The false indigo bush (Amorpha fruticosa) is known by many common names, including desert false indigo, dull-leaf indigobush, leadplant, and river locust. There are also other local plants with similar sounding common names, such as wild indigo (Baptisia tictoria). This is a prime example of the importance of scientific names in accurately differentiating between species.

The leaves of false indigo bush, when present, are alternate and compound, with blunt oval-shaped leaflets 1 to 1.5 inches long. In May and June, the shrub produces spikes of purple flowers, which give rise to the small, resin-dotted seed pods pictured here. These resinous bumps, as well as those found elsewhere on the plant, contain a compound that has insect repellant properties. The genus name Amorpha comes from the Greek word “amorphos”, which means formless or deformed, due to the fact that the flower has only a single petal, unlike the typical pea flowers found throughout the legume family.

Whether or not this species is native to New England depends on the source you consult. USDA’s plant database designates it as native in every state within the contiguous U.S. except Nevada and Montana. However, other reputable sources, such as the Native Plant Trust, Lady Bird Johnson Wildflower Center and the Missouri Botanical Garden, call it non-native, weedy or invasive in the northeast. It is likely that false indigo bush’s original range did not extend into New England, but that it arrived here through its use as a landscape plant and has since escaped cultivation. Where it has naturalized, it is often found in wet areas along rivers, streams, ponds, and ditches. The plants pictured here were found at the upland edges of a tidal estuary. 

All plants require nitrogen to grow. This element can become a limiting factor to plant growth where it is in short supply. All members of the genus Amorpha, which belong to the legume family, have the unique ability to fix atmospheric nitrogen into the soil through special nodules on their roots that house nitrogen-fixing bacteria.