Aesculus hippocastanum aka Common Horse Chestnut

Horse chestnut, Aesculus hippocastanum, is a large deciduous tree in the soapberry family, Sapindaceae. It comes from the mountains of the central Balkans, including Greece, Albania and Bulgaria. Its range extends east into Türkiye. Gardeners now grow it throughout temperate regions of the world. In Britain people call it the conker tree, after a children's game played with its seeds.

In gardens the tree usually reaches 50 to 75 feet (15 to 23 metres) tall. Its spread runs from about 40 to 70 feet (12 to 21 metres). Exceptional trees can top 100 feet (30 metres). The crown is upright and oval to rounded. Lower branches droop, then their tips curve upward. Growth is moderate, and a tree may take 20 to 50 years to reach full size.

The bark is brownish grey and breaks into rough, scaly plates with age. These plates can peel to show orange bark beneath. Stout twigs carry large, glossy, sticky buds of reddish brown. The opposite leaves are palmate, with usually seven toothed leaflets. Each leaflet grows 4 to 10 inches (10 to 25 cm) long, with sunken veins. Leaves open light green and darken in summer. Autumn colour is often a dull yellow or brown.

In May the tree bears upright panicles of creamy white flowers, 5 to 12 inches (13 to 30 cm) long. Each flower has a yellow blotch that turns red with age. Bees visit the blooms. The fruit is a round, very spiny, leathery husk about 2 inches (5 cm) across. It splits in September or October to release one to three glossy brown seeds.

The tree grows best in full sun but tolerates part shade. It prefers moist, fertile, well-drained soil and adapts to clay, loam, sand or chalk. Soil pH matters little. It is very cold hardy, to USDA zones 3 or 4 through 8 and UK rating H7. Dry, hot or windy sites cause the leaves to scorch. A deep taproot makes established trees hard to move.

The tree suits parks, campuses, golf courses and other large spaces. It is too big for small gardens. Many towns plant it as a shade or street tree. The seed saponins can serve as a soap substitute. The husk and bark yield dyes. The wood is soft and of little value. Herbalists use the plant to tone veins, but only under professional guidance.

Leaf blotch and scorch damage the foliage most years. Powdery mildew, anthracnose, leaf spot, canker, coral spot and honey fungus can also occur. Pests include horse chestnut scale and the leaf-mining moth. Bagworms, Japanese beetles and borers attack it from time to time. Falling fruit makes a mess.

All parts of the tree are poisonous, and the seeds are the main danger. They contain aescin saponins and the glycoside aesculin. Eating them causes vomiting, diarrhoea, weakness and, in severe cases, paralysis. Dogs, cats, horses and livestock can suffer tremors and seizures. Some traditions roasted the seeds for a coffee substitute. Others leached them to make a starchy flour. Leaching removes the saponins, but eating the seeds remains risky.