Amorpha fruticosa

Amorpha fruticosa, the false indigo bush, is the largest and most widespread of the native false indigos, a fast, open, deciduous shrub that carries long spires of tiny deep blue-purple flowers, each lit with a single vivid orange anther, at the branch tips in late spring and early summer. From a suckering base rise arching stems six to twelve feet tall, clothed in soft, ferny, pinnate leaves that give off a clean, resinous scent when crushed. In full bloom the whole shrub seems to smoke with color, and the flower spikes hum with bees. A plant of floodplains, streambanks, gravel bars, and swamp margins across most of the United States and into Canada and Mexico, the false indigo bush is one of the toughest and most adaptable of native shrubs, at home in wet ground and dry alike. The name Amorpha, from the Greek word for shapeless, marks the odd single-petaled flower, so unlike the usual winged pea blossom of the bean family to which the plant belongs. As a legume, false indigo fixes nitrogen at the roots and builds the poorest soils, and the foliage feeds the caterpillars of the silver-spotted skipper and several other native butterflies and moths. In the garden, Amorpha fruticosa suits a wild or naturalistic planting, a streambank, a rain garden, a pond edge, or a tough sunny bank where the fast growth and suckering roots help bind the soil. The summer flower spikes feed bees and butterflies, and the shrub makes a fine screen or backdrop where room allows. Vigorous and free-seeding, false indigo can spread beyond bounds in the wrong place, and sale is restricted in a few states, so site the plant with that enthusiasm in mind and cut back the seed spikes where self-sowing is unwelcome. Give the plant full sun to part shade and almost any soil, wet or dry, lean or rich, and false indigo asks for nothing more. Prune hard in late winter to keep the open habit in check and to draw out strong new flowering wood, and pull suckers or spent seed spikes to curb the spread. A tough, fast, pollinator-rich native shrub for the difficult, damp, or neglected corners of a large garden.