Thermopsis caroliniana (T. villosa) is a good, dependable perennial that might be overlooked if it flowered later but right now is something of a star in the border. Native to the eastern states of the USA it is related to the blue baptisias and multicoloured lupins from neighbouring states. It appears as greyish purple, slightly hairy shoots in spring but these are soon covered in bright green, typically pea-like leaves and soon the dark, main shoots terminate in long (more than 30cm) spikes of lemon yellow flowers. Although it is drought-resistant and prefers well-drained soil it has done well on the ‘improved’ clay here and in one year has formed good clumps. Because of its early flowering it would make a good companion for crown imperials, late tulips or geums which are just starting to bloom.
It can be raised from seed and is best planted as young seedlings because of its long taproots. It does not seem to set seed for me but if deadheaded produces a second flush of smaller spikes before it goes into leaf production and creates a spreading mound of attractive if not earth-shattering foliage.
All in all it is a useful and attractive plant that is no trouble and longer-lived than true lupins.