The Gentle Giant

The cup plant is big and beautiful. Photo: meadowviewfarmandgarden.com

The gentle giant I’m referring to is a very large and very attractive, easy-to-grow perennial called the cup plant (Silphium perfoliatum). You commonly see it in botanical gardens, but not as often in home gardens and I think it’s time to change that, so I’m sharing here what I know about this stunning goliath.

In the wild, the cup plant found throughout much of eastern North America. Once limited mostly to moist prairies, flood plains and open woodlands, it’s one of those plant that has greatly profited from human intervention. As the great eastern forests were cleared and farmland installed, it moved into hedgerows and ditches and settled in along railroad tracks. 

This plant is easily recognizable by its huge opposite dark green and shiny leaves, triangular in outline, because they are perfoliate, that is to say that they are welded together at the base, which gives the impression that they are pierced by the thick stem which is, curiously, winged and square. No other plant looks like that, guaranteeing a rapid identification.

Birds and Bees

The toothed leaves arch up and out, creating a cup at the base of the conjoined leaves where rainwater collects. This depression that fills with water gives the plant its common name: cup plant. The cup appeals very much to birds, who can drink from it or even bathe in the water without having to land on the ground (always risky for birds, since most of their predators are terrestrial). And it makes a great lesson in ecology to share with young children. Your project could be “today, let’s plant a birdbath.” 

In addition, birds, especially goldfinches, also appreciate its seeds, produced in October. So, it’s a great plant to use if you want to attract birds to your garden.

But the flowers also attract butterflies and, especially, bees. Bees of all sizes and shapes visit the nectar-rich flowers, including honeybees. In fact, in Germany it is becoming more and more popular as a honey flower, especially useful because the plant blooms for such a long time (2 months and more) and its nectar produces a honey of excellent quality.

Compass Plant

Note too that the pairs of leaves are always placed at right angles and legend has it that they always point to the four cardinal directions, hence its other common name: compass plant, one it shares with other plants in the genus Silphium. I’m not sure how you could use this living compass to find your way home if you get lost in your garden, though.

Giants Need Space

I said the cup plant was a giant and I’m sure most people would agree with me. When in bloom, it measures between 6 to 10 feet (180 to 300 cm) tall and 3 to 4 feet (90 to 120 cm) wide. If you space plants about 30 to 40 inches apart (75 to 100 cm), you can create a beautiful perennial hedge! Given its size, it’s otherwise mostly used as a background plant for large perennial borders or as a stunning stand-alone plant. Its thick stems are very robust and therefore no staking is normally required.

The inch-wide (2.5 cm) flowers borne in dense clusters at the top of the plant are typical of those of the Asteraceae family (daisy family): an inflorescence composed of hundreds of florets forming a central disc which is surrounded with elongated yellow ray flowers. As a result, the bloom resembles a small sunflower (Helianthus annuus) and that plant is, indeed, a close relative.

Given the height of the plant, the blossoms are something you have to look up to see. It starts to bloom from late June in the South to the end of July in the North and usually continues into September.

The flowers do differ from those of a sunflower in one odd way. While the sunflower produces male and female flowers in its disc (and thus fills in with seeds) while its ray flowers are sterile, the florets in the center of a cup plant bloom are all male and produce no seeds, but instead the ray flowers are all female. As a result, seeds form all around the flower head rather than in the center.

How to Grow a Gentle Giant

The cup plant prefers full sun, although it will readily tolerate partial shade. It likes deep soil that is always a bit moist, which is why, in the wild, it’s often found growing in ditches and along rivers and streams. Despite this, thanks to its long taproot, a well-established plant (3 years old or greater) will be quite drought-tolerant … as long as the drought doesn’t last all summer!

It adapts to pretty much any soil: clay, sand or loam, rich or poor, acid, neutral or alkaline. It’s also a very long-lived plant, essentially permanent: you sometimes find specimens in old gardens that are over 50 years old!

Not much fertilization or indeed any special care seems necessary: it pretty much takes care of itself.

As for cold hardiness, I’ve seen it listed as hardy to USDA hardiness zones 3 to 8 (AgCan zones 4 to 8), but I see it growing happily a full zone colder. 

If you find it too large, pinch it back hard in June (see the article Time to Pinch Back Floppy Perennials) and that will reduce its height by about a third.

Finally, the cup plant doesn’t seem prone to disease or insect pests and, in general, deer tend to avoid it.

So, all you have to do is plant your cup plant, water it regularly the first and perhaps second season, and you’re left with a tough permanent perennial as big and robust as a shrub.

Making More Cup Plants

Mature specimens sometimes produce suckers at the base and they can be harvested and planted elsewhere. Normally, though, this plant is mainly propagated by seed.

Ideally, you’d give the seed a 2- to 3-month cold stratification. This replicates the conditions it receives in nature, where the seeds fall to the ground in the fall and overwinter in cold and moisture to germinate the following spring.

Sow the seeds either outdoors or in a cold frame in the fall and they’ll germinate with the return of warmer weather. Or start the seeds indoors in January, sowing them in moist soil, then place the container in the fridge for 2 to 3 months before exposing it to warmth and light. You can read more about cold stratification here: Some Like It Cold: Cold Treatment for Seeds. 

Seed-grown plants fill in quite nicely by the second year, but really reach full height and bloom in year 3.

Both Useful and Ornamental

Curiously, the other potential uses of the cup plant have received little interest in its native North America, but in Europe, it’s a rising star in farmers’ fields.

In Germany and France, especially, it’s increasingly used for the production of biofuel, producing biomass equal or even superior to that of corn. And as the cup plant is a long-lived perennial that grows back annually for decades, it beats corn—an annual that has to be reseeded yearly—hands down in the productivity department. Plus, unlike corn, it requires no fertilizer or insecticide treatments, crowds out weeds on its own and needs no cultivation, thus reducing erosion. As a result, its potential is seen as enormous.

It’s also being tested as a fodder plant for cattle and sheep and gives two generous harvests per year.

Also, permaculture specialists are looking into the use of this plant as a possible vegetable. The young stems and leaves are perfectly edible, especially tasty when cooked. (Older ones become tough and unappealing.) But if you cut back the stems regularly, the plant keeps producing more. The question is therefore how many crops you can harvest without weakening the plant. And of course, continuous harvest will mean the plant won’t bloom and therefore won’t be of interest to birds and bees.

Native Americans once used this plant for medicinal purposes and that could be looked into. Among other possibilities, wounds to the plant produce an aromatic resin that can be used as a breath-freshening chewing gum.

Where to Find It?

This is not a common plant in garden centers and you’ll likely have to order it from a mail-order catalog. Look for native plant nurseries and those specializing in wildflowers, especially prairie plants. Sometimes, though, the easiest thing to do is to go out and harvest seeds from the wild or from abandoned gardens.

Other Gentle Giants

Once you’ve tried growing the cup plant, you may want to give other Silphium species a try. I’ve found prairie dock (Silphium terebinthinaceum) quite spectacular with its huge tropical-looking leaves while the deeply cut leaves of the prairie compass plant (S. laciniatum) are just as stunning, although plant itself can be floppy if you don’t pinch it back. And there are over 20 other species you could try, most being giant plants with yellow (or, more rarely, white) daisy like blooms. 

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So, if you have the space, give the cup plant a try: it makes a fascinating, useful and beautiful garden plant.