How to Grow Silphium Plants

Guide to Growing Rosinweed, Cup Plant, Compass Plant, Pilotweed, and Prairie Dock

Silphium are tall hardy perennials that range from 1.5 m to 3.6 m in height (5 to 12 feet).

They come into bloom from midsummer through to early autumn, when they carry flowers reminiscent of sunflowers in yellow.

Their large size means that Silphium can be grown at the back of a garden border.

Some common names for members of the Silphium genus include Pilotweed, Cup Plant, Rosinweed, Indian Cup, and Compass Plant.

Silphium integrifolium
Silphium integrifolium deamii (Deam's Rosinweed) by gmayfield10.

Silphium Growing and Care Guide

  • Common Names: Rosinweed, Cup plant, Prairie dock, Compass plant, Pilotweed.
  • Life Cycle: Hardy perennial.
  • Height: 16 to 157 inches (40—400 cm).
  • Native: Northern America, North Africa.
  • Growing Region: Zones 3 to 9. UK Hardiness: H7

  • Flowers: Mid-summer through early autumn.
  • Flower Details: Yellow. Ray florets, central disc. One to fifteen flowers per inflorescence.
  • Foliage: Herbaceous. Lanceolate-ovate. Smooth to toothed edge. Coarse. Large.

  • Sowing: 1/2 inch (12 mm) deep. Autumn, using fresh seed. Germination time: three weeks. Chipped seeds should first be sown into flats. Next, sink the flat into the ground in an area that offers shade, preferably close to a wall that faces north. Provide a glass/plastic covering. Keep moist. Bring indoors over winter. Transplant to their final location in the spring or autumn. Space at 36 to 60 inches (90—150 cm).

  • Requirements and care: Full sunlight or light shade. Good drainage. Acidic to neutral soil pH 5-7. Moist soil for best results. Spring feed with low-nitrogen fertilizer. Divide every two or three years to maintain vigor. Provide support. Propagate: by dividing in the spring in cooler areas or in the autumn in warmer areas.

  • Family: Asteraceae (Compositae)
  • Closely Related Species: Daisy, Aster, and Sunflowers.
  • Miscellaneous: Used in Ancient Greece and Rome as a herb for birth control. It was so popular for this use by the Romans that the members of the genus native to Northern Africa went extinct. One of the favorite foods of the pesky (or loveable, depending on point of view) white-tailed deer (Odocoileus virginianus).

How to grow Silphium

When growing Silphium plant species from seed, it is best to sow them outdoors using fresh seed in the autumn.

The seed should be chipped, and then sown at a depth of 12 mm (1/2 inch) into a flat. The flat should then be sunk into the ground in a completely shaded part of the garden; next cover the flat with glass. Germination should take about three weeks.

The plants can then be transplanted into either a sunny or partially shaded part of the garden in the autumn. Or bring the flats indoor for the winter and transplant to the garden in the spring.

Ideally the soil that Silphiumplants grow in will be slightly acidic to neutral (pH5 to 7) and moist.

Caring for Silphium

It is fairly easy to look after Pilotweed, Rosinweed, and other Silphium plants, they like a moist soil so water frequently. Every spring apply a fertilizer that has a low nitrogen count.

Silphium are large plants so supply a stake. Every three years or so divide the plant to maintain vigor. If you require more plants then they can be propagated by division, this should be carried out either in early autumn or spring.

I hope that you enjoyed this guide on how to grow Silphium plants. You may also enjoy the following Gardener's HQ growing guides: How to grow Crepis and Ammobium plants.