Horse chestnut, <em>Aesculus hippocastanum</em>: planting, growing and care

Botany and description

Genus Aesculus comprises around thirty species of deciduous trees or bush, many native to North America: in that case they are the buckeyes. Other species come from Europe and temperate Asia: these are the horse-chestnuts. Formerly placed in family Hippocastanaceae, the genus is now, according to APG III classification, included in family Sapindaceae: unrelated to firs, this name refers to genus Sapindus, the soap tree of Asia.

Horse-chestnut (Aesculus hippocastanum), the tree commonly thought of when mentioning horse-chestnuts, is an imposing tree that can reach 40 metres and develops a globular crown on a tall trunk. Alongside this giant are dwarf horse-chestnuts or buckeyes, often from North America, sometimes not exceeding two metres in height and with a very rounded habit.

Bark ranges from smooth grey-brown (e.g. Aesculus indica) to deeply fissured (e.g. Aesculus × carnea) depending on species. Buds are reddish-brown, sometimes sticky and protected by two layers of scales.

Foliage is deciduous, opposite and long-petiolate. Leaves are compound, made of 5 to 7 leaflets that may be petiolate (with a petiole) or sessile (without petiole). Depending on species, leaflets are oblong (longer than wide with rounded ends) to elliptical (length twice width with tapered ends), often dentate on the margin, glabrous on the underside and pilous along the veins. Central leaflets are longer than lateral ones, the whole leaf resembling a hand. At bud burst period, leaves may be tinged pink then turn green in summer before taking warm autumn hues: yellow, orange and sometimes red depending on species and cultivars.

Aesculus hippocastanum – botanical illustration

The distinctive inflorescences of Aesculus are called thyrsi. These erect, pyramidal thyrsi develop in spring (between April and June). On one plant, hermaphrodite flowers can occur alongside male and female flowers: this condition is called trimonoecy. In fact, male flowers possess aborted female parts and vice versa. Horse-chestnuts and buckeyes are insect-pollinated and require insects for pollination. That is why their flowers are strongly scented and nectariferous.

Each flower has a bell-shaped calyx with 5 unequal teeth. Corolla consists of 4 to 6 petals of unequal size, white, yellow, pink or red and often spotted on two petals: yellow spots on petals turn red when flower has been pollinated, signalling to visiting insect that the flower will no longer produce nectar and is not worth visiting. Insects perceive yellow but not red. Stamens number 5 to 9 and vary in length depending on species. Remaining floral parts form a pistil with 3 carpels, fused and topped by a style as long as the stamens and a stigma acute or bifid.

Fruits resulting from fertilization are actually dehiscent capsules: the husks correspond to the fruit covering and derive from transformation of the carpel walls (note: chestnut burr arises from transformation of flower bracts). These husks differ by species: sometimes spiky, sometimes smooth, and oval, round or pyriform (pear-shaped). Husks contain a seed (rarely two): the conker. Dehiscent, they usually open along three seams corresponding to the number of carpels. Dispersal is, as you might expect, barochorous — fruits fall by gravity right beneath parent plant. Unless you kept childhood habit of filling pockets with conkers and… losing them everywhere afterwards.

Growth of these trees and bush is generally fairly rapid. Trees may live 200–300 years, while bushes have an average lifespan of about 50 years. Sexual maturity in horse-chestnut occurs around 15–20 years.

Fruits of horse-chestnut: the famous conkers and their husks!

Brief overview of main species

Aesculus hippocastanum

Horse-chestnut is widely used as a street or large park tree. Paradoxically, it is fairly sensitive to urban pollution and prone to various diseases. It is a tall tree, up to 30 m with a spreading rounded crown. Flowering occurs May to June, scented and ranging from creamy-white to orange-red.

Aesculus x carnea

This horse-chestnut is a hybrid between Aesculus hippocastanum and Aesculus pavia, appearing in early 20th century. It is smaller than A. hippocastanum, about 9 m tall, but tolerates drought better than the latter. Foliage is mid-green with sticky buds. Flowering is pink and appears in May–June.

Aesculus pavia

This small horse-chestnut, or rather buckeye, is native to eastern United States and easy to grow. It is hardy enough to be cultivated widely here. It can be a 7 m tree or a 3 m bush depending on cultivar. Foliage consists of small leaves with 5 leaflets that turn a fabulous red in autumn. In June, a mass of tubular scarlet flowers appears.

