September 2002
Plant of the Month
False Indigo
(Amorpha fruticosa)
shrub
zone: 4
by Stan Hokanson


Amorpha fruticosa – False indigo
Commonly called false indigo, bastard indigo or indigobush amorpha, Amorpha fruticosa is a deciduous, perennial leguminous shrub which prefers to colonize open wet woods, shores of ponds and lakes, and moist ground near streams and ravines. False indigo occurs locally with a notable population occurring along Lake Minnetonka's Gray's Bay on Highway 101. The native range for the species extends from southern Canada to northern Mexico and encompasses each of the contiguous 48 states except for Montana and Nevada.
False indigo has pinnately compound leaves consisting of 10-33 oval to elliptically shaped leaflets, each 0.5 to 1.5 inches long and 0.25 to 0.5 inches wide. Mature plants can vary considerably in regards to morphology; however, a mature plant generally has a spreading crown comprised of 1-10 stems growing 3.5 to 15 feet high with a spread from 3 to 12 feet.
Flowers are a cluster of racemes, 3-6 inches long, which are borne around mid-June in the Twin Cities. Although flowers are generally a dark purple, colors range from white through varying shades of blue to purples and nearly black. Flower colors are offset by showy orange anthers. The flowers result in a single legume shaped pod containing a single seed which persists into winter.
False indigo has been described as an ungainly “leggy” shrub. Although this description is somewhat accurate, the species displays considerable variation in morphology. We have been observing a planting of over 800 A. fruticosa plants collected as seed, mostly from the northern plains and upper Midwestern states and passed on to us by Dr. Nancy Ehlke and her graduate student Lee DeHaan from the Department of Agronomy and Plant Genetics. We have observed plants ranging from a couple of six-foot stems with leaflets occurring on the outer six inches of the stem, to short statured three-foot plants that are completely covered in leaflets. Flower colors have ranged across the described spectrum with flower densities ranging from light to quite dense.
Propagation of this species is reportedly quite easy. Seeds have impermeable coats and dormant embryos. Light acid scarification for 5 to 8 minutes in sulphuric acid followed by cold stratification is recommended, although simple cold stratification prior to spring sowing may suffice. Softwood cuttings have been reported to root easily with no treatment. Hardwood cuttings were reported to root well in peat:perlite media following 480 ppm Ethrel treatment. The species can also be propagated from layers, suckers and divisions.
Although I would like to say we were first with the idea of developing this native plant, it was once said, “there is nothing new under the sun” and such is the case with false indigo. Several selections of the species have been named including ‘Albiflora’ (white flowered), ‘Coerulea’ (pale blue flowers), ‘Crispa’ (crinkled leaves), ‘Dark Lance’ (dark purple flowered selection) ‘Lewisii’ (large flowers) and ‘Pendula’ (pendulous branches). Although numerous sources of seed and seedlings exist (including several in the state of Minnesota!) an informal survey of nursery catalogues suggests none of the named cultivars are currently available in the nursery trade.
Previously developed false indigo cultivars disappeared from the trade for a reason. The species will need some further study and refinement to make it a widely utilized landscape plant. Although the flowers are quite attractive, the duration of flowering is fairly short, approximately 1.5-2 weeks. Earlier or later flowering selections will need to be made to extend the bloom season for the species. Even though we have identified more compact plants, the species propensity towards "legginess" necessitates that cultural management schemes, including the utility of fall or spring mowing to reduce "legginess" will need to be developed. Best management practices for nursery production of the species will also need to be worked out.
Despite some obvious shortcomings, the species has considerable potential as a landscape plant for the upper Midwest. Its frequent colonization of wet sites belies the fact that it will also grow well in poor, dry soil conditions. The plant is capable of fixing nitrogen and some research suggests the species is resistant to air pollutants, salt stress, and drought. Due to its ability to adapt to tough landscape conditions, false indigo could prove useful in roadside plantings and possibly in large commercial settings where plants receive little maintenance. Similarly, the plant could figure prominently in low maintenance naturalized or native landscapes. The large racemes are reminiscent of Buddelia, Clethra and Vitex and could offer a reliably Zone 4 hardy alternative for these attractive, relatively cold sensitive plant materials.
Despite its ignominious monikers, false indigo could fill a niche in the landscape for the upper Midwest as it exists today. With some breeding, selection and cultural management, this frog could one day become a charming prince.

By Stan Hokanson
University of Minnesota

Locate an Expert | Useful Information | What's New | Resources | About MNLA | Contact Us
MNLA, PO Box 130307, St. Paul, MN 55113 | Phone: 651/633-4987 888/886-6652 Fax 651/633-4986


Copyright © 2000, Minnesota Nursery and Landscape Association