Horse chestnut, American chestnut are distinctly different kinds of tree

The ink had not dried on my last column before I realized the need to explain the difference between its subject, the American chestnut, Castanea dentata, and a far more common tree with the misleading common name of horse chestnut, Aesculus hippocastanum. Your life may depend on knowing the difference, since the nuts of the latter species are extremely toxic to humans.

The two trees are not remotely related. American chestnut is in the oak and beech family of trees, while horse chestnuts are in their own family. Comparison of foliage characteristics provides a reliable way of telling the two species apart. American chestnuts have simple leaves, narrow and coarsely toothed along the margin, while horse chestnuts have palmately compound leaves, each leaf composed of five to seven leaflets fanning out from a central point on the leafstalk.

Floral characteristics are also distinctive. American chestnut flower clusters are long, pencil-thin spikes that appear in summer, after the leaves are fully grown. The flowers of horse chestnut appear with the new leaves in spring.

In middle to late May a horse-chestnut tree becomes a living candelabrum, large spiked flower clusters exploding from the branch tips. Each panicle bears a hundred or more bright white flowers, the petals splashed with yellow and deep pink. The blossoms give off a sweet aroma, promising a nectar reward for bees that move the pollen around as they forage.

Flowers are followed by prickled capsules with green-brown husks. At maturity the capsule is golf-ball size and contains one or more shiny, 1- to 2-inch reddish-brown seeds, or nuts.

While they look good enough to eat, the nuts contain esculin, a chemical that destroys red blood cells, causing vomiting, paralysis, and sometimes death in humans. Squirrels, on the other hand, are not bothered by the esculin and devour the seeds as they ripen and fall from the tree.

If I come across a horse chestnut when the seeds are falling, I squirrel away one or two myself, carry them around in my pocket for good luck until they develop wrinkles and lose their shine. This is a carry-over from my youth in the South where Ohio buckeyes and yellow buckeyes, native species related to horse chestnuts, are abundant trees and their long-lasting seeds always among the contents of a country boy’s pockets.

In winter, horse chestnuts can be distinguished by the large, sticky apical bud that terminates each stem. Also, the fallen leaf leaves a distinctive horseshoe-shaped scar on the winter twig of a horse chestnut.

Horse chestnuts grow wild in Greece, Albania and Bulgaria. Since the 16th century they have also adorned the gardens and city streets of northern Europe, lining the Champs-Elysees in Paris and a mile-long avenue of trees in London’s Bushey Park. They made their first appearance in North America during the 18th century. While they have proved to be a good urban tree in this country, they are not without problems. In 1996, hundreds of horse chestnuts were cut down in Brooklyn, N.Y., to limit the Asian longhorn beetle infestation.

I have a fondness for the old horse-chestnut trees scattered about Maine landscapes, trees with swooping branches that shade the front of old homes. They are a good excuse to leave the main highway for the narrow lanes. Their massive trunks are typically flush with the edge of the road, an indication that the tree was there first, the road made to accommodate it.

Send queries to Gardening Questions, P.O. Box 418, Ellsworth 04605, or to rmanley@ptc-me.net. Include name, address and telephone number.