Know Your Natives – Red Buckeye

Red buckeye (Aesculus pavia var. pavia*) of the recently expanded Soapberry (Sapindaceae) family–it now includes the maples from the former Aceraceae as well as the buckeyes and horse-chestnuts previously classified in the Hippocastanaceae–has large, showy red inflorescences in early spring. The genus name, a classical name for an oak tree, is based on the Latin for “edible acorn”; however, red buckeye’s nut-like seeds are poisonous. The specific epithet honors Petrus Pavius, a 16th-century Dutch botanist. In the U.S., this species occurs principally from south-central Texas to southern Illinois, east to the Atlantic and Gulf Coasts. In Arkansas, it occurs statewide except for portions of the western Ozark Highlands and low lying areas along the Mississippi River. Red buckeye’s preferred habitat is the understory or margin of mixed woodlands, on lower slopes, in valleys and along stream banks, in full to mostly partial sun, where deep mesic soils are well drained. Other common names include scarlet buckeye and woolly buckeye. “Buckeye” refers to the appearance of the seed.

Red buckeye may be a shrub or small tree with a height of 20 feet or more (term “tree” used herein). Plants can produce flowers and fruits by their third year when they may be only a foot tall. Mature trees produce a dependable fruit load. Seeds dropped to the ground germinate readily, if they remain moist. Seedlings quickly develop a tap root and leaves.

Mature trees tend to be broad with a rounded top, especially in sunnier sites. Stout new branch segments (herein, term “branch” includes “twigs”) with scattered round lenticels (pores) are typically reddish at first, but become green over the growing season and then greenish gray to gray in subsequent years. Older branches are smooth and round with the small lenticels becoming raised. Younger trunks, with brown and gray splotches, are mostly smooth to slightly fissured and still marked with lenticels. Older trunks have fissured bark with flat blocky plates.

Overwintering apical buds may occur singly (when branch was not terminated by an inflorescence the previous year) or as an opposite bud pair (when branch was terminated with an inflorescence the previous year). Apical buds at the tree’s perimeter are typically reproductive, comprising both embryonic leaves and an inflorescence, and are impressively large. Buds have an outside layer of brown, tightly imbricated scales which, in late winter, spread open as underlying scales grow into strap-like, drooping, pinkish bracts. Scales and bracts quickly fall away as the branch matures.

Red buckeye’s structure is open, with denser branching on sunnier sites. Each year’s new growth produces mostly straight stout segments from a fraction of an inch to a foot long. New spring branch segments have downy hairs (puberulent), which is lost over the growing season. The greatest growth rate occurs on branches that terminate across the crown and, even more so, around the tree’s mid-section. Expansion of the mid-section is due to the dominance of the lower of the two opposite terminal buds, so that near-horizontal branches develop. Heavy fruit loads at the branch tips further enhance this spreading, shrub-like habit. Branches that grow from less dominant buds do not die; they just grow more slowly. When a vertically positioned branch terminates with a pair of opposite buds, two divergent branches develop. Young branches have prominent shield-shaped leaf scars that fade away the second year.

Photo 1: Older trunks become fissured with blocky plates. Inset photo shows a seed (the “buckeye”) and a terminal branch segment: beyond the opposite buds (the lower is dominant) is a dead peduncle (stalk of inflorescence), one branch of which still bears the remains of the fruit wall from which one or more seeds have fallen.  Photo – mid-March.
Photo 2: When branches regularly produce terminal inflorescences and the lower bud of the remaining terminal bud pair dominates year-to-year, near-horizontal branching develops. Photo – Jan 20.

Red buckeye has opposite, palmately compound leaves with five or seven leaflets joining at apex of the petiole (leaf stalk). Leaves are widely spaced along the branch (to 2 inches or more apart) and can grow to be quite large (to about 14 inches wide and long) on long slender petioles (to about 6 inches). Leaflets, broadly lanceolate, on ¼-inch petiolules (leaflet stalks), often droop from the petiole apex. They have a shiny medium green adaxial surface with a yellowish green midrib, a yellowish green abaxial surface, and light reddish petioles with the color extending onto the petiolules. The adaxial surface is typically glabrous (without hairs) while the abaxial surface and petioles have a downy pubescence. Leaflet venation is pinnate, depressed adaxially and expressed abaxially, with the nearly straight secondary veins extending to the margin. All leaves have axillary buds, but typically most new growth arises from the single and paired terminal buds. When soil becomes dry over summer, leaves become yellow and may fall off (in some years, quite early), even as fruits continue to mature. If leaves are lost during summer, additional leaves do not develop until the following spring.

