Aster family (Asteraceae)
Description: This herbaceous perennial plant is 3-5' tall and unbranched, except near the inflorescence. The stout central stem is usually covered with stiff short hairs, but sometimes becomes glabrous with age. It is usually light green, but sometimes turns red in the presence of bright sunlight. The opposite leaves are up to 5" long and 2½" wide. They are broadly lanceolate to ovate, and have stiff small hairs on both the upper and lower sides, providing a sandpapery texture. The margins of these leaves are usually smooth, or they may have tiny teeth. As they ascend the stem, the opposite leaves rotate their direction by 90°. A panicle of composite yellow flowers appear at the top of the plant, resembling small sunflowers.
Each flower is about 2–3" across, consisting of numerous disk florets surrounded by 12-25 ray florets. Only the ray florets are fertile. There is no noticeable floral scent. Often, there are side stems that bear smaller panicles of flowers. The blooming period occurs from mid-summer to fall, and lasts about 1-2 months. The seeds are large, flat, and lightweight – they can be carried several feet by the wind. The root system consists of a taproot and short rhizomes, which enable this plant to form clumps. Several varieties of this plant have been reported by various authorities, some of which may be natural hybrids with other Silphium spp.
Cultivation: The preference is full sun and mesic to dry conditions. The soil can contain loam, clay-loam, or some gravel. Rosinweed is rarely bothered by disease and is easy to grow. It matures more quickly than many other members of the genus, such as Silphium terebinthinaceum (Prairie Dock) and Silphium laciniatum (Compass Plant). Another nice feature of this plant is that it rarely flops over in the flower garden, if the location isn't on a steep slope.
Range & Habitat: The native Rosinweed occurs throughout most of Illinois, except for a few southern and western counties (see Distribution Map). It is a fairly common plant. Habitats include mesic to dry black soil prairies, gravel prairies, clay prairies, hill prairies, openings in rocky upland forests, limestone glades, and areas along railroads, particularly where prairie remnants occur. This plant can survive significant degradation, and recovers readily from occasional wildfires. It competes well against most prairie grasses and forbs in mesic to dry areas.
Faunal Associations: The pollen and nectar of the flowers attract long-tongued bees primarily, including honeybees, bumblebees, Little Carpenter bees, Epeoline Cuckoo bees, Miner bees, and large Leaf-Cutting bees. Insects rarely attack this plant, although the Silphium Beetle (Rhynchites sp.) may feed on the flowers and seeds, and the caterpillars of the rare Tabenna silphiella (Silphium Moth) eat the epidermis of the leaves. The larvae of a Gall Wasp (Antistrophus sp.) may feed within the stems, forming galls that are invisible from the outside. They attract the hyperparasitic wasp Eurytoma lutea, whose larvae feed on the larvae of the Gall Wasp. Some butterflies occasionally visit the flowers, including Sulfurs and Painted Ladies. Other visitors include short-tongued bees and various flies. The seeds are eaten occasionally by Goldfinches. Small herbivores, such as rabbits, are less likely to eat this plant because of its height and the coarseness of its leaves. However, some large herbivores, such as cattle, readily consume the foliage, stems, and flowers.
Photographic Location: The photographs were taken at Meadowbrook Park in Urbana, Illinois.
Comments: Like other Silphium spp., Rosinweed has a fragrant resin while in flower, which was chewed as gum by Amerindian children. It is less dramatic in appearance than some of its gigantic cousins, but matures more quickly and tolerates drought as well or better. Rosinweed resembles many Helianthus spp. (Sunflowers), but its disk florets are sterile and ray florets are fertile. The Sunflowers, on the other hand, have fertile disk florets and sterile ray florets. Rosinweed tends to produce flowers earlier than the Sunflowers, but sometimes their blooming periods overlap. While this plant can form sizable clumps, it doesn't spread as aggressively by means of underground rhizomes as many Sunflower species, nor is it known to be allelopathic.