Flower:

Showy, loose to dense, rounded to semi-pyramidal branching clusters 6 to 10+ inches long at branch tips. Flowers are intermixed staminate (male), pistillate (female) and perfect (both male and female parts), 1 to 1¼ inches across with 4 or 5 petals. Petals are spreading, irregularly rounded, white with a red to pink or yellowish blotch towards the base, the outer surface smooth or covered in white woolly hairs. The 7 stamens are longer than the petals, flaring out and up from the center, hooking up near the tip, and white with deep orange tips (anthers). The single style is white with an obscure stigma at the tip and longer than the stamens. The calyx cupping the flower is bell shaped with 5 lobes, about ¼ as long as the petals.

Leaves and bark:

Leaves are opposite, long stalked, palmately compound with 5 to 9 leaflets, commonly 7. Leaflets are 4 to 9½ inches long, 2 to 4½ inches wide, somewhat wedge-shaped, widest near the tip (obovate), rounded at the tip end with an abrupt taper to a short point, and a long taper to the base. Edges are a bit wavy, finely toothed or double-toothed. The upper surface is hairless or with sparse white hairs, the lower densely covered in rust-colored hairs, more so near the base and in vein axils. Fall color is gold to red-orange.

Young twigs and branches are stout, brown with very fine short hairs and scattered lenticels (pores), becoming smoother with age, the lenticels more pronounced. Terminal buds are very large especially in full sun specimens, to 1½ inches long, smaller below, egg shaped with a blunt tip. Bud scales are also broadly egg-shaped, tips rounded, dark reddish brown with a sticky, resinous coat, tight against the bud. Leaf scars are broadly smiley faced with 6 to 8 conspicuous vascular bundle scars.

Branch bark becomes rougher with conspicuous light and darker gray vertical lines, older bark breaks up into thick, flaky gray plates. There are few specimens to observe a typical trunk diameter for Minnesota, but in its native European range it can reach 16 feet diameter at breast height (dbh) though 6 feet is more typical.

Fruit:

Fruit is a fleshy, globular capsule 2 inches diameter, golden brown, the surface leathery and covered in stiff spines to 1/3 inch long. Seed is a large, shiny, dark reddish-brown nut with a large circular "eye" that sometimes covers nearly half the surface. Each fruit has 1 to 3 nuts.

Notes:

Introduced to North America for ornamental purposes from milder, temperate regions of southeastern Europe and Asia, Horse Chestnut has historically performed poorly in much of Minnesota due to winter hardiness problems. Where it is successful as a specimen tree, the site is typically sheltered from weather extremes. The specimen we photographed was on the University of Minnesota's St. Paul campus, sheltered in between classroom buildings. Still relatively small, it has already suffered some trunk die-out and, as can be noted in our winter image below, it also did not have an adequate growing season to fully harden off and shed its leaves before the onset of winter. Where it is hardy, it is noted for its extremely broad, dense crown and beautiful spring floral display. Interestingly, Horse Chestnut is considered Threatened or Endangered in much of its native range, where it is a tree of deciduous forests and the rocky slopes of valleys and canyons.

It is published here in our field guide mostly to prevent confusion with the far more common US native Ohio Buckeye (Aesculus glabra). Horse Chestnut has white flowers and leaflets are all typically widest near the tip with an abrupt taper to a short point, where Ohio Buckeye has pale yellow flowers and leaflets have a longer taper at the tip. Horse Chestnut buds are also up to 1½ inches long, rather shiny and sticky with 6 to 8 vascular bundles in the leaf scar, where Ohio Buckeye buds are about ½ inch long, dull and smooth and have 3 bundles in leaf scars.