Silphium Plant Growing & Care Guide for Gardeners

Article by Dean Ravenscroft, Ph.D. - Updated 27th Oct 2023

In this Gardener's HQ guide, we'll explore cultivating Silphium plants in your garden, indoor spaces, and other settings.

Compass Plant, Rosinweed, & Cup Plant: Cultivation & Garden Use

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 deamii (Deam's Rosinweed) by gmayfield10.

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.

Common Questions

How many members does the Silphium genus have?

The Silphium genus, commonly known as Rosinweed or Compass Plant, consists of about 15 to 20 species.

Do members of Silphium make a good garden or landscaping plant?

Yes, Silphium species can make a great addition to gardens or landscapes due to their tall, striking presence and their attractive sunflower-like blooms.

Which Silphium species are most frequently grown by gardeners?

Silphium perfoliatum (Cup Plant) and Silphium laciniatum (Compass Plant) are commonly grown by gardeners.

Are members of the Silphium plant genus fragrant?

Silphium species are not typically known for their fragrance. They are usually grown for their large, attractive flowers.

What is the perfect location to grow Silphium?

Silphium prefers full sun and deep, well-drained soil. They are often used in prairie-style plantings, meadows, or large perennial borders.

Is Silphium invasive in the USA, if so in which states?

Currently, Silphium is not considered invasive in the USA. Always refer to local guidelines for the most accurate information.

How do I remove Silphium plants from my garden?

To remove Silphium, dig up the plant making sure to remove the deep root system completely to prevent regrowth.

Summary

The Silphium genus consists of perennial plants native to North America. These tall, sunflower-like plants are known for their large, yellow flowers and their deep roots, which allow them to withstand drought conditions.

Plant Silphium seeds or young plants in spring or autumn. They thrive in full sun and prefer well-drained soil. Once established, they are drought-tolerant and require minimal care, making them an excellent choice for wildflower gardens or naturalized plantings.

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.