Should colorful invasive plants be controlled?

Many of the invasive plants that we deal with in restoration ecology are rather unremarkable in appearance, so one doesn’t mind getting rid of them. But there are also invasive plants that are colorfully attractive and there may be a temptation to let them be. Two species in this category are dame’s rocket (Hesperis matronalis) and creeping bellflower (Campanula rapunculoides). Both are decorative and would appear to be welcome additions to a native garden or prairie. But they aren’t, and should be eradicated.

Fortunately, because these two species are colorful, they are easy to spot. It would be an embarrassment to have a whole field of one of these plants.

Dame’s rocket

Although this plant is often mistaken for a woodland phlox, it is really a mustard (note the 4 petals). Like garlic mustard, it is often found in wooded areas, but it also invades open areas such as roadsides. This is one of those species that is often included in seed packets labeled “wildflowers”. To many people, “wildflower” implies “native”, but this species was introduced from Europe in the 1600s and is highly invasive.

Like garlic mustard, dame’s rocket is a biennial, and this gives a clue to control methods. The number one point is not to let it set seed. Cut the flower heads off or hand pull the plants before seed formation. Like garlic mustard, bag flowering plants, because even pulled plants can go on to make viable seeds.

Large infestations can be sprayed with glyphosate. Also, first year plants remain green in the fall long after native species have senesced and can be sprayed with glyphosate (again, like garlic mustard). [Native species that have senesced will not be affected by glyphosate. The rule with glyphosate is: if it’s green, it will be killed. Thus, when spraying with glyphosate keep an eye out for the green of fall regrowth of native species.]

In contrast with garlic mustard, colorful dame’s rocket is very easy to spot. With care, it should be possible to eradicate dame’s rocket from a site.

Dame's rocket moving into a wooded area

University of Wisconsin-Extension has a good flyer on dame's rocket, available at this link.

Creeping bellflower

Creeping bellflower is a perennial which spreads rapidly via rhizomes. It has the potential to form large clones. Its rhizomes can be up to 6” deep, with vertical storage roots. If cut or mowed, the plant will readily regenerate from rhizomes or perennial roots. The number one point is do not let it get started, because once established it is very difficult to eradicate.

Because of the deep roots and rhizomes, it is virtually impossible to get rid of creeping bellflower by digging without tearing up the whole yard. My approach is to handle it like a woody plant, which means cut the stems and treat the cut stems with 15-20% Garlon 4 in oil. Just a single spritz at the center of each cut stem is all that is needed. The herbicide will be translocated to the roots and rhizomes. [I verified this procedure by marking treated plants and checking them the following year.]

Link to the spritz technique

Large patches can be sprayed with glyphosate or an herbicide labeled for broad-leaf species.

I should emphasize that creeping bellflower is a very undesirable plant, despite its colorful character.

University of Wisconsin Extension has a good flyer on creeping bellflower, available at this link.

Addendum

After the above post was made, we discovered two small populations of creeping bellflower in the Valley Prairie  at Pleasant Valley Conservancy. They were immediately killed  using our standard Garlon 4 in oil spritz technique. This involves finding the lower part of the plant with the bare stem and basal leaves, and spritz both. Within a week they were dead.

Kathie exhuming the base of each  creeping bellflower stem and spritzing with Garlon 4 in oil.
Five or six individual stems growing out of a cup plant patch.

American bellflower

There are two “native” species of Campanula which are attractive and desirable in a restoration. Harebell (C. rotundifolia) is a small, delicate perennial that is found individually or in small patches in dry or rocky habitats where it is free from competition from larger species. American bellflower (C. Americana) is an annual or biennial that reproduces exclusively from seed and is found scattered in wooded or savanna areas.