Eryngium yuccifolium aka Rattlesnake Master

Eryngium yuccifolium is a perennial in the carrot family. Gardeners know it as rattlesnake master or button snake-root. It comes from the central and eastern United States. It grows in prairies, glades, and open woods. In gardens, plants reach 4–5 feet [1.2–1.5 metres] tall. They spread 2–3 feet [0.6–0.9 metres] wide. Some records reach 6 feet [1.8 metres] in height. RHS gives a smaller height range of 1.6–3.3 feet [0.5–1 metre].

The plant forms a clump of stiff, narrow leaves. Their sword-like shape recalls yucca, though the plants belong to different families. Leaf colour ranges from green to blue-grey. Parallel veins run along each leaf, with spines along the edges. The longest leaves reach 3 feet [0.9 metres]. Green, upright stems rise from the leaf rosette. These stems branch near the top and carry rounded flower heads. The coarse leaves give the plant a strong shape in borders.

Tiny greenish-white flowers crowd into heads about one inch across. Each head looks like a small, pale thistle. Flowers have a sweet scent and bloom from June to September. NC State gives the shorter season of June to August. Bees, flies, wasps, and butterflies visit the flowers. Soldier beetles also feed on the pollen. The plant hosts larvae of the rattlesnake-master stem-borer moth. Brown schizocarps follow the flowers, with two seeds in each fruit. The dried heads add interest through winter.

Grow this plant in full sun with soil that drains well. It prefers sandy soil with little organic matter. It also grows in loam, clay, and shallow rocky ground. It tolerates drought, though flowers fade when water becomes too scarce. Rich soil or shade can make the stems flop. Its deep taproot makes moving an established plant difficult. It can seed itself where conditions suit it. Missouri Botanical Garden lists USDA zones 3–8; RHS rates it H4. RHS also advises keeping it free from winter wet.

Use it in native gardens, prairie plantings, rock gardens, or borders. The flowers work fresh or dried in arrangements. Keep the stiff leaf spines away from paths. Voles may eat the crowns in winter. Slugs, snails, and eelworms can also cause harm. Root rot and powdery mildew are possible problems. Native Americans used leaves for woven goods and roots in medicine. PFAF lists no known food uses or hazards. That does not establish safety for eating or medicinal use.