Invasive sunflowers (Helianthus divaricatus)

Last year we thought we might be able to keep woodland sunflower under control, but this year it is worse than ever. Perhaps the long, cool spring is responsible. The scene is especially bad in what we sometimes call the Basin Savanna, the area just above the Pocket Prairie. This is predominantly a white oak savanna, although there are scattered bur oaks.

The photo here is an example of what this area looks like when the sunflower gets to spread its wings.

Large clone of Helianthus divaricatus. So dense that no other species can compete.

We are learning how to control this plant, although it is time-consuming and expensive.  Pulling is completely ineffective since pieces of rhizome are left behind and quickly send up new shoots. Spraying of the whole clone can be done. (Any herbicide that kills broad-leaved plants will work.) However, killing the whole clone leaves behind a large bare area in which other invasives can become established. Seeding the bare area with "good" species can be done, but the natives take several years to get established and in the meantime undesirables such as bluegrass or smooth brome generally take over.

For small clones, such as the one shown below, herbicide will work because at this early stage of the growth process a residue of good plants is still present beneath or nearby. A brief "spritz" in the meristem region will kill each stem  in a day or two. (The small red areas show where the herbicide was sprayed.)

A small clone in which every plant has had the herbicide spritz (note red areas).

 The photo below shows another situation that is most disturbing. A site where purple milkweed (Asclepias purpurascens), an endangered species, has been growing for a number of years is being taken over by the sunflower. To stop this, I carefully spritzed each sunflower plant, taking care not to hit any milkweeds. (There were four milkweed stems near the single sunflower stem.)

A woodland sunflower that has come up inside an area of the endangered purple milkweed. The sunflower plant was carefully spritzed so that the milkweed was protected.

We are at the beginning of a long process. In 2009 we located 76 sunflower clones by GPS. Many of the smaller clones have been eradicated, but the large ones are still present. Our preferred control method now is to use the herbicide spritz on the circumference of a clone, thus stopping it from spreading. This won't eradicate the clone but will keep it under control.