Having trampled through many prairies in eastern
Nebraska, I have become very familiar
with the big burly plants from the genus Silphium, also known as
rosinweeds. In the prairie, these tall, bold-textured plants with bright yellow
sunflower-like blossoms take center stage in late summer, a focal center among
a sea of grasses. In the garden, these impressive, deep-rooted plants make a
dramatic and pleasing addition to the back of a mixed border. The tall, stiff
flower stalks add an interesting vertical element to the garden and the big
leaves add contrast and weight to fine-textured grasses. On the prairie “plant calendar” the Silphium
blossom is a sign that summer is almost over, while the sunflower or Helianthus
blossom ushers in the fall.
The name sunflower refers to their habit of facing the sun from sunrise to sunset. Most of us are familiar with the common annual sunflower growing along field margins and roadsides, but the perennial sunflowers are also important prairie forbs. Ripening in fall, the seeds are ready just in time for migrating birds to gorge on as they make their way south.
In the flower garden these beauties spread rapidly by rhizomes to form impressive colonies. Sunflowers work best either competing with equally aggressive prairie plants like big bluestem and Indiangrass to keep them in check, or planted in an island surrounded by mowed grass. They’re very drought-tolerant and able to break through heavy soils; but if the soil is too rich or they get too much water they will flop over when blooming.
If you’re looking for a classic, tough prairie plant to add some color late in the season, find a place in your sunny border for these bold beauties.
Compass plant, Silphium laciniatum. The deeply cut, rough, leathery leaves of compass plant can grow up to 20” long, like a giant oak leaf. Mature plants send up a massive flower stalk that can reach up to 10’, with clusters of large yellow flowers along the stem. Prairie grasses like little bluestem and prairie dropseed make nice, fine-textured companions to the big leaves. Most Silphiums need at least three years to mature but will reward you by living for decades.
Rosinweed, Silphium integrifolium. The dark green, thick leaves of rosinweed, stiff and rough to the touch, are a nice complement to the bright yellow flowers clustered at the top of the stems. A mature plant will give rise to many stout, erect stems that may reach 5’ high and 3’ wide. A dependable performer and one of the best-behaved for the garden, growing well in a variety of soils from moist to dry and excellent in clay. Try as a backdrop for other perennials or grasses.
Downy sunflower, Helianthus mollis. Beautiful butter-yellow flowers, blooming in August, complement the soft, grayish-green leaves. Growing up to 6’ tall, this species is also called ashy sunflower because the fuzzy leaves look like they’ve been rubbed in ashes. Creeps slowly by rhizomes, making this one of the best-behaved of the sunflowers. It prefers dry, well-drained soils.
*The following natives are aggressive and best limited to prairie gardens; they are not recommended for home garden usage.
*Cup plant, Silphium perfoliatum. According to Neil Diboll of Prairie Nursery, “the cup plant is the single best species that you can plant for wildlife.” The bright yellow flowers are a favorite of butterflies and it ranks as the number one plant for birds, providing food (birds devour the nutritious seeds in fall), water (leaves clasp the square stems to form little cups that can hold rainwater) and the large foliage also provides cover. Give it plenty of room, because this big boy can grow up to 8’ high and it self-seeds readily in open soil.
*Common sunflower, Helianthus annuus. This species is the wild-growing ancestor of the commercially grown sunflower. It was reportedly cultivated by American Indians who selected for plants with large seeds.
*Sawtooth sunflower, Helianthus grossererratus. This sunflower is very vigorous, forming large, robust colonies. It can be recognized by its coarsely toothed leaves and large clusters of bright yellow 2-3” flowers. This species grows naturally in rich bottomlands and wet prairies.
*Maximilian sunflower, Helianthus maximilliani. This impressive sunflower has bluish-green, sickle-shaped leaves folded into a trough shape and arched. In late summer stout stems may reach over 6’ high, with big yellow flowers along the top 3’ of stem. The stiff stems serve as perches for seed-eating songbirds in fall. Very aggressive.
*Prairie sunflower, Helianthus pauciflorus or H. rigidus. This widespread species is also called stiff sunflower in reference to its stout, erect stems. The blossoms, occurring singly at the tip of the central stem flower, are about 2½–4" across and have dark red centers. It grows to 6’ high. Easy to grow, but can spread and become very aggressive.
*Jerusalem-Artichoke, Helianthus tuberosus. This sunflower is neither from
nor an artichoke. It is a perennial sunflower that produces tubers 3-5” long
that have a sweet nutty flavor. Cultivated for centuries, the “earth apple” has
large coarse leaves, rough leafy stems and clusters of bright yellow flowers in
fall. Very aggressive and can take over an area if left undisturbed.