Golden banner (Thermopsis rhombifolia var. rhombifolia) is another of my rescue plants. This member of the legume family (Fabaceae) is closely related to lupines (Lupinus) and wild indigo (Baptisia).
A species of the high prairies in xeric and badlands habitat, golden banner occurs from Saskatchewan to Alberta and south along the east slope and foothills of the Rocky Mountains of Wyoming, to the Spanish Peaks in Colorado, and into Utah in the Uintah Mountains. I did not find the plant there, however. Where I found golden banner was along an abandoned railroad grade in Wisconsin some 900 miles east of its Rocky Mountain home. How it got to Wisconsin is unknown but the yellow pea-like flowers are attractive and bloom very early in the spring so if it were introduced the reasons are not hard to understand.
Average height of the golden banner plants is about one foot tall with one to several leafy stems coming from points along the rhizomes or from a caudex. The leaves are composed of three leaflets like a clover. The leaflets are rhombic in outline, dark green with prominent venation. At the base of each leaf on the stem are two rhombic-shaped stipules. The bright, clear yellow pea-like flowers are on short loose spikes of about seven to ten flowers. On the banner petals are a few small purple-brown freckles. The seed pods have an interesting crescent shape and may be smooth or with minute soft hairs.
Golden banner is a slowly growing plant and it has taken mine almost 20 years to spread thinly across the grassy wildflower garden where it grows with liatris, smooth blue aster, little bluestem, smooth scouring rush, gray sagewort, monarda, and dwarf bilberry. My plants have never produced viable seeds. Golden banner can be propagated by rhizome divisions taken in mid-summer and planted in a loose well-drained potting soil until the plants are growing strongly at which point they should be planted into their permanent location.