Once in a while we like to throw a curveball and feature a plant that most people do not grow or
may not even know. Thermopsis is a little-known legume that strongly resembles the lupin and can lend the garden lovely lemony shades of yellow.
Thermopsis is a genus of summer-flowering rhizomatous herbaceous perennials that are in the Papilionaceae section of the pea family. Native wild flowers of North America and eastern Asia, they are fully hardy plants, coping down to -20°C in habitats including the Rocky Mountains and Siberia.
The plants produce racemes of lupin-like yellow or purple flowers and attractive trifoliate leaves. They are a good choice for wildlife, attracting great interest from bees and butterflies.
Although not commonly grown in UK gardens, some species are available from nurseries with
wide-ranging stock, such as Hardy’s Cottage Garden Plants and Cotswold Garden Flowers. The most widely available species appears to be T. villosa, sometimes sold under its synonym T. caroliniana, which refers to its region of origin, eastern North America, where it is known as the Carolina false lupine.
It is a clump-forming perennial with upright, dense spikes of yellow flowers that are borne in summer and closely resemble the lupin famous in cottage gardens. The foliage is trifoliate and greyish/green. The species name refers to the long shaggy hairs that cover the seed pods.
Other species include T. chinensis, which has attractive greyish/purple buds and soft yellow spires of flowers that are tough enough to survive frost in early spring, lending a brightness and verticality to the perennial border when other plants are still yet to unfurl.
Another is T. lupinoides, which being found in the wild in Alaska and Siberia is also fully hardy. It is a clump-forming species that has dark-green to black, slightly hairy stems carrying narrow heads of pale-lemon/yellow flowers in conspicuous dark-green calyces in late spring.
Thermopsis are easy to grow in average, dry-to-medium, well-drained soils in full sun. They tolerate summer heat well and drought. While generally early to flower, after deadheading the
foliage remains an attractive feature in the border.
There are some species such as T. montana that become invasive, so watch out for which type you are growing. Unless you are happy for them to colonise an area, such as an informal wild section of a garden, it is best to stick to the clump-forming species. They also have long tap roots, which can make them difficult to transplant.
What the specialists say
Rosy Hardy, co owner, Hardy’s Cottage Garden Plants, Hampshire
"Thermopsis are an underused plant in the UK. You need to be careful to select plants that are clump-forming and not the ones that will rampage, that is unless you have the area for this, such as a wild garden.
"The species that stands out for me is T. lanceolata. It’s a lovely plant with soft yellow flowers and a good growth habit.
"I would warn you that Thermopsis are slow to establish. I would leave all top growth to protect crowns during the winter. Plant them in a reasonably fertile soil as this helps them on the way."
Ben Wighton, assistant head gardener, Lincoln’s Inn, London
"I have only grown T. villosa but I am tempted to try others as they have lovely pea flowers and nice glaucous foliage. Some of the species flower early so they can fill gaps in the herbaceous border when there are really only hellebores and early-spring bulbs giving a display.
"I do worry about invasive species as there seem to be quite a few, and species appear to have been confused and wrongly named in the past. But if I stick to the clump-forming ones like T. lanceolata and T. lupinoides they could be a great addition to the border."
Species and varieties
T. lanceolata is an herbaceous perennial that produces bright-yellow flowers from blue/green calyces between March and May. Easy to grow and non-invasive. Height and spread: 90cm.
T. lupinoides is a hardy species from Siberia that produces dense spikes of yellow flowers between March and May. Its leaves are silvery and pubescent. Easy to grow and non-invasive. Does not like waterlogged soil. Height: 40-60cm.
T. macrophylla is endemic to a small region in California. A very tall herbaceous perennial that produces bright-yellow flowers in many spikes. Seed only germinates after being heated in wildfires. Height: 2m.
T. macrophylla ‘Agnina’ is a very tall herbaceous perennial with spikes of large, yellow, pea-like flowers produced between May and June. The trifoliate leaves are bluish/green and felted. Height: 2m.
T. macrophylla var. venosa is a tall herbaceous perennial that forms spikes of large, yellow, pea-like flowers between May and June. The foliage is trifoliate and silvery. More showy and hardier than Lupinus arboreus. Height: 1m.
T. mollis, or the soft-haired Thermopsis, is a tall herbaceous perennial with erect branching stems topped with yellow racemes. Height: 1.5m.
T. montana var. montana is a hardy North American wild flower that has dense spikes of yellow lupin-like flowers between May and June and narrow green foliage. An invasive species. Height: 40cm.
T. montana var. ovata is a North American wild flower with tall racemes of yellow lupin-like flowers between May and June. It has green foliage with silvery hairs. An invasive species. Height: 45cm.
T. purpurea is a rarely found short bushy plant from China with silvery hairy foliage and heads of purple/black pea flowers between June and August. Likes well-drained soil but not hot dry places.
T. rhombifolia var. montana is found growing wild in the USA from the Rocky Mountains to New Mexico. Yellow flowers borne between June and July contrast attractively with mid-green leaves, which are hairy underneath. Height: 90cm. Spread: 60cm.
T. villosa (syn. T. caroliniana) has long heads of bright-yellow flowers between May and August. Easy to grow, non-invasive. Height: 85cm.
Thank you to Floramedia, which supplied the images for this article from its photo library