Yellow Wild Indigo (Baptisia sphaerocarpa) of the Pea (Fabaceae) family is a shrub-like, herbaceous perennial with bright yellow pea-type flowers. Genus name is derived from Greek for “to dye” (bapto), based on another member of the genus (Indigofera tinctoria) used to produce a blue dye. Specific epithet is derived from Greek words to denote the plant’s round (spherical) fruit. Native to central US, principal area of occurrence includes southeast and northeast Texas, southeast Oklahoma, southwest and northwest Louisiana, coastal Mississippi and portions of Arkansas. In Arkansas, area of occurrence includes Arkansas Valley, Ouachita Mountains and far-northwest Ozark Plateaus. Other common names include Yellow False Indigo, Yellow Wisteria and Bush Pea. Preferred habitats are sunny sites with wet to dry, sandy to clayey soils of prairies, roadsides and woodland borders.

A gnarly root-crown is supported by a network of wide-spreading and descending tough, yellowish, ropy roots along with fibrous roots. The terete ropy roots have a strippable epidermal layer and a smooth white, tough core. Being a legume, the plant “fixes” nitrogen from the air into nodules on its roots with the aid of bacteria.

Mature plants, 3+ feet tall with an equal width, have closely spaced, erect to spreading stems from the central root crown. The number of stems and plant width increase year-to-year while the plant retains a compact, broadly rounded shape – especially in sunny sites. Yellowish green stems, with well-spaced axillary branches and secondary branches, both set at 45⁰, have fine longitudinal ridges of equal size and spacing. Stems and branches, also with strippable epidermal layer and a very tough white core, are glabrous (lacking hairs).

New stems, appearing in March, are pubescent, but become glabrous as they grow. Bases of stems, early in their growth, are protected by several elongate reddish, alternate bracts. These lowermost bracts are initially tightly clasping, but spread as stem girth increases. With lengthening of stems, a few clasping bracts above the lowermost ones may be divided at their apices into sharp points with a leaf between. Such leaves have their petiole (stalk of leaf) embedded into a bract’s fabric. Diameter of stems/branches decreases slightly at each more-distal node. Stems, branches and petioles are yellow-green. Bracts are early-deciduous.

Above the clasping bracts, compound leaves have a petiole with a broadened base that fades into the node of the stem/branch. A pair of lateral, early-deciduous bracts (stipules) occurs on the petiole (larger leaves) or on the node (smaller leaves). Stipules, becoming increasingly small distally, are early deciduous. Petiole lengths, decreasing distally, range from ⅛-inch to sessile.

First leaves are green and pubescent (see Photo 5), but become glabrous with age. Later leaves are a dull grey-green on adaxial (upper) and abaxial (lower) surfaces and may have minute, downy pubescence before also becoming glabrous. In sunny sites, tripartite compound leaves (having 3 leaflets) extend to ends of branches with an isolated 2-leaflet-leaf in between. In less sunny sites, 2-leaflet-leaves and simple leaves occur. Floral stalks may terminate some stems/branches while other stems/branches are vegetative and terminate with a single leaf.

In regards to tripartite compound leaves, the symmetrical central leaflet (to 3 inches long and 1 inch wide) tends to be somewhat broader and slightly longer than the similarly sized, slightly asymmetrical lateral leaflets. Leaflets are mostly elliptic with a rounded, slightly notched apex (emarginate apex), and a wedge-shaped (cuneate) base. Leaflets have smooth upper and lower surfaces and uncut (entire) margins may be narrowly revolute (downturned). Pinnate veins, mostly the same grey-green as blade fabric, are obscure on the upper surface and slightly expressed on the lower surface. Equally spaced secondary veins are arcuate toward apices. Leaflets are on petiolules (stalks of leaflets), about 1/16 inch long, that join at a common point atop the stubby petiole or at the node (sessile leaf).

The inflorescence is positioned above the foliage; however, with branch-growth, the infructescence becomes somewhat hidden. Elongate-pointed flower buds, subtended by a single early-deciduous bract, are enclosed by a bluish calyx that becomes green as the corolla emerges. Pea-type flowers reach anthesis in late April into May on erect racemes that terminate floral stalks. Individual flowers are on a short stalk (pedicel). A typical floral stalk, same yellow-green as stems and branches, have a 1-2-inch-long leafless base and an 8-10-inch long rachis bearing to 24+ flowers. Flowering, more profuse in sunnier sites, proceeds up-raceme with most flowers at anthesis at the same time. A mature plant may have dozens of racemes equally spread across the round-topped, somewhat squat shrubs.

