A Shrub to ‘Dye For’

Simply stated, one of my great passions in life is to better know and understand plants.  When those, such as myself, are first consumed by this passion, we are like high school students – we believe we can learn everything possible about the topic in a few months, or at worst, a year.  My analogy is based partially upon personal recollection – when I believed that there were only 5 species of Maples – and the fact that I presently have two sons in High School!  Having recently turned fifty, I have long since come to the realization that those high school misconceptions were so delightfully wrong.  I will always be learning more about plants.  One genus that recently came to my attention and we have started to use in the Rutgers Gardens is Indigofera.

The Genus Indigofera receives its name from the dark purple dye of Indigo that was historically extracted from the leaves of Indigofera tinctoria, a tender plant native to China.  The genus is a member of the Fabaceae, or Pea Family, indicating that it is able to ‘fix’ or extract nitrogen from the air, allowing it to live in lower fertility soils. Of the roughly 700 species available, two are relatively available to the New Jersey gardener:  Indigofera amblyantha and Indigofera gerardiana.   Both species are easy to grow in full sun or light shade.  They are pH adaptable and soils need only to be well drained.  The chief cause of death is soils that drain poorly.  

Indigofera amblyantha is the taller of the two, reaching heights of 4-6’.  If you wish to keep the plant more compact, it is best to cut it back severely in early spring, much as you would a Butterfly Bush. This will also improve the appearance on the late winter garden, since Indigo is not a plant of winter beauty!  The species epithet, amblyantha, means a blunt flower, referring to the relatively short yet dense flower spikes, which reach 3-5” long. The 3/8 to1/2” long pink, pea-shaped flowers appear from June through September and although they are never as brazen as Forsythia, they definitely brighten up the summer border.  The foliage is comprised of compound leaves, with attractive and dainty bluish green leaflets of 9, 11 or 13 per rachis (the central ‘stem’ of a compound leaf).  Although labeled as hardy to zone 6, I recently planted a large mass of 24 plants in Sussex County, NJ.  With little time to root out and become established, all the plants survived this past winter.

Indigofera gerardiana is named after the English Herbalist, John Gerard (1545-1607).  It is a far more compact plant, only growing to 3 feet tall and it literally flowers from May to frost in New Brunswick!  If you decide to cut the plant back in late winter to improve the garden’s appearance, the plant will only grow to 2’ tall and will not commence flowering until June.  The ½” long pink flowers appear along racemes of 3-6” in length, and provide a nice impact.  This species is supposedly more cold hardy, to zone 5.

There are a myriad garden uses for these plants.  They look great when massed, especially near a walk where the delicate flowers can be easily appreciated.  Also, in an effort to get the flowers closer to the viewer, I have combined Indigo in large containers with various annuals with smashing results.  In the mixed border, the pink flowers look great with silver foliaged plants, such as Stachys (Lamb’s Ears) and Artemesia (Wormwood), or with purple foliage or flowers, such as Cotinus coggygria ‘Atropurpurea’ (Purple Smoke Bush) or Purple Snake Root (such as Cimicifuga simplex ‘Hillside Black Beauty’). 

For the gardener, Indigofera is a plant without compare.  It provides color and an extended bloom, as well as an important history of the indigo dye.  It is indeed fun to know that there is always another great plant to be discovered! 

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Last Modified: 12/6/2007, rugardens@njaes.rutgers.edu.
©2008 Rutgers, The State University of New Jersey