These two species have very similar looking inflorescences, with yellow disks and anywhere from 8 to 20 rays; note that despite the specific epithet “decapetalus” (meaning ten petals), thin-leaved sunflower does not always have ten rays.
Both grow from two to six feet tall. Despite this and the similar flowers, it’s easy to tell them apart if you look at some of the details.
Habitat: woodland sunflower, as the name suggests, tends to be found in the shade of woodlands, often in drier soils than the other species. Thin-leaved sunflower is more often found in open areas between trails and the river, where there’s more sun and moisture.
Woodland sunflower has long-tapering, lance-shaped leaves that are either sessile or with very short petioles. The margins have a few slight teeth. The leaf surface is rough to the touch, like sandpaper.
The leaves of thin-leaved sunflower have long petioles, often winged (especially the larger leaves, which are sharply toothed), and the top leaf surface is smooth.
Woodland sunflower can be found in the eastern US and a few states west of the Mississippi. In the Potomac Gorge, it’s not too common. It’s listed as “special concern” in Rhode Island.
Thin-leaved sunflower has a similar range and is much easier to find in the Gorge, especially along the eastern half of Billy Goat C. There are no conservation issues.
Sixty more native species and naturally occurring hybrids of Helianthus are in the continental US and Canada. Seventeen total can be found in Maryland; of these, three are listed as S1 (“highly state rare”) by the DNR.