY LOU MAYFIELD
The official definition of a “weed” is a plant that’s growing where it’s not wanted, in competition with cultivated plants. Native plants that are growing where they are not welcome, are often described as “aggressive,” since they are not like invasive plants, which come from other countries. An exception is the California Poppy, which is native to the “lower 48 states,” but classified as “invasive” in Alaska, Canada, and Hawaii.
When I brought some Jerusalem Artichoke (Helianthus tuberosus, a native sunflower) home from the Plum Grove wildflower garden, I was warned that it was a plant that spread aggressively, through the root system and the seeds. Even though it’s a native plant, I’ve had to stay on my toes, to keep it from taking over the flower bed.
A surprise was the Bee Balm (Monarda dydyma), which I planted on the northeast side of the house, and it didn’t thrive because it wasn’t getting enough sun. So I planted it by my back door, on the southeast side of the house, and it thrived in the full sunlight. So I had the bright idea of mixing it with my perennial border, on the southwest side of the house. Not so bright! It was out-competing the native plants I had growing there, including Blue and Cardinal flower Lobelias (Lobelia siphilitica and L. cardinalis). So as soon as it finished blooming, they all came out and I’ll have to stay ahead of the ones that come up in that location, next year. I still have the patch by my back door, for the hummingbirds and butterflies.
Don’t be confused by common, or even scientific names. According to USDA Plants Database, “Canada” thistle (Cirsium arvense) is not native to the Americas! It took over our neighbor’s yard, and even though he has weeded it all out, we still get seedlings. The Native “Field” thistle is very similar, but is taller and has larger blooms. https://xerces.org/sites/default/files/publications/20-017.pdf
The Common Milkweed, whose scientific name suggests that it originated in Asia (Asclepias syriatica), is native to the Americas and is no longer considered a “noxious” weed due to its value to Monarch butterflies. On the other hand, it’s critical that hay farmers keep it out of their fields because large quantities are toxic to livestock. I haven’t had any trouble with it or the “Butterfly Milkweed” (Asclepias tuberosa) spreading outside their designated flowerbeds. https://xerces.org/sites/default/files/2018-05/15-057_01_XercesSoc_Pollinators%2BRoadsides_Are-Milkweeds-Really-Weeds_web.pdf
Canada goldenrod can also be very aggressive. One gardener simply controls her patch the way I do with the Jerusalem Artichoke and Bee Balm.
Another way to discourage over enthusiastic native plants is to “solarize” the area with black plastic, which kills everything underneath without introducing plant toxins. On the other hand, Goldenrod can out-compete Japanese Knotweed (Polygonum cuspidatum).
Some other “aggressive” native plants make excellent groundcovers, outcompeting the invasives. It’s called “competitive exclusion.” I know of one yard in Iowa City that is awash with Virginia Bluebell flowers in the spring. Ostrich Fern can be used to control invasives in shady areas. Other vigorous natives include Mountain Mint, Wild Bergamot, White Snakeroot, Jewelweed, and Virginia Creeper. We have some Virginia Creeper on a trellis near our house, and we have to cut it back to keep it from “invading” the space between the house and the siding!
Golden Ragwort (Packera aurea) can out-compete Garlic Mustard (Alliaria petiolate). Native woodferns (Dryopteris spp), combined with weeding, can overcome Japanese Stiltgrass (Microstegium vimineum). Southwestern wild rye (Elymus glabriflorus) can crowd out Canada thistle. https://www.humanegardener.com/how-to-fight-plants-with-plants/
Sometimes cultivars are sold as native plants, such as Blackeyed Susans (Rudbeckia fulgida) “Goldsturm.” This perennial provides a nice groundcover in my yard, spreading by both root and seed, but does not come true from seeds If I don’t deadhead the flowers, I can expect the patch to eventually be replaced to the native form. http://downtoearthgardenclub.org/2017/07/black-eyed-susan/
Some woody natives can also be aggressive. Our Common Elderberry (Sambucus nigr and Red Osier Dogwood (Cornus sericea) produce seedlings and send out runners that are attached to the original bush. The unlucky ones wind up in the lawn, where they are mowed down. The others will keep sprouting because cutting them down does not remove the connection with the parent plant. https://ipaw.org/aggressive-native-plants/
Finally, there are the native plants that are not welcome anywhere. The Giant and Common Ragweed, are the cause of late summer allergies, not the Goldenrod, which blooms at the same time and is more noticeable because of the color.
Johnson County Master Gardeners of Iowa 