Photo 3: Leaves and terminal inflorescences grow rapidly in early spring. Strap-like, light colored bracts and short brown bud scales are poised to drop off. Photo – March 29.

Reproductive buds produce both leaves and a terminal inflorescence, a panicle, attached immediately above the uppermost pair of leaves. From late March into April, for about two weeks, twenty or more upright flowers clusters grow along the rachis of each inflorescence. Each cluster bears one to five flowers on short pedicels (flower stalks), with larger clusters at the base of the panicles and single flowers near and at the top. Panicles bloom sequentially from base to apex and from the rachis outward.

Red buckeye produces both staminate flowers (fertile stamens and infertile pistil) and perfect flowers (both stamens and pistil fertile) in the same panicles. Staminate and perfect flowers have the same appearance, except styles of the perfect flowers are noticeably exserted. Perfect flowers occur at the lower portion of panicles and close to the rachis. As perfect flowers develop fruit, staminate flowers and their supporting pedicels, as well as other non-fruit-bearing portions of the inflorescence dry and drop off.

Photo 4: Peduncle occurs as a continuation of the branch below. Note early-developing axillary buds; the lower bud is already dominant. Perfect flowers, with exserted white styles, can be seen on left side of rachis. Photo – April 20.

Flowers are spectacular: to 1½ inches long, set in dull red, elongate tubular calyxes, tipped with five triangular lobes. The corolla is composed of two protruding upper and two protruding lateral petals with enlarged rounded apical portions and sharply narrower bases that disappear into the calyx. Upper petals are shorter with a larger rounded apical portion while lower petals are longer with a smaller apical portion. Color of rounded apical portions of petals varies from dark to light red with color of narrow portion being lighter red to yellowish. Rounded apical portion of upper petals flares upwards and backwards while rounded portion of lateral petals extends directly forward. Exterior of petals is about the same color as the calyx. The exposed portion of petals is about half as long as the calyx. Five to eight stamens have light yellowish green filaments supporting reddish anthers. Anthers open lengthwise to expose reddish pollen. Styles of fertile pistils are long, slender, very light reddish to white, and taper to pointed stigmas exserted well-beyond the anthers. Infertile pistils of staminate flowers are short and white. Petals and other internal flower parts are partially or totally covered with downy or twisted pubescence.

Photo 5: Display showing stamens (#1), petals (#2), calyx (#3), fertile pistil with ovary and style (#4), and an infertile pistil (#5). Note pubescence. Photo – April 7.

Fertilized flowers produce large (to 2 inch in diameter), irregularly shaped, ovoid, yellow-brown seed capsules that have thick, smooth, dull, leathery husks. Typically, only one to a half dozen fruits per panicle will reach maturity. In early fall, capsules split (dehisce) along two or three seams, allowing their one to three seeds to drop out. Individual seeds are generally rounded, but may have a peaked end (one seed per capsule) or be rounded with a flattened side or two (two or three seeds per capsule). Seeds are large and nut-like, an inch or more in diameter. The seed coat is shiny reddish brown with a light tan hilum, the scar left where the seed stalk has fallen away. Shrunken seed capsule walls and ultimately the entire inflorescence drop off during winter. Seeds are poisonous to humans.

Photo 6: Fruit capsules will dehice along seams to release seed. Thereafter, entire peduncles drop off. Inset photo shows germinated seed in mid-March. Main photo – Jul 28.

In a formal or natural garden, red buckeye would be a superb addition. It has interesting features throughout the year: showy late winter/early spring growth, spectacular flowers, opposite palmate leaves, intriguing fruit with unique seeds, and attractive winter-time structure of stout twigs and plump, handsome buds. Flowers provide an early food source for hummingbirds which, in turn, provide pollination. This shrub-trending plant can be nudged into a tree form by early removal of lower branches. A heathy tree can produce a large number of seeds which, with help from squirrels, can result in extra plants in the area. Trees in open areas may suffer wind damage due to heavy fruit load and may lose all leaves in mid-summer, due to drying soil.

One other buckeye species occurs naturally in Arkansas, the much larger Ohio buckeye (Aesculus glabra). Its greenish yellow flowers and spiky seed capsules help separate it from red buckeye. See previous article on Ohio buckeye. Occasional hybrids between red and Ohio buckeye can be found (these plants are called Aesculus ×bushii).

* A second variety of “red” buckeye, yellow buckeye (Aesculus pavia var. flavescens), found in west and central Texas, has same characteristics as red buckeye, except for its yellow flowers. Red and yellow buckeye hybridize naturally. 

Article and photographs by ANPS member Sid Vogelpohl

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