Collector’s Choice: Indigofera

Within the Fabaceae, or so-called pea family, there exists an abundance of plant species that are part of our collective consciousness—clover, peanuts, and sweet pea among them. They all share the telltale traits of this family: often pinnately compound foliage, a keeled zygomorphic (irregularly shaped) flower, and an elongated seed pod known technically as a legume. Spines and tendrils are not uncommon.

The genus Indigofera represents a virtual minefield of plant taxonomy, with over 800 species worldwide, primarily native to warm regions of the world. The classic rich violet dye, indigo, gave its name to the genus because the “type” species, Indigofera tinctoria, was the traditional source of this pigment. Although most species hail from the tropics, a handful of highly ornamental indigoferas are adaptable to temperate climates and have caught the eye of keen horticulturists.

As a general rule of thumb, the hardy indigoferas are sub-shrubs—those indecisive creatures that are uncertain whether to remain above ground during the winter or retreat to safety. They are notoriously late to come into growth in spring, confusing greenhorn gardeners with a sincerely dead appearance. Growth will ultimately resume from close to ground level. Most species should be pruned back that far as a matter of routine late winter maintenance.

The first of the lot with which I became acquainted was I. incarnata ‘Alba’ (syn. I. decora f. alba), growing in the rock garden at the University of British Columbia Botanical Garden. A low and somewhat suckering species that rises to only two feet, it is clad with bright green pinnate foliage, throughout the growing season. In midsummer, when presenting its lax, elongated clusters of white flowers, it is a dead ringer for a diminutive ground-hugging white wisteria. The typical form has pink flowers and, though equally stunning and certainly gardenworthy, is less often seen in North American horticulture.

I first met I. heterantha while walking through the gardens at Highgrove House, the private residence of the Prince of Wales, with the late Rosemary Verey. There was a large swath of it growing near the terrace and Ms. Verey—being privileged to do such things—offered me some seed. The resulting plants have entertained me for over 15 years, rising to three and a half feet by summer’s end, carrying finely fretted pinnate foliage and an endless progress of pink flowers presented in short racemes. We have it planted in a gravelly summer-parched bed, not far from our 1970s rambler. There it looks as becoming as it might next to a future monarch’s castle,

I had read of I. pendula in Roy Lancaster’s classic Travels in China (Antique Collectors’ Club, 1990) before I encountered it growing in open pineland near Lichiang in Yunnan Province in 1996. It is a spectacular species, although limited, sadly, to gardens with more benign climates. Plants from my seed collections on that trip continue to garner this species admirers. It blossoms for a full 12 weeks throughout summer and early fall. Long elegant chains, expanding to 15 inches by summer’s end, carry light pink flowers along an open upright framework to 12 feet. It often befuddles visitors to our garden at Heronswood; they believe they are seeing an abberant form of wisteria. Though this may be cut back hard in late winter, like other indigoferas, I believe it is best grown as a multistemmed small tree.

While touring Chanticleer Garden near Philadelphia with its director, Bill Thomas, I was immediately smitten with another indigofera I bad not yet encountered. Indigofera amblyantha (shown) is an upright subshrub to four feet. In summer it is cloaked with the usual pinnate foliage and with densely packed recemes carried perfectly upright on its stems. The trait of this species that most pleases the talented gardeners at Chanticleer is one that is common to the genus: the spent flowers self

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