Aesculus parviflora

The bottlebrush buckeye is native to south-east United States, from Georgia to Florida. It is a suckering horse-chestnut that forms a large dense rounded bush up to 4 m wide by 2.5 m high. Deciduous leaves emerge bronze-tinted then turn dark green before becoming golden yellow in autumn. In June–July, splendid upright spikes about 30 cm tall resembling candelabra and white with bright red stamens appear. They attract many butterflies. Note that conkers are small with light-brown skin and enclosed in a pear-shaped, non-spiny husk.

Aesculus indica

This tree grows on Himalayan slopes and is a species threatened in its natural habitat. It is fully hardy and readily reaches around 20 m in height. Summer flowers are delicate and pinkish-white.

Aesculus californica

This small tree tolerates drought particularly well. Its relatively modest size, up to 5 m, makes it suitable for small gardens. Foliage is grey-green and flowers are scented, creamy with pink tints.

Aesculus flava

This is a large horse-chestnut nicknamed the yellow horse-chestnut because of its yellow flowers. In cultivation it can reach up to 25 m with a 15 m spread.

Aesculus mutabilis

Aesculus (x) mutabilis is a hybrid between compact Aesculus sylvatica, native to Appalachian forests of United States, and Aesculus pavia. It is a small shrubby horse-chestnut about 2 m high and 2 m broad, fully hardy. Deciduous leaves are lime-green with bluish reflections and colour beautifully to yellow or orangey-pink in autumn. Small flowers appear May–June and are deep pink to salmon; the salmon hue is due to small yellow patches at petal tips.

Aesculus neglecta

Aesculus (x) neglecta is a recent horticultural hybrid sharing the same parents as the hybrid cited above, yet rather different. This medium-sized horse-chestnut forms a trunk topped by a rounded crown, reaching about 6 m high by 3 m wide in 10 years. Young shoots are strikingly coloured and unfold into large palmate leaves that quickly turn light green. From mid-September, this bush dons a red–orange autumn coat. Inflorescences, in the form of small erect panicles 10–15 cm long, open between May and June. Flowers are pale yellow, then deepen to salmon-pink.

Our best varieties

Aesculus pavia var. discolor Koehnei

This bush is a dwarf horse chestnut, compact and slow-growing. It flowers in May–June as upright panicles of deep pink to salmon. Its palmate foliage, bronze at bud burst period, turns green through summer before taking on a rust colour in autumn.

• Flowering time June, July
• Height at maturity 3 m

See the product sheet

Aesculus parviflora

Aesculus parviflora, or white buckeye, is a bush native to the southeastern United States, thoroughly hardy, with foliage bronze at first, then dark green and finally yellow in autumn. The bush bears panicles of white flowers with a feathery appearance in late summer.

• Flowering time August, September
• Height at maturity 3,50 m

See the product sheet

Aesculus mutabilis Induta

Adorable deciduous bush with a rounded crown, this dwarf horse chestnut will not exceed 2 m in height. Its palmate foliage, divided into five large leaflets of lime green, frames a superb spring flowering of red to salmon-pink candles, followed by the formation of small ovate conkers.

• Flowering time June, July
• Height at maturity 2,25 m

See the product sheet

Aesculus neglecta Autumn Fire

Fine ornamental tree, this horse chestnut is slow-growing and reaches only 6 to 8 m in height. Superb in spring with young bronze-copper shoots, it turns light green in summer before taking on a flamboyant orange-red autumn display. It flowers in May–June as pale yellow candles turning pink.

• Flowering time June, July
• Height at maturity 9 m

See the product sheet

Planting

Where to plant your horse chestnut?

As a rule, horse chestnuts or pavias are planted in any deep, well-drained soil that remains cool even in summer.

They are all hardy enough to grow anywhere in France and Belgium but they dislike coastal areas because of sea spray. They prefer regions with well-defined seasons: cold winters and hot summers.

Horse chestnuts are planted in full sun or partial shade. Only white pavias can thrive in full shade.

Note: as with all trees, allow for its development and choose your horse chestnut wisely according to its size and that of your garden.

When to plant your horse chestnut?

