It is mid-summer and my southern Ontario garden is dominated by yellows.
On the short end of the spectrum are Lanceleaf Coreopsis (Coreopsis lanceolata – up to 45 cm), Orange Coneflowers (Rudbeckia fulgida – 60 to 90 cm), Black-eyed Susans (Rudbeckia hirta – up to 100 cm) and Grey-headed Coneflowers (Ratibida pinnata – up to 150 cm).
A few mid-height yellows are also blooming, such as False Sunflower (Heliopsis helianthoides – up to 180 cm), Sneezeweed (Helenium autumnale – up to 130 cm) and Rosinweed (Silphium integrifolium – up to 180 cm).
But there are also a few yellow giants, such as Yellow Giant Hyssop ((Agastache nepetoides – up to 210 cm), Tall Tickseed (Coreopsis tripteris – up to 240 cm), Wingstem (Verbesina alternifolia – the books say up to 240 cm but in my garden this plant often tops 340 cm) and Green-headed Coneflower (Rudbeckia laciniata – up to 360 cm).
Among the giants is Cup Plant (Silphium perfoliatum) and it is this one that I will focus on today. This is one of the first of the yellow giants I planted in my garden and it holds a special place in my heart (despite the fact that is can be fairly aggressive – especially in a smaller garden). As usual, the Plant Description and In the Garden sections are courtesy of Shaun Booth from In Our Nature.
Scientific Name: Silphium perfoliatum
Common Name: Cup Plant
Family: Asteraceae (Aster Family)
Alternate Common Names: Carpenter’s Weed, Cup Rosin Weed, Indian Cup, Ragged Cup
Plant Description: Cup Plant features thick, four-sided, smooth stems that are green to reddish. Along the stem you will find large, opposite leaves that measure up to 20 cm long and just under 13 cm wide. These leaves are fused together around the stem and form a distinctive “cup” that holds rainwater. Leaves are coarsely toothed with pointed tips and rough surfaces. Stalked basal leaves also occur, but they usually die back by flowering time. Upwards of 30 sunflower-like flowers are found in branching clusters at the tops of the stems. Each flower is up to 9 cm wide with 17 to 40 yellow ray florets (petals) surrounding a green to yellow centre disk. Behind the flowers are smooth, egg-shaped bracts with flaring tips. Ray florets mature into flat, black, winged seeds that can be carried by the wind.
In the Garden: Cup Plant towers above surrounding plants to boast its bright yellow flowers. It makes for a dramatic structural plant, and the rigid stems persist well into the winter months. Great for the back of a border or used as a privacy “hedge.” Make sure to give it space!
Skill Level: Beginner
Lifespan: Perennial
Exposure: Full sun to part shade
Soil Type: Clay to sand — prefers loamy soil
Moisture: Wet to dry (may lose lower leaves if dry for too long)
Height: 75–270 cm though in my garden it has topped 350 cm (11’6”) in some years
Spread: 60–90 cm
Bloom Period: Jul, Aug, Sep
Colour: Yellow
Fragrant (Y/N): N
Showy Fruit (Y/N): N
Cut Flower (Y/N): Y
Pests: No serious pests or diseases; however, Cup Plant is sometimes attacked by red aphids, which gather on the underside of the leaves.
Natural Habitat: Moist woods, prairies, lowlands, river floodplains, forest openings, and forest edges
Wildlife Value: The flowers attract many native bees and several butterflies — it seems to be a favourite of the Eastern Tiger Swallowtail (Papilio glaucus) in my garden — and goldfinches are very fond of the seeds. I have often witnessed a variety of insects and small birds drinking water from the “cup” formed by the leaves
Butterfly Larva Host Plant For: None
Moth Larva Host Plant For: None
USDA Hardiness Zones: 3–9
Propagation: The seeds need to be cold, moist stratified for 60 to 90 days for successful germination, or you can sow them outdoors in the fall.
Additional Info: Cup Plant is reportedly sensitive to herbicide drift. Note that this plant is a very prolific spreader and is considered an invasive species in New York where it is illegal to grow it for sale or distribution.
Native Range:
