Celebrating Silphiums

Late July to early August is a great time of the year to be celebrating Silphiums. Scott wrote a Silphium post last summer highlighting the four species we can grow well in south central Kansas. I noticed during a recent walk around the Arboretum how brilliantly all four of these species are flowering right now and felt that they were worth recognizing once again. Review Scott’s post at the link above to become familiar with the four species we have in the Arboretum. I’ll touch on a few additional features of this genus in more depth.

Leaf Orientation and Morphology

Compass plant gets its name because the leaves tend to orient north and south and take advantage of cooler morning and evening sunlight to photosynthesize. When the sun is directly overhead during hotter times of the day, compass plant leaves have less direct sun exposure to minimize heat buildup and moisture loss. Cup plant does the same thing. Go HERE for an article in the American Journal of Botany for more on this topic.

Leaf morphology (or shape and form) plays its part too. Deeply lobed compass plant leaves have greater surface area than cup plant, which may translate to a more efficient heat radiating capacity (think of the function of a car radiator with all its coils and fins to maximize cooling potential).

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Compass plant leaf

 

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Cup plant leaf

Of the four species that Scott highlights, cup plant is the only Silphium species that doesn’t have a native range in Kansas (even though it seems to grow well here). Its range is east and north of Kansas where average precipitation levels are higher and temperatures are lower. Compass plant extends into drier and warmer climates and so this difference in leaf morphology between the two species may be a plausible adaptation for dealing with climate variation. Dense white hairs on Silphium leaves also help reflect sunlight and reduce wind speeds at the leaf surface. Both can reduce moisture loss.

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White hairs on compass plant leaf

Comparison to Sunflowers

Even though Silphiums are in the sunflower family (Asteraceae), they differ physiologically in their seed formation. You probably know that typical sunflower family flowering heads have both disc and ray florets. The ray florets act as sterile pollinator attractors and the disc florets are the seed producers. Our state flower, annual sunflower (Helianthus annuus), is one good example. Silphiums are just the opposite in that the attractive ray florets are the seed producers. Botany geeks are fascinated by these kinds of things, but I’m sure the more casual observer will find this equally as riveting.

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Fertile ray florets blooming on compass plant

 

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Flat, dark seeds have formed from compass plant disc florets

Easy to Grow

When it comes to growing native plants in your landscape, few are easier to establish than Silphiums. That’s the positive way to look at it. If you ask anybody who has had experience growing Silphiums as an ornamental in their manicured landscape, they will probably cite that they are invasive and problematic. Some use more pointed, even colorful language. You won’t notice this for the first few years while they are establishing. But when they start flowering in year four or five and seeds start dropping, that is when the invasion begins. Because each Silphium plant grows to a sizeable diameter of three feet or more, and the plants grow to a substantial height, they can become downright bullies.

So, we still encourage people to enjoy Silphiums for all the apparent reasons…colorful flowers during the heat of the summer, interesting foliage, and a great attractor of all sorts of insects via flowers and vegetation. But plant them in an area such as a prairie restoration or a less ornamental landscaped area where you won’t be as concerned with its aggressiveness or be judicious with deadheading. Keep scale in mind too with regard to the 6-9 feet height of Silphiums and the size of your planting area, as taller plants fit better in bigger planting areas.

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Compass plant has spread and come to dominate this 15 year-old planted bed.

 

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Rosinweed in the foreground and compass plant in the back seem to thus far be balanced in a diversely-planted and highly competitive five year-old reconstructed prairie environment.

 

Finally, if you have identified the appropriate area and decide to add Silphiums to your landscape, you might as well add some complementary purple flowers like ironweed and gayfeather. They bloom at the same time and add visual enjoyment and pollinator sources. These species along with Silphiums will add great interest to your landscape.