Horse chestnuts are preferably planted in autumn (November) or in winter outside frost periods. Spring planting can be considered for American horse chestnuts.

How to plant your horse chestnut?

• Immerse pot of your new Aesculus young plant in a bucket of water for a few minutes to re-moisten the rootball
• Dig a hole twice as deep and twice as wide as size of rootball
• Throw one or two handfuls of well-rotted compost into bottom of hole
• If soil is heavy, add some gravel to improve drainage
• Begin to loosen rootball a little to free roots. Do this gently by hand or with a small fork to avoid damaging roots
• Place remainder of rootball at bottom of hole, spreading out roots well
• Fill hole with excavated soil that has been loosened beforehand
• Firm soil gently around tree with hands (not with feet!)
• Give base 10 litres of water with a watering can to reduce risk of air pockets between roots and soil
• Apply mulch to protect young tree from drought or plant directly around tree some small groundcover plants (creeping bugle, sweet woodruff, Glechoma, Geranium macrorrhizum, …)

Beautiful Aesculus californica in its setting

Care and maintenance

Care

Water your horse chestnut regularly during the first years after planting. These trees or bushes are sensitive to dry soil.

Pruning

Prune only obstructive or crossing branches and any suckers on some species (notably Aesculus pavia). Avoid cutting branches over 5 cm in diameter. Horse chestnuts react poorly to pruning, often carried out harshly in urban environments. They form large callus ridges but the inside of the trunk or branch easily rots. Exercise caution. Best option is to choose a site large enough to accommodate an adult horse chestnut or select a species with a smaller habit.

Possible diseases and pests

Unlike other Aesculus species, which rarely contract diseases or are scarcely attacked by pests, Aesculus hippocastaneus or horse chestnut faces a few problems…

• Bacterial canker of horse chestnut

This disease devastates horse chestnuts in our towns and is caused by a bacterium named Pseudomonas syringae. It is mainly found on nursery-grown horse chestnuts (therefore with low genetic mixing), those that have suffered stress (water stress, pollution, root damage) or those damaged by inappropriate pruning. Symptoms are varied: delayed bud burst period, slowed growth, presence of an oozing canker and finally drying of foliage on part or all of the tree. Various treatments are currently being tested with little progress; ideal measures are to provide horse chestnuts with a healthy environment and appropriate care beforehand. This minimises risk of decline.

• Scale insects

Pulvinaria regalis scale is often found on horse chestnuts, usually without causing significant damage. Do not worry about it!

• Red-spot disease

In cool, wet springs, horse chestnut can be susceptible to red-spot disease or black-rot. In early summer, foliar spots first yellow then brown with a yellow halo appear. These are quickly followed by drying and leaf fall.

This disease is particularly problematic for young trees.

Horse chestnut red-spot disease (Guinardia aesculi on Aesculus hippocastaneum)

• Horse chestnut leaf miner

The horse chestnut leaf miner (Cameraria ohridella) is a lepidopteran (moth) native to the Balkans that causes significant damage to horse chestnuts, and sometimes to Aesculus pavia. Larvae mine the leaves, creating brown-reddish patches that lead to leaf fall during the summer. Egg-laying occurs in one to three generations from April to late autumn. Larvae develop and then pupate inside leaves over winter, emerging as moths in spring to restart the cycle. Best control is to encourage a garden welcoming to insectivorous birds (tits in particular) and, in severe cases, to collect and burn fallen leaves to destroy chrysalises.

Read also : Horse chestnut leaf miner: a pest to fear?

Propagation of horse chestnut

By sowing

In October–November or in March, you can sow the conkers in a substrate kept moist and fairly light: 40% potting compost, 40% pumice or sand and 20% loam. Once germination has started, in early spring, the seedling will develop a substantial taproot before branching. Your young shoot should be placed in partial shade and sheltered from wind. Do not leave this young horse chestnut in a pot for too long; ideally plant at start of following autumn in a sunny spot.

By grafting

To reproduce cultivars, propagation can be carried out using a shield graft onto a true Aesculus rootstock. This technique is fairly complicated and should be reserved for professionals.

By separation of suckers

In the case of paviers, germination is fairly slow but fortunately the bush produces suckers that can be separated from the parent plant in November or March. To do so, dig around the bush to expose one or more suckers and sever them with a spade or pruning shear.

