Wholeleaf rosinweed looks like a sunflower, but its relatively small central disk of infertile florets reveals it as a rosinweed, genus Silphium. It occurs statewide.
Wholeleaf rosinweed is not quite so tall as most of our other rosinweeds. A perennial wildflower, its stems can be solitary or clustered and vary in their amount of hairiness; hairs, if present, are short. The stem usually doesn’t branch except at the very top. The stems often turn reddish in bright sun.
The sunflower-like flowerheads are in loose, open clusters at the top of the plant, usually with 20–35 yellow rays.
Blooms July–September.
The basal leaves are often withered away by flowering time. The stem leaves are thick, sometimes leathery, sometimes roughened, sometimes smooth, sometimes hairy. The leaves are usually well distributed along the stems; they are mostly opposite, and each pair is turned 90 degrees from the pair above and below it. The stem leaves are stalkless and become slightly smaller at the top of the plant; the shape varies from lance-shaped to oval to heart-shaped to clasping, tapering to a sharply pointed tip.
Key identifiers:
- Height usually 3–5 feet.
- Ray florets mostly 20–35.
- Leaves opposite, stalkless, each succeeding pair changing orientation by 90 degrees.
- Disk of flowerhead relatively small (compared to true sunflowers); disk has staminate florets only, which do not produce seeds.
Two varieties of wholeleaf rosinweed occur in our state:
- Var. integrifolium is a roughened, hairy plant and is scattered nearly statewide.
- Var. laeve is glabrous (smooth) and strongly glaucous (white-waxy coated) and is scattered along the western edge of the state, including the loess hill prairies of northwestern Missouri.
This species sometimes hybridizes with cup plant (S. perfoliatum), and where the two plants occur together, their offspring, exhibiting traits of both parents, may be present.
Similar species: Six Silphium species are recorded for Missouri. Of these, wholeleaf rosinweed, prairie dock (S. terebinthinaceum), starry rosinweed (S. asteriscus), compass plant (S. laciniatum), and cup plant (S. perfoliatum) are relatively common. The sixth species, rough-leaved rosinweed (S. radula), is known only from a single collection from Vernon County in 1965.
To separate rosinweeds from the similar-looking sunflowers (genus Helianthus), see Ecosystem Connections.
Height: 16–80 inches (1⅓ to 6 feet).
Scattered nearly statewide.
Habitat and Conservation
Occurs in upland prairies, loess hill prairies, openings of mesic to dry upland forests, savannas, tops of bluffs, glades, banks of streams and rivers, and rarely margins of ponds and lakes; also in pastures, edges of crop fields, ditches, railroads, and roadsides.
Status
Usually this species is called simply “rosinweed.” Considering that the term applies to all the plants in the genus, including five common Missouri species, it emphasizes the usefulness and clarity of scientific names. The “integri” of the species name “integrifolium” denotes completeness and entirety, referring to the plant’s entire (undivided) leaves, which is one of its notable characteristics. Translating the scientific name to “wholeleaf rosinweed” helps distinguish the plant in lay terms.
Human Connections
Wholeleaf rosinweed contributes to the legions of sunshiny yellow composite flowers that adorn our state in midsummer.
This is a rather coarse plant, but the flowers are showy and it is a hardy, drought-tolerant addition to a natural landscape or as a backdrop to a perennial garden.
Plant scientists have been working to develop domesticated strains of wholeleaf rosinweed that can be used for oil (similar to how sunflower seeds can be made into oil), and for livestock fodder.
- A big benefit that rosinweed has over common sunflower is that rosinweed is a perennial plant that need only be trimmed, while common sunflower is an annual that must be reseeded each year.
- Not tilling the soil reduces erosion and helps maintain healthy soil chemistry and microbiome.
- Wholeleaf rosinweed is also drought resistant and is a good candidate for growing among solar panels.
Rosinweeds exude a gummy resin when cut (hence the name), and Native Americans and pioneers used this exudate as a kind of chewing gum. Most species had folk medicinal uses, too — for example, for pain relief or for treating urinary tract infections.
Ecosystem Connections
Butterflies, bees, and many other insects are attracted to the flowers. A variety of other insects deposit eggs on or in the plants, and their developing larvae eat the leaves or bore into the stalks.
The presence of herbivores, pollinators, and nectar eaters attract crab spiders, parasitic wasps, robber flies, and other insect predators, creating a micro habitat right on the plant.
Birds such as goldfinches eat the seeds, and grazing animals such as deer and cattle eat the foliage.
Wholeleaf rosinweed and other rosinweeds are in their own genus, Silphium, and not in genus Helianthus (sunflowers). Yet the two groups look so much alike! How can you tell the difference? It has to do with which parts of the compound flowerheads produce seeds:
- The disk (center) florets in rosinweeds are staminate (male, producing only pollen) and therefore don’t create seeds; meanwhile, the disk florets in sunflowers are pistillate and create seeds. Then, in rosinweeds, it’s the petal-like ray florets that are pistillate (female) and turn into seeds, while those in sunflowers produce only pollen.
- Due to the above fact, the disks in rosinweeds tend to be smaller in diameter than the disks of sunflowers.
- Looking beneath the flowerhead, the green, leafy outer involucral bracts in rosinweeds are comparatively large and broad compared to those in sunflowers.
