Woodland sunflower: still out of control

Present distribution of two species of invasive clones at Pleasant Valley Conservancy

The GIS-based map above shows the distribution of two invasive species that are especially difficult to get rid of. We learned how to get rid of sumac and it is mostly gone. Woodland sunflower (mainly Helianthus divaricatus) is another problem. Although we have found some techniques that work, it spreads so fast that we can't keep up with it using our limited resources.

The map above shows the distribution of woodland sunflower when we first started to work on it, in 2009. Marci and Amanda did the survey in August 2009, when this species is easy to recognize because it is at peak flowering. Each clone was located by GPS and entered into an Excel spreadsheet. There were 95 clones, with wide variation in size. I chose some of the smaller clones for experimental work, and was able to eradicate them. (More on that in another post.) However, many of the clones were too large to eradicate.

The photo below shows the extent with which this species has spread in our fine White Oak Savanna. Where the dense patches are present no other plants are growing. The area seen here once had fine populations of cream gentian and Culver's root.

Woodland sunflower spreading widely in the White Oak Savanna (Unit 12A). This photo was enhanced in Photoshop to bring out the color of the patches. Almost everything that is lighter green is sunflower.

This species of Helianthus grows rapidly and is able to replace almost any other species. In fact, some backyard gardeners plant it in order to eradicate garlic mustard! The stem densities are very tight and nothing else is able to grow between or underneath. (Some non-native grasses might struggle along.)

I have also seen this same species in other nearby sites. In fact, it has invaded the planted prairie of one of my neighbors. We have had it invade several of our planted prairies, moving out of nearby savannas. The photo below shows it growing out of an oak woodland into a nearby field.

Woodland sunflower spreading out of an oak woodland into a surrounding field.

Lately I have been using the Internet to find photos from various sites showing monocultures of woodland sunflower. Recently there was a photo of sunflower taking over and destroying the diversity of Vestal Grove, a classic oak savanna restoration in the Chicago area. The managers were wondering whether they ought to do something about it. Yes!

A brief search of the Internet will reveal that sunflowers in general, including this species, are allelopathic and produce toxins that will kill (or prevent the growth of) other species. They are also autotoxic and inhibit their own growth, so that the center of  a clone may be partially bare. However, according to my observations, they don't eradicate themselves, since they are always growing further and further away from the toxic zone.