Indigobush (False Indigo)

Size, growth and what to expect

Indigobush flowers young. In seed production plots at the Tallgrass Prairie Center, transplants flowered and set seed the year after planting. Growth is fast, and our growth estimator models it at nearly three feet a year on an average site. It makes a loose, airy shrub of 6 to 12 feet that spreads into a thicket of cane-like stems, branching only above 3 or 4 feet.

The purple spikes run 3 to 6 inches long and open in June, and the small warty pods that follow are under half an inch long, each holding a single seed. Bees and butterflies take the nectar, and it is a larval host for the silver-spotted skipper and the gray hairstreak. Nodules on its roots fix nitrogen that neighboring plants can share. The name false indigo comes from early settlers who used it as a substitute for true indigo dye.

It grows wild from New Jersey west to Minnesota and Wyoming, south to Florida and Texas, and from southern California to New Mexico. It grows on wet ground along rivers, streams, ponds and ditches, and on gravel bars.

How to grow Indigobush

Indigobush grows anywhere in full sun or part shade, on a dry bank, ordinary garden soil, or ground that floods in spring. Full sun gives the heaviest bloom and the sturdiest stems. Plants lean outward, so allow 6 feet between them and keep them off a walk.

Plant dormant stock from leaf fall to bud break. Saplings come bare-root or potted. Dig twice the width of the roots, keep the crown at ground level, backfill with native soil, and mulch short of the stems. No stake is wanted. Seed is a hard-coated legume, so nick the coat, soak it overnight, and sow in fall under hardware cloth, or mice will clean out the bed.

Water the first summer and it takes over from there. Rabbits clip young stems in winter, so cage new plants a season or two. Cutting it hard in late winter costs no bloom, since flowers come on the current year's shoots.

Each flower carries both male and female parts and bees pollinate it, so there are no male and female plants to pair. The warning with false indigo is seed. It fruits freely, birds and water carry it, and it has made a nuisance of itself along rivers outside its home range. Maine, Connecticut, Oregon and Washington ban its sale or its entry, so we do not send it to those four states. On a small lot, cut the spikes before the pods ripen.