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Western ironweed (Vernonia baldwinii)

 

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Kansas gayfeather (Liatris pycnostachya)

Silphiums: Four Pillars in the Tallgrass Prairie

2015 seems to be the year of the genus Silphium in the arboretum.  In recent years, I can’t remember them looking so bright or growing so tall.  With the spring and summer rains these sun-loving, yellow-flowered plants have reached a new level.  In fact, they are among the tallest plants of the prairie in late summer and autumn. We grow and sell four species: Prairie Dock, Silphium terebinthinaceum; Cup Plant, Silphium perfoliatum, Compass Plant, Silphium laciniatum; and Rosinweed, Silphium integrifolium.  Each of these are distinct and easily identified by their unique leaves.

 

Prairie Dock (Silphium terebinthinaceum)

I call this the Hosta of the prairie. It almost has a tropical-look to it, with large rough leaves up to one foot wide and two foot long.  It will make a statement in the landscape, but give it plenty of room and keep it away from walkways, because the long stems tend to arch over the path.  Individual clumps can become large over time reaching six feet in diameter.  The yellow flowers develop in mid-August atop tall leafless stalks.  This member of the tallgrass prairie is one of my favorite wildflowers.  In my opinion, Prairie Dock is a must in your wildflower garden.

Dyck Arboretum Blog: Silphiums, Prairie Dock

Prairie Dock

Dyck Arboretum Blog: Silphiums, Prairie Dock

Prairie Dock with Missouri Black-eyed Susan

 

Cup Plant (Silphium perfoliatum)

A natural bird bath in the landscape.  Each pair of leaves clasps around the stem forming a small basin.  When it rains, these crude cups fill with water that is then available to wildlife.   They stand tall in the landscape and therefore work well as a screen.  I have also used them as a dark green background for other shorter perennials like black-eyed Susan, and gayfeather.  The yellow blossoms can be seen starting in July and are visited by a host of butterflies.  Later, birds cherish the seeds.   Cup Plant thrives in heavier clay soils or even wet conditions.  It will be happy in any setting if given ample sunlight.

Dyck Arboretum Blog: Silphiums, Cup Plant with water

Cup Plant with water

Dyck Arboretum Blog: Silphiums, Cup Plant Flower

Cup Plant Flower

 

Compass Plant (Silphium laciniatum)

Do you need directions?  This is the plant that can help.  The interesting basal leaves look like flat hands.  Those lower leaves usually orient themselves north-south to minimize exposure to the intense summer sun, hence the descriptive common name.  These extremely tall (up to ten feet) wildflowers are found in prairies and glades throughout the eastern third of Kansas.  Each stem is covered with tiny white hairs that give it a rough, bristly feel.  The bright yellow flowers emerge along the upper parts of the plant in summer.  Split or broken stems exude a clear sticky resin much like pine sap.  Native Americans used this resin as a mouth-cleansing chewing gum.  I think I will stick with Trident®.

Dyck Arboretum Blog: Silphiums, Compass Plant

Dyck Arboretum Blog: Silphiums, Compass Plant

Compass Plant Leaf

Rosinweed (Silphium integrifolium)

Rosinweed is shorter, but just as tough as the other Silphiums.  Again, the name describes the resin exuded if the stem is bruised or broken.  The golden yellow flowers that mature at the top of the stems are beautiful in the summer.  It is a pollinator magnet, attracting bees, butterflies and even hummingbirds to the flowers.  It becomes a natural bird feeder in the fall and winter as the seeds are devoured by birds.  It is quite drought tolerant once established and is at home in a wide variety of soils.

Dyck Arboretum Blog: Silphiums, Rosinweed

Rosinweed

 

While each of these wildflowers are unique in appearance, especially as you look at the leaves, they all have that “clear, sticky juice” that exudes if the stem is damaged.  I love them in the landscape, but they need room because they grow so tall.  They are great in prairie settings or areas on the periphery of your yard.  You can’t go wrong – just give them plenty of sunlight so the sunflower-like blooms can brighten your summer landscape.

Each of these plants can be purchased at our FloraKansas Fall Plant Sale, September 11 to 13.