Herbicides for control of woodland sunflowers

Woodland sunflowers (Helianthus spp.) are showy plants but highly invasive. Although they are found mainly along wood edges, they often remain after woods are cleared of invasive shrubs and trees. They not only continue to thrive, but are able to spread into native prairies. Last year I observed two patches of woodland sunflower in the Pocket Prairie which had spread by underground runners from a nearby savanna area.

Although the flowers may be nice, too much of a good thing is bad. Woodland sunflowers are definitely plants out of place in prairies and savannas and are therefore classified as "invasives". Once they get started in an area they crowd out all other native plants, thus greatly reducing species diversity.

I have been unsuccessful in eliminating these plants by mowing, a technique that often works for Canada goldenrod. They quickly recover and come back the following year with undiminished vigor. Hand pulling may work, although it is difficult to remove all of the underground root stock.

The best bet would seem to be a broad-leaf herbicide. I did some preliminary experiments last year with clopyralid (Transline), an herbicide very effective against members of the aster family. The patches were killed, but I was not sure if the underground root stock was eliminated. This year I am repeating the clopyralid work and adding another broad-leaf herbicide, triclopyr (Garlon 3A).

The two photos below show the response just a few days after spraying. Garlon (upper photo) and Transline (lower photo). The plants are obviously sensitive. The plots are permanently marked so that I can return to them next year.