Silphiums

In the late summer, there are lots of bright colored flowers to see, and among the showiest of these are the four silphium species.  Including compass plant, Silphium laciniatum, prairie dock, Silphium terebinthinaceum, cup plant, Silphium perfoliatum, and rosinweed, Silphium integrifolium, they stand tall with bright yellow daisy-like flowers that wave in the breeze.  The name silphium comes from Greek meaning a resinous plant.  The four plants we will look at today are found in a variety of habitats, and are all easily grown in our gardens.

The cup plant likes sun and moist soil and can be found along stream and river edges, as well as in open woodlands and savannas.  It is 4′-10′ tall, with a single, four-sided stem that only branches for the multiple flowers it supports.  Leaves are opposite, about 4″x 8″ and join together on one side around the stem, forming a cup-like structure capable of holding water.  Cup plants grow in large colonies providing an area for birds to rest out of the hot sun while searching for insects among the foliage and having drinking water readily available in the cups.

Prairie dock is found in wet or dry savannas.  It has a vase-like cluster of leaves at the base of the plant with flowers on naked stalks from 3′-10′ high.  A stout taproot extending nearly 12′ deep supports the plant.  They prefer full sun and moist soil, but can tolerate drought well.  Prairie dock often displays large, glossy black blotches on its foliage.  These are formed by leaf beetles that lay their eggs on the leaf and cover them with a sticky substance that dries to a hard shell for protection.  After the eggs hatch, the larvae will eat the leaf underneath while protected from above.

The compass plant, another species with a long taproot, grows 6′-12′ tall with leaves along the whole stem that gradually get smaller towards the top.  A mature plant can live over 100 years, exhibiting up to 30 flowers in late summer for about six weeks.  Young compass plants will align their leaves from north to south so they receive the cooler morning and evening sunlight.  As early settlers crossed the prairie, they tended to use the alignment of the leaves for finding their way.  However, older plants do not turn their leaves as readily, much to the woe of many wayfarers.

Rosinweed has the shortest stature, standing 3′-5′ tall with a stout, central stem that branches at the base of the flowers.  This is the only one of the four with a fragrant scent from the flower.  Although it grows in sizable clumps, it is not an aggressive spreader from underground roots.  Most clumps are composed of individual plants.  The seeds are a favorite of goldfinch.

All of the silphiums are similar in appearance, with yellow flowers growing on tall stalks.  They bloom in mid to late summer for four to six weeks.  They are easy to maintain and resistant to drought, but may have a tendency to topple over when planted on slopes exposed to heavy wind.  All are native to Illinois.  Their foliage is rarely consumed by small mammals due to its coarse texture, but may be eaten by larger herbivores, including cattle.  The disk florets, inner part of the flower, are sterile, while the ray florets,  outer parts, are fertile.  This is the exact opposite characteristic of that found in sunflowers. 

Silphiums are a favorite of pollinator species including honeybees, bumblebees, miner bees, and ruby-throated hummingbirds.  The flat, dark brown seeds are especially favored by goldfinch.  Consider adding one or two of these plants to your garden for a bright background display.