Astery Things #7: Two Sunflowers

Last week when I drove to Lock 6 in search of the two Verbesina species, I was pleasantly surprised to find two Helianthus species growing in the same area. Sunflower species can be tricky to differentiate, but I’m fairly confident that these are H. decapetalus and H. tuberosus. Here’s a closer look at both.

First thing to note is that these were likely planted here, and are growing in rather rough conditions (lots of foot traffic, dogs, invasive alien plants, lawn mowers), so neither stand is showing the plants at their best. Depending on what source you consult, both thin-leaved sunflower (H. decapetalus) and Jerusalem artichoke (H. tuberosus) stand anywhere from half a meter to two meters tall. Many sources seem to agree that generally, the former is not as tall as the latter.

The leaves of each species look very similar. The largest leaves of H. tuberosus are usually somewhat larger than the largest leaves of H. decapetalus, but keep in mind that there is a lot of morphological variation in Helianthus species. Note that in this picture, the H. decapetalus leaf has more pronounced teeth, which is another general characteristic. Serrated margins like this are best observed on lower stem leaves. Also, note that both leaves are winged to some extent. Both leaf margins and presence of petioles are characteristics that can vary widely, even on the same plant.

Here’s another look at the leaves. Can you see that there’s a textural difference? Although not entirely smooth, H. decapetalus leaves are much smoother than H. tuberosus leaves, which feel like sandpaper. (In the above left photo, H. tuberosus is on top and H. decapetalus is on bottom. In the above right photo, H. tuberosus is on the right and H. decapetalus is on the left.)

Another, more obvious difference is seen in the stems.  H. decapetalus stems are mostly smooth, maybe with a few short hairs. H. tuberosus stems are covered in short hairs that stick straight out.

About the flowers… Clearly they look a bit different. Don’t let the specific epithet decapetalus fool you, because this species does not always have ten ray florets (“petals”). It will usually have about ten rays, though. H. tuberosus has about ten to twenty, though some sources say as few as six and as many as twenty-five. In these two populations, the ray florets look a little different, a little wider and rounder on H. decapetalus, and a little longer and narrower on H. tuberosus.

As with the verbesinas in my last post, I caution you against using any one characteristic, or any one flower head or leaf, or any one section of stem, to determine the identification. You really need to look at the entire plant – several leaves, several flower heads, several stem sections – to be sure.

Helianthus identification is fraught with peril. It’s highly likely that I have H. tuberosus here, but there is one other Helianthus in this area that has hairy stems: H. hirsutus. These two look similar at the stem, but the latter species has much longer and narrower leaves, which are either sessile or with very short petioles. H. hirsutus is listed S1/highly state rare in Maryland. I’ve never found it, so I can’t share any pictures to illustrate the point.

Another tricky ID involves H. strumosus (pale-leaved sunflower), which is hard to distinguish from H. decapetalus. The major difference is seen in the phyllaries (aka involucral bracts) on the underside of the flower head. In H. strumosus, they don’t usually spread out past the width of the disk of the flower head, while in H. decapetalus, they do spread beyond the disk. I’ve never seen H. strumosus. The picture above shows the involucral bracts of H. decapetalus.

Here’s one more Helianthus, just for fun: H. divaricatus (woodland sunflower). The differences should be obvious.

Flower of the Day: Jerusalem Artichoke

aka sunchoke,topinambour; Helianthus tuberosus; Asteraceae (aster family)


The Jerusalem artichoke is from the New World (not Jerusalem), and is only distantly related to the globe artichoke (Cynara cardunculus) that you buy in the grocery store, though they are both in the aster family.  Globe artichokes are a type of thistle, actually, while this plant is a type of sunflower with an edible root.  It grows up to 10 feet, with characteristically rough, hairy stems and leaves:



Jerusalem artichoke is native to most of the US except the desert southwest, and is considered a weed by some authorities.  In this area you can find it on the forest edge along the riverbanks.

Flower of the Day: Hairy Hawkweed

aka queendevil; Hieracium gronovii; Asteraceae (aster family)


Unlike the big sunflowers and coneflowers I’ve posted about in the last several days, this little plant grows in the dry soils along the rocky bluffs well above the river.  Like its close relative rattlesnake weed (fotd 5/31), this composite flower is comprised only of rays.

The flowering stem of hairy hawkweed grows only one to three feet tall, and might show a few small leaves; otherwise the plant has only a very low basal rosette of leaves:


It’s found from Quebec and Maine (or not; it’s listed as possibly extirpated in Maine) south through Texas and Florida, but not in Vermont and New Hampshire.


By the way, I swear I did not re-use the picture from May 31! These two plants are in the same genus and the flowers are almost identical.  Indeed, when I first spotted this plant I thought I’d found an extremely late-blooming rattlesnake weed; I only realized it was something different when I looked at the basal rosette of leaves.


Flowers of the Day: Sunflowers

woodland sunflower, Helianthus divaricatus 

thin-leaved sunflower, Helianthus decapetalus

Asteraceae (aster family)


Both of these woodland species share a similar range across the eastern US. The woodland sunflower likes conditions a little shadier and a little drier, while thin-leaved sunflower likes a bit more sun and moisture.  Woodland sunflower will grow three to six feet tall, with flowers one and a half to three inches across.  Thin-leaved sunflower will grow to five feet tall and can have slightly larger flowers (two to three and a half inches across).

(both images above: woodland sunflower; below: thin-leaved sunflower)

The most obvious differences between the two lie in the leaves. Woodland sunflower leaves are blunt at the base, sessile (the leaf base touches the stem of the plant), and are exceptionally long-pointed at the tip.  Thin-leaved sunflower has a slightly long-tipped leaf that narrows dramatically into long, winged petioles.