Silphium laciniatum aka Polarplant

Silphium laciniatum, or compass plant, is a tall herbaceous perennial. It belongs to the daisy family, Asteraceae. Its native range spans southeastern Canada and central and eastern America. It grows in prairies, glades, and open, disturbed sites. These include roadsides and land beside rail lines. It is a plant of temperate climates, not native to Britain.

Plants grow upright on stiff, hairy stems that hold resin. Sources describe heights of 6–12 feet [1.8–3.7 m]. RHS gives an upper range near 13 feet [4 m]. Plants spread about 1.6–3.3 feet [0.5–1 m]. A deep central taproot anchors the plant. Chicago reports roots reaching 15 feet [4.6 m] below ground. The large basal leaves have deep cuts toward the midrib. They reach about 1.5–1.6 feet [0.45–0.5 m] long. Leaves get smaller farther up the stems. The hairy leaves have a scent, and the foliage dies back. Their north-south alignment gave rise to the name compass plant.

Yellow flower heads look like small sunflowers, with yellow centres. They reach about 5 inches [0.13 m] across. Loose groups of blooms rise along the upper stems. Flowering starts in summer and can last into early autumn. Chicago lists a bloom season from July through October. Mature plants may carry as many as 30 flower heads. The flowers draw butterflies to beds and native plant gardens.

Give compass plant full sun and moist soil with good drainage. RHS also allows partial shade and recommends deep, fairly fertile soil. Loam and clay suit it, with neutral to alkaline pH. Missouri Botanical Garden notes that it also tolerates poor soil. Chicago lists USDA zones 3–7, while RHS rates it H7. Plants take time to settle and may skip flowering initially. The first blooms may wait until year two or three. Plants can live for decades; Chicago reports lifespans near 100 years. Use its height at the back of borders or in prairie plantings. Raise new plants from seed or by division. Cut stems after bloom, or leave them until before spring growth.

Sources report few major pest or disease problems. PFAF lists no weed concern. Resin from the upper flowering stem has served as chewing gum. PFAF also records past medicinal uses of resin, roots, and leaves. These include remedies for coughs and preparations that cause vomiting. The same source reports possible toxicity, so food use needs caution.