Flower of the Week: Horse Chestnut

Horse chestnut (Aesculus hippocastanum) is a common ornamental, deciduous tree. With expanding branches and dense shades, they grow perfectly as street border trees or ornamental trees in yards.

Horse chestnut’s compound leaves are palmate. Each leaf has 5-7 serrate-margined leaflets, which makes it easy to identify. However, these trees are more impressive in April and May each year when the flowering season arrives.

Each spring, horse chestnut blooms with large numbers of white flowers. In different maturity stages, the centers of those flowers may look hot pink or yellow, and present a truly graceful picture. The white flowers group into big, eye-catching conical inflorescences that are about 20-30 cm long and cluster on the branch tips. Certain cultivar varieties even bloom in light yellow or pink, earning even higher ornamental value.

Horse chestnut originated from the Balkan Peninsula and is closely related to the Buckeyes from North America (including 9 species such as A. glabra, A. californica, etc.). The Northern American variety’s flowering season takes place much later than its European cousin and is more cold-resistant.

A Star Among Gardening Trees

Before its introduction into Europe, horse chestnut had already garnered fame to the East in Turkey, where people medicalized the fruits to treat broken wind in horses. That’s why this tree is called “horse chestnut” in English.

Britain was the first country to introduce horse chestnut into horticultural practices, with use dating back to 1580. Thanks to its adaptability to the Western European climate, beautiful shape, and delightful flowers, the tree immediately gained popularity in every Western European country once introduced. Crossing oceans, horse chestnuts are also now widely planted in New Zealand and Canada.

Outside the gate of the Trinity College Cambridge stands an ancient horse chestnut. More of them flank the famous Avenue des Champs-Elysees in Paris as border trees. Horse chestnut has even become the city symbol of Kyiv, the capital of Ukraine.

There was another famous horse chestnut in Amsterdam, the capital of the Netherlands, called the “Anne Frank Tree”. During WWII, Anne Frank’s family took shelter in the vicinity of this tree. During her two years hiding away from the sun, the tree was the only thing outside that Anne could see, and it was mentioned in the Diary of Anne Frank quite a few times. Unfortunately, this tree fell in a windstorm in 2010.

Is "Hose Chestnut" Edible?

The shells of horse chestnut’s fruits are aculeate and crack open in the fall when the fruits ripen, exposing brown, chestnut-like seeds. The fruits often fall off the trees and scatter everywhere.

However, no matter what people may want to believe or what excuse they make – “It looks just like a chestnut!” or “People say the bark and fruit of horse chestnut are used to extract medicine” – the seeds of horse chestnuts are unfortunately inedible. They contain a toxin called Aesculin, which is poisonous to humans.

However, thanks to the horse chestnut seeds’ bitter taste, accidental consumption rarely happens. Though they share the same English name and look similar, horse chestnut is not related to chestnut. Just take a look at their leaves and you can tell the difference right away. Horse chestnut leaves are large, palmate, compound leaves, while chestnut leaves are oblong, simple leaves.

Leaves and fruits of the chestnut tree

Distinguish between them next time you go camping, and don’t pick the wrong nuts to bring home.

Let's Play With Its Seeds

Though inedible, the seeds of horse chestnut are far from being useless. At a minimum, they are fun to play with.

In the UK, horse chestnut seeds are called “conkers”. Once they are ripened in the fall, children play with these seeds in a traditional game called “conker”, where they drill a hole in a horse chestnut seed and fix it to a string. Then, children throw their own seeds to hit the string-pendant seed.

This seemingly simple game is not exclusive to kids. Adults love it, too. Since 1965, a “World Conker Championships” has been organized and held annually in Ashton, Northamptonshire.

The origin of the word “conker” is also interesting. Some people believe it means “hard”, originating from the French word “conque” which means “conch”. But others insist the word derives from a different French word, “cogner“, which means “hit”. It remains a mystery to this day whether the game inspired the naming of the seed, or the seed named the game.

Can I Grow Horse Chestnut?

Horse chestnut likes light. It is slightly shade-enduring and rather cold-resistant. It loves deep soil with good drainage and can also tolerate barren soil. It can adapt to a wide range of soil types, and only has medium requirements for fertilizing and watering. Because the trees tend to lose vitality after fruit harvest, it’s best to purchase and plant commercial, nursery-grown stock for a better rate of survival. The seedling needs to come with a rootball 7 times the trunk diameter at breast height. One-year-old and older trees can be planted in early spring, but once rooted they can’t be transplanted again. Pay attention to watering and fertilization of the plants. Apply organic fertilizers to the adult trees each winter, after the leaves shed.

Horse chestnut doesn’t do well in the heat but is quite cold-resistant. It secretes viscous resin to protect the leaf buds from frost damage. In spring, the resin melts off, and branches sprout from the horseshoe leaf scars. Though the 2003 summer heat in Europe only lasted a short week (Paris had a high temperature above 37°C for just three days), plenty of horse chestnuts turned yellow early, and some died entirely. Thus, in summer, take precautional measures to protect the trees from getting sunburnt.

Pruning and beautifying are best done each winter after the leaves fall, or each spring before budding. The treetop of horse chestnut is naturally round, so it’s best to keep the original top shape, and the general principle is to keep the treetop well-ventilated and pervious to light.

Size: 70-80 ft in height, 40-50 ft in the spread

Hardiness: USDA Hardiness Zone 4-8

Light duration: Full sun to partial shade

Soil: Moist but well-drained soil

Bloom time: Spring