→ Learn more in our tutorial : How to propagate horse chestnut?

Companion planting in the garden

Ideas for autumn…

Aesculus pavia ‘Koehnei’ is a delightful horse chestnut that really comes into its own in autumn when it can offer bright orange foliage. To accompany it, nothing beats a small tree also native to North America, such as Franklinia alatamaha. This tree has vanished in the wild but survives in parks and gardens thanks to enthusiasts. Not only does it provide red‑orange foliage in autumn, it even has the luxury of dazzling with very late flowering, sometimes lasting until frosts, with flowers resembling single camellias: white with golden stamens. At the feet of our two companions, a tapetum of Cornus canadensis, one of the rare representatives of the genus that is a perennial, will look superb. In autumn, these tiny miniature dogwoods do not go unnoticed when white bracts have given way to red fruits and the foliage takes on purple hues. A few clumps here and there of Deschampsia caespitosa ‘Goldschleier’ will add lightness with their large inflorescences in loose panicles where gold and silver mingle. And finally… How about trying bergenias? No, wait, don’t run away! There are now very attractive varieties nothing like bergenias of old. For example this superb variety named ‘Eden’s Dark Magic’, whose glossy dark‑green foliage edged with purple takes on spectacular purple and brown tones in autumn.

An example of an autumn planting: Deschampsia caespitosa ‘Goldschleier’, Aesculus pavia ‘Koehnei’, Franklinia alatamaha, Cornus canadensis and Bergenia ‘Dragonfly Sakura’ (or ‘Eden’s Dark Margin’)

Or for spring…

But Aesculus are also planted for their spectacular flowering, such as the small slow‑growth tree Aesculus neglecta ‘Autumn Fire’. Its name, as you will have guessed, comes from its fiery orange autumn foliage, but it is equally beautiful in spring when young leaves unfurl in bronze‑copper tones. This is quickly followed by a flowering that starts pale yellow then turns salmon as days pass. It is truly a gem to plant on its own… or not. Well accompanied, it will be even more striking! Some bushes with golden‑yellow foliage will complement the young leaves and flowers of the horse chestnut perfectly: for example Cornus alba ‘Aurea’, a timeless classic always attractive from January to December, together with a handful of Spirea japonica ‘Magic Carpet’ at the border. A touch of blue will be perfect to contrast with all these elements. Take our exclusive Hydrangea macrophylla ‘The Original’ with blue flowers, a very robust and very floriferous hydrangea. Fancy blue foliage? Could a small Picea pungens ‘Glauca Globosa’ delight you? This small conifer is marvellous in spring when it takes on bluish tones.

An example of a spring planting: Aesculus neglectum ‘Autumn Fire’, Picea pungens ‘Glauca Globosa’, Cornus alba ‘Aurea’, Spirea japonica ‘Magic Carpet’ and Hydrangea macrophylla ‘Endless Summer The Original’ (blue)

Did you know? To be completed

• Aesculus flowers are nectariferous and melliferous, but that’s not all: bud scales provide bees with propolis — a resinous substance that enables them to seal their hives, plug cracks and secure wax ray florets.
• Botanists tore their hair out for a long time trying to determine where the horse chestnut came from. So much so that an expedition was sent to India in the 19th century in search of the tree’s origins. With no real success, although Aesculus indica, a close relative, does indeed grow in the Himalaya from Afghanistan and Pakistan to western Nepal.
• Horse chestnut proved so popular that a census from 1870 shows nearly 80% of trees planted in Paris public spaces were horse chestnuts.
• The genus name Aesculus actually means ” succulent “, which is paradoxical for trees and bushes whose fruits are not edible.
• Fruit contains saponin and, during the Second World War, a soap substitute was made from it. During that war, conkers were also used to prepare a nourishing starch for humans, after treatment to remove tannic acid and that saponin. Even today in France, conkers are harvested for the preparation of dry extracts.
• Aesculus parviflora was introduced to England in the 18th century thanks to British botanist John Fraser after his first voyage to America and quickly became a sought-after plant in nurseries. So much so that it eventually received the Royal Horticultural Society’s Award of Garden Merit (RHS).
• If you find yourself in a riddle duel, you can always throw out this formidable riddle to get out of it: “What is small and brown?” The answer, of course, is a conker.