Flowers have an upright, broadly flared upper petal (banner or standard), a pair of forward-projecting lateral petals (wings) and a central, forward-projecting pair of partially connate petals (keel). The tightly composed flowers are about 1 inch long, from calyx base to keel tip, and ½ inch wide across banner front. Jointed pedicels (stalk of a single flower) are to about ¼ inch long. The entire corolla is the same bright yellow except for a band of purplish freckles across the midsection of banner. The banner (viewed from front) has a thickened, recessed, central channel (fits against projecting keel) and a notched apex. The pair of free-standing, elongate-ovate wing petals project forward and outward from the prominent keel. Keel, a pair of petals weakly connate along their lower side, is open along its apex and along its upper edge (for insect access). Wing and keel petals are attached to the floral cup (hypanthium) by a single narrow, basal fabric strips (claw). Wing petals, near there broad base, have a bean-shaped raised area on their inner sides that fits against an indention on the outside of the keel.

Flowers have 10 stamens (filaments + anthers) encircling a pistil (ovary + style + stigma) set in a bell-shaped (campanulate) calyx. The ⅜-inch long calyx, formed by fused sepals, has 4 triangular, sharply pointed ⅛-inch lobes – one below the keel, 2 at the wings and a larger one above the banner. The pale greenish yellow filaments are tipped with bright yellow, oblong, upright anthers. The green pistil consists of an elongate ovary on a short stalk (floral axis) topped with a distally up-arched, smoothly tapering, slender style ending with a tiny pointed stigma. The slender stamens, aligned with the pistil, position anthers around and just below the stigma which may be exserted beyond the keel.

Fertilized ovaries develop into thick-walled spherical capsules with persistent styles and calyxes. Initially yellow green, capsules become medium brown with maturity as stems deteriorate in early fall. Capsules (to ¾-inch in diameter) have an encircling suture, extending from the pedicel and through the style. The placenta, along and to both sides of the upper suture, bears ovules/seeds on short stalks (funicles). Developing seed, tightly arranged side-by-side, extend down from the capsule’s upper side into the void below. Capsules become elevated above the calyx on slender, ⅜-inch-long floral axes. A mature capsule may contain 6-14 seed.

Ovate seeds are bean-like with a protruding end where the funicle attached seed to the placenta leaving a scar (hilum) on the seed. The flattened seeds, shrinking as they dry, are ⅛+ long and about 1/16 inch wide. Seeds, with dimpled sides, are lightly textured and yellow-brown. The thick-walled, dry capsule slowly divides along the entire suture with seeds gradually dropping out as capsules persist into winter. Dead stems break-off at the ground and may be blown-about by wind as remaining seeds are dispersed. Capsules remain attached to dead stems as stems and capsules decay.

Yellow Wild Indigo would be attractive as a specimen plant or in a group in various mesic to wet, sunny gardens where space is available for long-term growth. Also, appropriate in a prairie setting. Plants in less sunny sites tend to be more open and may not flower. Persistent seed capsules are attractive on dead stems from fall into winter at which time stems break off at the ground. Spring-time emergence of new stems is interesting while color and texture of later foliage of the sturdy plants add color and texture. While the showy bright yellow flowers provide nectar for a variety of large bees and butterflies, plants also serve as host plant for skippers, cloudy wings and elfins. In a sunny untended space, a colony of seeded plants can develop overtime. The deer resistant plants, with wide-spreading roots, survive dry years. Dead stems with capsules are suitable for dry arrangements.

Other Wild Indigos in Arkansas: White Wild Indigo (Baptisia alba var. macrophylla), Blue Wild Indigo (Baptisia australis var. minor), Cream Wild Indigo (Baptisia bracteata var. leucophaea) and Nuttall’s Wild Indigo (Baptisia nuttalliana). All are herbaceous perennials with mostly tripartite compound leaves. In regard to growth habit, Yellow Wild Indigo is most similar to Nuttall’s Wild Indigo. Yellow Wild Indigo, when in bloom and fruit, can be distinguished from all other species by its erect racemes of bright yellow flowers and brown spherical seed capsules. Of the other indigos, only Nuttall’s Wild Indigo has axillary flowers – whitish flowers.