Tom's Blog

Tuesday, June 3, 2008

Woodland sunflower as an invasive plant

Ever since we began restoration of our oak savannas we have been plagued with the spread of woodland sunflower (Helianthus divaricatus and several other species). It took us a while to realize what was going on here. After all, this is a native plant, and one that forms pretty yellow flowers. What's to dislike?

Woodland sunflower is a clonal species, spreading especially rapidly by means of underground runners (rhizomes). In this regard, it resembles Canadian goldenrod, another native species which is also a pest.

When we first started to see increasing amounts of woodland sunflower, we thought it was a benign plant, and would gradually disappear when it was exposed to full sunlight. Unfortunately, this has not been the case. In fact, in 2007 I saw a clone spread across a mowed fire break into one of our planted prairies and displace the native species. Also, in open woodlands, such as our oak savannas, it quickly becomes widespread.

In 2007 we started controlling this species by spraying medium-sized patches with clopyralid (Transline). This herbicide is quite effective, although it hits most broad-leaved species. I also did some experiments to show that mowing this species will not eliminate it, whereas hand-pulling will. Although exceedingly labor intensive, hand-pulling all the plants in a clone should eliminate it.

This year we decided to have our 5-person intern crew hand-pull some larger clones to see how the other species would respond. We were pleased to see that within the sunflower clone there were quite a few "good" species, including flowering spurge, tick trefoil, purple milkweed (yes!), showy goldenrod, etc. etc. Obviously spraying would have eliminated these "good" broad-leafed plants as well.

Another consideration: rather than wait for a clone to take over and then eliminate it, why not nip it in the bud? In this way I hand-pulled the scattered sunflower plants that had cropped up within a nice patch of yellow pimpernel (Taenidia integrifolium), a great savanna species that is not especially common in our area. Obviously, this is the way to go, although it requires careful observation to detect the few plants that need pulling. Later, if they flower, it should be easier to spot them.

I'd love to hear from others who have had trouble in their savanna restorations with woodland sunflower.


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