Buckeyes and horse chestnuts are all members of the genus Aesculus, a large group of many garden worthy plants that ranges in size from low and shrubby, to tall and imposing, and includes North American natives, European and Indian exotics, and many horticultural hybrids. All share the characteristic of producing beautiful candelabras of flowers perched among coarse, leafy branches, with hand-shaped foliage, from May through June, with exact time of bloom dependent on the species or cultivar. All of the trees discussed here are fully hardy throughout the southeastern US and adaptable to most southeastern landscape conditions as long as they are not grown in exceptionally dry soils. They will prosper in full sun or partial shade with full sun giving the best flowering and also minimizing fungal disease problems on the foliage. Aesculus are sexually propagated by seed layered in a moist medium, like peat moss, and stored for three to four months at 40°F, while named cultivars are generally grafted onto seedling understock (with the exception of A. parviflora).
Among the best known of the group is the common horse chestnut, A. hippocastanum, a native of Greece and adjacent mountainous areas. This large, bold textured tree will reach 50' to 80' in height with an almost equal spread in its wonderful, beehive shape. Creamy colored flowers with a yellow throat and reddish speckled center are borne in 6"–8" tall pyramidal clusters in June. Flowers mature to 2" diameter fruit in a shell covered with dangerous spines. Fruit split open to reveal the handsome, chestnut-brown, shiny nuts that are the treasure of children the world over. This is an excellent tree for large public spaces and has been widely planted in parks and recreational areas in both the eastern United States and Europe. It is not a good choice for small properties (there are many good, small Aesculus for those settings) but is wonderful in large, rolling pastures or other open lawn areas. Fall color is essentially nonexistent except for perhaps some gold-yellow. The foliage is susceptible to powdery mildew and leaf blotch and often browns and dies early in the fall. The excellent cultivar 'Baumannii' has large, double white flowers that are sterile and therefore produce no fruit (which means no spiny fruit to rake or step on).
Among the numerous North American native Aesculus, there are five species native to the southeastern United States (with somewhat overlapping ranges) that are also good garden plants themselves or are parents of the better horticultural hybrids. They range from small to large trees with beautiful flowers that can be found in the woods from Georgia through the Carolinas and up into Ohio. Each of the five species has its own geographic area of concentration but they do overlap and and are easily confused unless they are in flower (and even then they are often difficult).
A. flava, yellow buckeye, is a large tree ultimately reaching 50' to 70' in height. Yellow flowers appear in May and are quite showy in 6" upright clusters. Fruit is smooth and interesting in appearance. Foliage is less susceptible to fungal disease than others and may turn a burnt orange in the fall. Yellow buckeye has a northerly native range from areas of northern Georgia to the mountains of North Carolina and Tennessee, up through Pennsylvania, Ohio, and Illinois.
A. glabra, Ohio or fetid buckeye, is another native of the northern extension of their range stretching from Tennessee up through Ohio into the Midwest. It can be distinguished from its other native cousins by a fetid odor released when the stems are bruised or broken. Flowers are greenish-yellow with orange throats and not as showy as other species but are delightful additions to the late spring woodland landscape. The foliage is a very attractive rich green and emerges relatively early. It is generally seen as a young tree in native settings but will reach 20 to 40 feet in height with a pleasing, rounded habit. The grey bark is often corky.
A. parviflora, bottlebrush buckeye, is a spreading, multi-stemmed, low, shrubby plant only reaching 6' to 10' in height. The form is quite fluid and beautiful. Its native range is from South Carolina through Alabama and Florida. Foliage is moderately pubescent (fuzzy), rarely disease susceptible, and often turns a lovely, clear yellow in the fall. Flowers are white and appear quite late in June on exceptionally large, 8", erect clusters. This buckeye is extremely tolerant of shade and an excellent choice for small, shady gardens where its dramatic flowers are a wonderful summer show. It is important to note that it will sucker and spread, and so needs root pruning in very restricted areas. The botanical variety, A. parviflora f. serotina flowers several weeks later than the species and has smooth foliage. Fruit is smooth and light brown but fruitset is often sparse. The cultivar 'Rogers' is a selection of var. serotina from the University of Illinois with incredibly grand, 18" tall flower clusters that bloom very late. This species can be propagated from softwood cuttings rooted under mist.
A. pavia, the red buckeye, is another low-growing, shrubby species that forms clumps reaching 10' to 20' in height with equal or greater spread. Foliage is handsome in season but develops no appreciable fall color. Flowers are generally a very beautiful salmon to cherry-red color but there is a yellow-flowered native strain as well, A. pavia var. flavescens. This species is found from the coastal plain of North Carolina south and west all the way to Texas. Red buckeye usually has less powdery mildew than horse chestnut but may have severe leaf blotch problems late in the summer. There is no good fall color. The cultivar 'Atrosanguinea' has very deep red flowers while 'Humilis' is an especially low-growing, almost prostrate form with small clusters of true red blooms.
There is some argument among botanists and horticulturists as to whether A. splendens (now recognized as Aesculus pavia 'Splendens') is truly a separate species or only a botanical form of A. pavia. Its chief differentiating characteristic is a very heavy pubescence on the undersides of leaves. It is also a red-flowered form.
A. sylvatica (previously named A. georgiana), the Painted Buckeye, is abundant in the Piedmont woods of North Carolina, Georgia, Tennessee, and Alabama, with some of the population in Virginia. Flowers range from yellow, to yellow-green, to pinkish yellow, in broad, 5"–6" high clusters, with color variation often on the same plant. This understory tree is generally 5' to 12' tall with a somewhat rounded, open habit. Foliage is an attractive emerald green but shows little fall color.
Some of the best Aesculus for the garden are the horticultural hybrids. Aesculus ×carnea is a spectacular medium sized tree reaching 20'–40' in height with a broadly conical shape. The glossy foliage is a beautiful grass green and makes an excellent foil for the 8" upright candelabras of salmon-pink to light red flowers. Disease is less of a problem than with common horse chestnut but powdery mildew is nonetheless likely to appear. This does not interfere with the overall growth or flowering of the tree but results in early leaf drop. The cultivar 'Briotii' is a floriferous, salmon flowered form, while 'O'Neill' offers truer red flowers in particularly large clusters to 10" in height, and 'Rosea' has pink-red flowers.
Aesculus ×hybrida is something of a catchall name for the many hybrids of A. flava with A. pavia. Flowers are yellow-red to salmon-red to pink. The true names of these hybrids are very confused and they are often mixed with other hybrids having A. pavia as one of the parents. Aesculus ×arnoldiana 'Autumn Splendor' is a cross of A. ×hybrida with A. ×glabra from the Minnesota Landscape Arboretum which offers the unusual trait of bright red fall color.
The horse chestnuts and buckeyes are a large and often confusing clan but the selections and hybrids found in reputable nurseries and garden centers offer unusually bold-textured spring and summer foliage combined with striking floral displays for late spring and summer. This unique combination of landscape characters makes Aesculus a marvelous choice for most gardens. Whether you choose a low-growing beauty for close inspection, or a magnificent large tree to display those wonderful, dramatic flowers, Aesculus will bring a bold show of color and texture to your summer garden.