Woodland sunflower; an invasive native species

Woodland sunflower (Helianthus divaricatus, strumosus, etc.) is a highly invasive native species which is in full bloom at present. This is the time when it is easy to see how widespread it has become in some of our savanna restorations. We have been aware of this species for some years, but other problems always have had higher priority. Also, there is no real guidance in the literature on how to control it.

It is a fiercely clonal species, and forms such dense patches that nothing else can become established, as this photo shows.

In a shaded site such as this, flowering is sparse. However, since it invades primarily by spreading rhizomes, flowering is not essential. The stem density here is amazing, lots worse than Canada goldenrod or other clonal species. Since it may also be allelopathic, nothing else has a chance to get started.

Small patches can be hand pulled or dug. It is essential to get the whole root mass, since if cut or broken above ground it will quickly resprout. Note that it cannot be controlled by mowing, despite some published recommendations. The underground root system must be eliminated and the only way to do that for large patches is by spraying.

I have been spraying patches with Garlon 3A (4% aqueous). I found that the best time to spray is late May, after all of the stems in a patch have emerged. However, it is necessary to return to sprayed areas later in the summer, as there are seem to be a few new shoots emerging that escape the herbicide. Therefore, I mark every sprayed patch and plan to return again next year just to be sure.

Once the plants are dead, I have been seeding the sprayed area with native grasses, because the area will otherwise simply remain bare.

Eradication of woodland sunflower is another one of these long-term restoration problems!