Cup Plant – Grow and Care Guide – Silphium perfoliatum
August 17, 2019
The Cup Plant is a tall, sturdy perennial flower native to the Midwestern United States, with isolated pockets in surrounding states as far as Tennessee, North Carolina, and Pennsylvania. It produces many yellow flowers that are about 4″ across (10 cm) and blooms for about a month.
Its common name (Cup Plant) comes from the characteristic where two large leaves join together at its thick stalk, forming a cup. This cup can hold rainwater, and is thought to possibly be an evolutionary trap for insects trying to climb the plant to eat the flowers. I’ve not seen it yet, but I suspect that birds might be treating this as a natural birdbath in our micro-prairie.
Cup Plant Facts
Can grow 10′ (3 m) tall in optimum conditions
Has pairs of leaves that join at the stem and collect rainwater
Is a BUTTERFLY MAGNET
Can become invasive, and become a garden thug
Blooms 4-6 weeks
The Cup Plant is very tall by August, reaching 8-10′ tall in optimum conditions. The only branches will be short stems near the top that contain multiple flowers.
Stalk / Stem
The stalk is very large, solid, and square shaped. There will be pairs of leaves going up the stalk that are joined and can hold water. The pairs and leaves and square stem make it very easy to identify this plant in the wild or in your garden, once the stalks start shooting up.
The leaves along the stalk are lanceolate (lance shaped) with large coarse teeth. Rub your hand on a leave and you will be surprised as to how rough the surface feels, almost like very fine sandpaper.
The flower is similar to a common sunflower in appearance. They are typically 3-5″ in diameter. There are 20-40 petals per flower, and may not be tightly grouped and have gaps in them. I’ve seen much variation in flower appearance when it comes to how close the petals are together.
This plant has a taproot, and small rhizomes. Once established, it would be very difficult to move this plant. The small rhizome roots will make new plants, contributing to this flowers nature of crowding out competition.
This plant is at home on moist prairies of the Midwest. Or should I say, back when there was still prairies in the Midwest. But it likes full sun and moist soil. The more sun it gets, the taller it will grow. Less sun and less water will mean fewer blooms and a shorter plant. It can tolerate partial shade and droughts. But in general you should just use your intuition and provide water if the soil seems dry.
When I visit relatives back in Iowa, I see this plant at the bottom of ditches, in small prairies near woods. It just seems to randomly pop up in places, but never alone. I will generally see several healthy plants within a few feet of each other.
How to care for the Cup Plant
You don’t really need to do anything to keep the plant healthy, as long as it gets full sun and is in moist soil. It is a really tough plant that can beat out most competition. No fertilizer should be needed to keep this plant healthy and vigorous for a really long time.
This plant can flop over in high winds, especially after blooming. As the seed heads seem to form it must get a bit top heavy. So, trim it back or stake it to keep it upright and tall.
The main thing to watch out for is numerous seedlings that will germinate in the Spring. This plant has a reputation for being very aggressive, and I’ve seen it first hand. My plants are three years old now and I probably pulled 100 or more seedlings this Spring to try to keep it in its place. Many references state that it will spread via rhizomes, but I have yet to see this happen. It may help that I planted it in the rockiest part of my yard (and it is pretty rocky). It is also hard clay soil, so that may help keep rhizomes down.
How to Establish Cup Plant
I’ve never seen this plant for sale in a nursery. So buying some seeds is probably your best option. The plant won’t bloom until the second year, but at least it will be cheap to grow!
Plant seeds directly in the fall, just pressing them firmly into the soil. The seeds need a cold moist stratification period of at least 2 months. So if you are going to start them in pots, I suggest that you winter sow the seeds, pressing them into potting soil and placing outside.
The way I established my plants was winter sowing seed, caring for the young seedlings. Then I transplanted them outside in August (had to clear the garden space). The second year my plants grew about 4-5′ tall and flowered, although they weren’t that showy. But all good things come to those who wait, and by year 3 – Woohoo! My patch of Cup Plant was the neighborhood butterfly Mecca! The tallest plant was over 8′ tall and everything was full of beautiful yellow flowers.
This flower can be the center or focal point of a flower bed or micro-prairie / wildflower garden. Anywhere you want to attract butterflys would be a great place for this flower! Its towering height is very prominent. It could also be use along the edge of a forest, as then mowing would keep any unwanted seedlings in check. And this plant certainly reproduces quite vigorously. So understand that before adding it to your garden.
I have five plants at the far Eastern edge of our backyard micro-prairie. That way I have easy control if they get out of hand (they haven’t yet). And they won’t steal sunlight from most of my other plants in this way. You can learn how to make your own backyard Micro-Prairie at the link below;
Here is a brief video summarizing this article. This is what I have growing at our house, at the far edge of our backyard micro-prairie. I placed them there so that if they started to spread too much, I could easier control them. In 3 years, I’ve yet to see any plants sprout via rhizome roots.
This plant will be visited, or mobbed by bees and butterflies when it is in full peak bloom. And I mean MOBBED. I’ve not seen any flower as popular with pollinators (of all types) as the Cup Plant. It is really amazing and hypnotizing to watch.
I have not noticed any animal eat this plant. No deer, no rabbits – nothing. So that is nice for a change. It could be the rough texture of the leaves.
Pests and diseases
I’ve not seen any problems on any of my Cup Plants. Not one. They are very healthy and strong.
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