Description: A mature specimen of this native perennial plant ranges from 6-12' tall. The central stem is thick, light to medium green, and has conspicuous white hairs. There is some branching into flowering stems in the upper part of the plant. The basal leaves are 12-24" long and about half as wide. They are covered in fine white hairs, broadly lanceolate in overall shape, but deeply lobed or pinnatifid. The leaves become much smaller as they ascend up the stem. The inflorescence is very tall and elongated, with yellow composite flowers about 3-4" across. They resemble wild sunflowers in overall size, shape, and structure. However, like other Silphium spp., the small tubular disk florets are sterile, while the ray florets are fertile. There is little floral scent. A mature Compass Plant has 6-24 of these composite flowers, which bloom during mid-summer for about 1½ months. The seeds are large-sized, but flat and light, and can be carried several feet by the wind. A large central taproot can extend 15 ft. into the ground. A resinous substance is produced by the upper stem when the plant is blooming. This plant can live up to 100 years.
Cultivation: The preference is full sun and moist to slightly dry conditions. A deep loamy soil is preferred for the central taproot. It takes several years for a seedling to develop into a full-sized mature plant. Mature plants are easy to maintain, resist drought, and can handle competition from other plants. If planted on a slope, there is a tendency to flop over, particularly while blooming.
Range & Habitat: This is a typical plant of black soil prairies in the tallgrass region. It often co-occurs with Andropogon gerardii (Big Bluestem). Other habitats include sand prairies, savannas, glades, and areas along railroads. Compass Plant is fairly common throughout most of Illinois, except in the SE and scattered western counties, where it is uncommon or absent. This plant recovers from occasional fires readily.
Faunal Associations: Long-tongued bees are the primary pollinators of the flowers, including bumblebees, Miner bees, large Leaf-Cutting bees, and others. Short-tongued Halictine bees and Syrphid flies also visit the flowers, but they are less effective at pollination. Occasionally, Sulfur butterflies and Monarchs may visit the flowers for nectar. Several species of insects are specialist feeders of Compass Plant. This includes the uncommon Okanagana balli (Prairie Cicada), whose grubs feed on the large taproot, while a Rynchites sp. (Silphium Beetle) and its larvae feed on the flower heads and stems. The larvae of Antistrophus rufus and Antistrophus minor (Gall Wasp spp.) feed within the stems, forming galls that are not visible from the outside. Nonetheless, they attract the hyperparasitic wasp Eurytoma lutea, whose larvae feed on these gall formers. Similarly, the larvae of Mordellistena aethiops (Tumbling Flower Beetle sp.) feed within the stems, while the adults may eat the flowers. The oligolectic aphid Iowana frisoni sucks the juices from the flowering stems.
Comments: The common name derives from the belief by pioneers that the leaves of Compass Plant pointed in a north-south direction. While this is probably true more often than not, it is not always reliable. The resin was used by Indian children as a chewing gum. With its imposing heighth, interesting leaves, and abundant yellow flowers, Compass Plant is an extraordinary plant. No tallgrass prairie is complete without a sizeable population of them.