Know Your Natives – Arkansas Yucca

Arkansas yucca (Yucca arkansana) of the Agave (Agavaceae) family, formerly of the Lily (Liliaceae) family, an evergreen shrub, occurs in Texas, Oklahoma, southeastern Kansas, southern Missouri and Arkansas.  In Arkansas, it occurs throughout much of the Interior Highlands.  The genus name originates from a misapplication by Carl Linnaeus in 1753 of a common name for cassava (Manihot esculenta), “yuca”, to the plants for which he named and described the genus Yucca.  Arkansas yucca, also called soft-leaf yucca and soapweed, is found in well-drained soils in sunny prairies, glades and rocky hillsides as well as along partially sunny edges of woodlands and thickets.

A mature Arkansas yucca has a several-feet-long taproot-like rhizome (round in cross-section) along with near-surface, short lateral side-rhizomes with scattered fibrous roots.  Rhizomes, that may be one or more inches in diameter, have a brown epidermis with a white potato-like interior.  Ends of lateral rhizomes turn toward the surface, where apical buds each produce a single rosette of leaves.  A mature plant may have a half-dozen scattered leaf rosettes so that a single plant appears to be a colony.  Single, stout, woody flower stalks grow from the apical bud within leaf rosettes.  After flowering, those rosettes and their rhizome segment die and decay, leaving a circular scar or nub on the surviving portion of the root.

Arkansas Yucca - Yucca arkansanaPhoto 1:  Rhizome segment at coin is decaying after producing a flower stalk marked with a red asterisk.   Lateral roots grew from either side of dead segment.  Taproot is below coin.

Leathery, flexible, radiating leaves, up to about 2 feet long, are widest at mid-leaf with a width of about ¾ inch.  Reduction of leaf width towards its pointed, pliable tip is very gradual.  White-edged margins are straight and entire with scattered, long, loosely curly white filaments that exfoliate from margins.  Leaves, a dull medium green above and below, become narrow near their bases before finally widening within the rosette base.  Leaves are convex below and concave above, with a thickened stout midrib.  Toward mid-leaf, the blade gradually widens and the midrib becomes less prominent and less stout where leaf may bend down and become twisted.  Parallel venation is not prominent on the upper or lower leaf surfaces.  Bases of previous years’ leaves gradually weaken so that old leaves gradually lower to lie on the ground where they remain for several years, gradually decaying.  Plants are usually stemless (acaulescent), although a short leaning (decumbent) stem, mostly hidden by decaying leaves, may develop.   Leaf rosettes may be 2 feet tall and wide.

Arkansas Yucca - Yucca arkansanaPhoto 2:  Long, radiating leaves are thin and pliable with a flexible point.  Photo taken in late February.

Arkansas Yucca - Yucca arkansanaPhoto 3:  Curly marginal hairs are not twisted as on some other species.  Appreciable stems do not develop as leaf rosettes add new central leaves from year to year.

The inflorescence in early spring consists of a raceme on a tall, stiffly erect, somewhat woody and glabrous stalk (rachis) that grows from the apex of a leaf rosette.  Stalks, usually 2 to 3 feet tall (but may reach 6 feet), have narrow, triangular and purplish bracts that are erect, both below and within the raceme.  As erect flower buds on the upper portion of the stalk enlarge behind bracts, those bracts wither and the pedicels bend down at the base of flowers so that, at anthesis, flowers droop and face the ground.  Individual flowers are on short pedicels (attached directly to the stalk) or in a group of several flowers (small panicles) on short pedicels growing from a short peduncle (which, in turn, is attached to the stalk).   Individual plants typically do not bloom every year.

Arkansas Yucca - Yucca arkansanaPhoto 4:  In this early May photo, purplish bracts on elongating flower stalk subtend and cover developing flower buds.  Bracts are relatively short, as compared to other species.

