We have a large collection of silphiums at JLBG, but unfortunately most have limited garden value since they splay apart and often completely fall over when in flower. While they’re loved by native bees, we have been frustrated to not find many that are mainstream garden worthy. One that has been impressive in our trials, however, is the southeast US native whorled rosinweed, Silphium trifoliatum var. latifolium. These 15 year-old clumps at JLBG, which originated from Scott County, Mississippi are now 6-7′ tall, and quite garden worthy. Hardiness Zone 5a-8b, at least.
Being a non-Latin guy, I know the plant by its common name of cup plant since rain water will accumulate in the junction of the leaves and the stem. It can flop if it is planted by itself or at the edge of a bed. I have it in a 30×80 prairie garden. It flops at the edges but in the middle with the support of other prairie plants it remains upright. I also notice that the older cup plants are stronger and less prone to flop. From my perspective the downside of the plant is it is a prolific seeder and has to be managed by dead heading or removal of seedlings. Bees and butterflies absolutely love the plant and finches eat the seeds. I wish the finches would eat more.
Yes, the common name cup plant is used for the species, Silphium perfoliatum. Most other species don’t form a cup where the leaves emerge from the stem.