Horse chestnut

Horse chestnut: how do you recognise it?

Habit

Large tree of 20–30 m with a broad, domed and dense crown.

Leaf

Opposite, palmately compound with 5–7 obovate, toothed leaflets 10–25 cm long.

Bark

Grey-brown, flaking in plates on old trees. Large, glossy and very sticky winter buds.

Flower

Upright white ‘candles’ with yellow to red spots, in May.

Fruit

Green spiky husk containing one or two glossy brown conkers (not edible), in September–October.

Difference from sweet chestnut

Sweet chestnut has simple, long and narrow leaves with bristle-tipped teeth, and edible chestnuts.

Difference from red horse chestnut

Red horse chestnut has red flowers and a husk with few or no spines.

Flowering: May · Pruning: July–August and November–February

Identify by feature: Opposite leaves · Nuts, acorns and chestnuts

Looks like it, but isn’t

• Sweet chestnut Castanea sativa: simple, long and narrow leaf with bristle-tipped teeth; edible chestnuts. Difference from sweet chestnut
• Red horse chestnut (Aesculus × carnea): red flowers and a husk with few or no spines.

Why grow it?

• A stately shade tree with hundreds of white flower candles in May.
• The flowers are loved by bumblebees and bees.
• Conkers for crafts in autumn.
• Grows on almost any soil, in sun or partial shade.

Horse chestnut: where does it grow best?

• Sun to partial shade.
• Almost any soil that isn’t too dry; deep and fertile is ideal.
• Allow for a crown 15 m wide or more: only for a park, a square, a field or a very large garden, well away from buildings.
• Under the dense crown it is dark and little grows; in autumn a lot of leaves and husks come down.

Light

Sun or partial shade

Legal basis

Any garden soil

Moisture

Normal

Size

25 m tall, 15 m wide

Hardiness

Throughout Belgium, no protection needed

Goes well with

• Wood anemone: Flowers in March–April under the crown, before the leaves open.
• Lesser periwinkle: An evergreen ground cover that tolerates the shade under the crown.
• Holly: A native evergreen shrub for the shady edge under large trees.

Good and bad companions

Goes well with

• Tolerate the shade under this tree: oregon grape, rhododendron, oleaster, japanese skimmia, garden privet, camellia, laurustinus, box-leaved honeysuckle

Best kept away from

• Sun-loving plants get too little light under the dense crown: english lavender, heather, butterfly bush, blackthorn, highbush blueberry, sea buckthorn, japanese rose, broom

Horse chestnut: when and how to plant it?

1. Plant from November to March when the ground isn’t frozen.
2. Choose a young tree: it establishes better than a large one.
3. Dig a large planting hole, plant at the same depth as before and firm the soil.
4. Put a stake on the windward side and tie the trunk loosely; remove stake and tie after 2–3 years.
5. Water generously after planting and during dry spells in the first year.

Care

Watering

In dry spells during the first two years; after that it copes.

Feeding

Not needed.

Fallen leaves

Rake up the fallen leaves in autumn: the pupae of the leaf miner overwinter in them.

Inspection

Have an old tree checked by an arborist every few years, especially near a play area or path: the wood is soft and large branches can break off.

Tip: Not enough room for a horse chestnut? Red horse chestnut stays smaller and suffers much less from the leaf miner.

Horse chestnut: when to prune it?

Pruning months: July–August and November–February.

When: Dead, diseased and broken branches can be removed at any time of year. Formative pruning in winter or summer.

How:

• Avoid large saw cuts: they rot easily. So prune mainly young trees, while the branches are still small.
• Train a young tree on a single trunk and gradually remove branches that hang too low.
• Cut branches just outside the branch collar, never flush with the trunk.

Caution: Have trees with bleeding canker (rust-brown sap stains on the trunk) checked by an arborist.

Pruning: when and how · Pruning calendar and basic rules

Propagation

Sowing

In October, gather fresh conkers and sow them straight away, 3–5 cm deep, in a pot or where they are to grow; don’t let them dry out. They germinate easily in spring.

Horse chestnut: how does it get through the winter?

Hardy throughout Belgium: no protection needed.

• Fully hardy.
• In winter you recognise it by its large, glossy, sticky buds.

Diseases and pests

• Horse chestnut leaf miner: Brown blotches in the leaves from June, caused by tiny caterpillars feeding inside them; by August the tree is sometimes completely brown. Unsightly, but rarely fatal. Rake up the fallen leaves and get rid of them with the green waste: that removes the overwintering pupae. Tits eat the caterpillars.
• Bleeding canker: Rust-brown to black, sticky sap stains on the trunk and dying bark, caused by a bacterium. There is no cure; have the tree checked by an arborist.
• Leaf blotch: Large red-brown blotches with a yellow edge on the leaves in summer, caused by a fungus. It looks like leaf miner damage and is harmless to the tree. Clear up the fallen leaves.

Through the year

• January: Remove dead branches; winter formative pruning.
• February: Formative pruning still possible; plant in mild weather.
• March: Last month for planting; the sticky buds swell.
• April: Leaves open: each one unfolds from the bud like a little hand.
• May: Flowering: white candles full of bumblebees and bees.
• June: Watch for the first brown blotches of the leaf miner.
• July: Summer pruning of young trees.
• August: Summer pruning still possible; the leaves sometimes already turn brown.
• September: The conkers fall.
• October: Gather conkers for crafts or sowing.
• November: Clear up fallen leaves against the leaf miner; planting season and winter pruning begin.
• December: Remove dead branches; winter formative pruning.

What can you use it for?

Garden and landscape

Large shade tree for parks, squares and playgrounds.

For wildlife

The flower candles are loved by bumblebees and bees.

Also used for

Conkers for crafts; once grated for washing laundry (they contain saponins).

Caution: Conkers are poisonous to people. The wood is soft and not very durable.

Good for wildlife

Bumblebees

Nectar and pollen for bumblebees, also from deep flowers and early in spring.

Bees

Nectar and pollen for honeybees and wild bees.

Did you know?

• Brown blotches on the leaves in summer are the work of the caterpillars of the horse chestnut leaf miner.
• Brown blotches on the leaves in summer are the work of the caterpillars of the horse chestnut leaf miner. This little moth was only discovered in the 1980s near Lake Ohrid in North Macedonia, and then spread across the whole of Europe.
• The yellow spots in the flowers turn red after a few days: the flower then no longer produces nectar, and bees pass it by.
• The leaf scar on the twig looks like a horseshoe, complete with ‘nail holes’.
• Conkers contain soap-like substances: they were once grated for washing laundry.
• The tree comes from the mountains of the Balkans. From Constantinople it reached the Arras-born botanist Carolus Clusius in Vienna in 1576.
• From the Secret Annex in Amsterdam, Anne Frank looked out on a horse chestnut that she described in her diary. The diseased tree blew down in 2010; saplings from its conkers now grow worldwide.