Arkansas Yucca - Yucca arkansanaPhoto 5:  Flowers, on short pedicels and peduncles, occur in a raceme-type inflorescence.  Bracts on the lower portion of the stalk do not dry while flowers are in bloom.  Site shown is mostly shady with loam soil.

The bell-shaped (campanulate) flowers, up to 2½ inch long and 2 inches wide, have six greenish-white, thickened tepals (three sepals and three petals) that have narrow bases, gentle tips and broadened mid-sections.  The whitish green superior ovary is topped by a pale green lobed style that terminates in a three-lobed stigma with a recessed central orifice.  Six round, white, post-like filaments, attached to flower receptacle below the ovary, bear pollinia (packets of pollen) at their rounded apices.  Yucca moths are the only insects that can successfully pollinate yucca flowers and developing yucca fruit is the only larval food source for yucca moths.*

Arkansas Yucca - Yucca arkansanaPhoto 6:  Heavy flowers cause pedicels to bend down.  As seed capsules develop, strengthened pedicels bend upward.

Arkansas Yucca - Yucca arkansanaPhoto 7:  The lobed green style sits above a large ovary that is surrounded by post-like filaments and thick tepals.  Honey ants can be seen feeding on flower bud resin.

As flowers pass anthesis, pedicels twist upward so that developing green, tough, seed capsules are upright.  Capsules, about 2½ inches long and 1 inch wide on woody pedicels and stalks, are composed of three elongate carpels that are divided into two locules, each with a stack of seeds.  A capsule, with an exterior that is rounded along each elongate carpel, contains 100+ seeds.  When capsules dry in late fall, they become papery and split from the top between carpels.  As capsules open, hundreds of flattened, rounded papery black seeds are shaken out on windy days and become scattered.   Dead stalks and racemes are somewhat woody and may persist for several years.

For a partially shady to sunny garden or natural area, Arkansas yucca can provide a dominant accent.  It has year-round presence, but its occasional flowering would be the highlight of a year.  Its large flowers dramatize its flower parts and its mutulistic relationship with yucca moths can be easily viewed.  In addition to the yucca moth, yucca plants are host plants for caterpillars of giant-skipper butterflies, including Megathymus yuccae, which is rare in Arkansas.

Identification of yucca species and their varieties has found disagreement among various authorities.  In Arkansas, four additional species are recognized: Yucca filamentosa (common names: Adam’s-needle, Spanish-bayonet or yucca), a Southeastern U.S. native introduced to Arkansas;  Yucca flaccida (same common names: Adam’s-needle, Spanish-bayonet or yucca), also a Southeastern U.S. native introduced to Arkansas; Yucca louisianensis (common names: Louisianna yucca, Gulf Coast yucca or sandhill yucca), a native to south Arkansas; and Yucca freemanii (common name: Freeman’s yucca), a native species discovered new to Arkansas in Miller County in 2014.  Characteristics that can help distinguish Yucca arkansana from these other species:  1) leaf rosettes of a plant are separated by a short distance, instead of bunched-up, 2) there are relatively few leaves per rosette and these are thin and pliable with weak pointed tips, 3) curly marginal leaf hairs are not twisted, 4) flowering stalks are generally less than 4 feet, 5) inflorescence is a raceme, instead of a panicle, and 6) color of style is a contrasting green within a whitish green flower.

*   Yuccas and yucca moths, totally dependent on each other for natural reproduction, represent an example of mutualism.  The moth species that pollinates Arkansas yucca (as well as some other yuccas) is Tegeticula yuccasella.   Female moths pack pollinia into a ball that they carry in their coiled palps to another plant.  Moths use their egg-laying abdominal appendage (ovipositor) to insert an egg into a chamber (locule) of the ovary.  The transported pollinium ball is then forced into the recessed orifice of the stigma to effect pollination.  The moth caterpillar, as it grows, ties a dozen or so ovules together as they are eaten, leaving hundreds of viable seed.  Mature caterpillars drop to the ground to pupate in the soil until the next year’s flowers are available.

Article and photographs by ANPS member Sid Vogelpohl

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