Sweet Chestnut: Growing and Harvesting Your Own Marrons

There is that moment in October when you stand under an old sweet chestnut and hear a rustle: first the prickly husk, then the glossy brown marron peeking out of it. Anyone who has roasted their own marrons once understands why the long wait is worth it. Because this tree needs patience. It grows old, it grows large, and it rewards you only after a few years. In return it then bears for decades, often across a whole gardening lifetime.

Sweet or horse chestnut? The difference that counts

This is the most important thing first, because it causes confusion every autumn. The horse chestnut (Aesculus hippocastanum) stands in parks and beer gardens, and its big brown conkers lie around everywhere in autumn. They are pretty for crafts, but mildly toxic and not for eating.

The sweet chestnut belongs to the beech family and is not related to the horse chestnut at all. Two features tell them apart reliably: the husk of the sweet chestnut is densely covered in long, fine spines like a sea urchin, while the horse chestnut has only a few short, soft warts. And the leaves are completely different. The sweet chestnut has long, single leaves with a serrated edge, the horse chestnut has finger-shaped compound ones. Once you have seen the two side by side, you will never confuse them again.

By the way: marron and sweet chestnut mean the same thing. Strictly speaking a "marron" is an especially large-fruited, easily peeled selection, but in everyday use we use both words for the fruit of the sweet chestnut.

The right site: acidic, warm and plenty of space

This is where it is decided whether your tree thrives or struggles. The sweet chestnut comes from the warmer regions of southern Europe and Asia Minor, and you can tell. It wants warmth, sun and above all the right soil.

Soil is the most common reason for disappointment. The sweet chestnut is decidedly lime-sensitive. On limey, high-pH soils it develops chlorosis, the leaves yellow and the tree stays weak. Ideal is a deep, free-draining, rather sandy-loamy soil with a pH of 5 to 6.5. It does not tolerate waterlogging at all. If your garden soil is heavy and limey, the sweet chestnut is sadly the wrong choice. In that case a walnut is worth a look, as it copes with more lime.

And then the space. A sweet chestnut is not a tree for a small terraced garden. As a seedling it grows 20 to 30 metres high and wide, grafted cultivars stay smaller at 8 to 15 metres but still need plenty of room. Allow at least 5 to 8 metres from the neighbour's fence and picture how the tree will look in 30 years, not today.

Planting and pollination

The best time to plant is in autumn between October and November, or in early spring while the soil is not frozen. Container stock can go in almost year-round, bare-root trees belong in the leafless season. Which option makes sense when we described in detail in the piece on planting a fruit tree.

1. Dig the hole generously

   Dig a planting hole twice as wide as the root ball. Loosen the base deeply so the taproot finds its way down.

2. Improve lime-free

   Work in mature compost and some acidic bark humus. Avoid lime, wood ash and limey substrate.

3. Set it at the right depth

   Set the tree as deep as it stood in the pot. The graft union stays above the soil.

4. Stake and water in

   A tree stake gives the young trunk support. Then water in thoroughly until the soil settles.

5. Mulch

   A layer of leaves or bark mulch keeps in moisture and keeps the soil life alive.

Now the point where many hobby gardeners fail: pollination. The sweet chestnut is self-sterile. A single tree flowers splendidly but sets barely any fruit, because it cannot pollinate itself. You need at least two different cultivars that pollinate each other, and the wind plus insects do the rest. Ideally the trees stand no more than 30 to 50 metres apart. No space for two? Often there is already a sweet chestnut in the neighbourhood, and then a single tree at your place is enough.

Flowering comes late, usually only from late June into July. Long, yellowish cream-coloured catkins then hang on the tree and give off an intense scent that some noses find strong. From the female flowers at the base of the catkins the prickly husk develops over the summer. Only two to three years after planting grafted cultivars does yield set in, with seedlings it can take ten years and longer.

Care through the year

The good news: an established sweet chestnut is little work. It is robust, healthy and above all wants to grow in peace. Care focuses on the first years.

Watering matters mostly while the tree is young. In the first three to four years you water thoroughly during dry spells so the taproot grows deep down. Once the tree is established, it looks after itself almost entirely. For feeding the same rule applies as for the soil: lime-free. In spring before bud break, so March or April, you work compost and horn shavings shallowly into the tree disc. Wood ash and lime stay off limits, otherwise chlorosis threatens again.

Pruning is minimal. Unlike an apple, you need no annual training cut. Remove dead, diseased or rubbing wood, and that is usually it. Save larger cuts for late winter, just before bud break, so the wounds heal well. Keeping the crown airy prevents fungal diseases and makes the later harvest easier, because the husks fall to the ground unhindered.

Harvest in October: the prickly challenge

Now comes the finest part. Harvest falls in October, usually between CW 40 and CW 43. You do not pick from the tree but gather from the ground. Ripe marrons fall on their own, either in the opened husk or as a glossy nut that has sprung from the split husk.

The spines are no joke, they sting nastily. Sturdy garden gloves are a must. Many gardeners roll the closed husks carefully with a shoe or a board until the marron comes loose. Gather daily if you can, because nuts lying on the ground are quickly found by mice, wild boar and insect larvae. Fresh, good marrons are plump and heavy, old ones become light and hollow.

1. Gloves on

   Nothing works without sturdy gloves. The husk spines are sharp and break off easily.

2. Gather daily

   In harvest season go under the tree every day if you can. Freshly fallen is best.

3. Do the float test

   Drop the marrons into a bucket of water. Whatever floats is hollow or larva-infested and goes.

4. Soak briefly

   One to two days in water (changed daily) kills off larvae and makes the nuts keep longer.

Storing, preparing, enjoying

Freshly harvested marrons are delicate, because they hold a lot of water and mould quickly. Unpeeled and lying in the open they keep only a few days. For longer storage you spread them out dry and airy for a few days, then they go into a net or an airy crate in a cool, dark place. That way they keep for several weeks. If you want to save the harvest over winter, freeze them raw and peeled or cooked, and they last for months. Detailed tips on drying and keeping you will find in the piece on harvesting and storing nuts.

In the kitchen marrons are wonderfully versatile. Classically they are roasted: before roasting or cooking you score each marron crosswise on the domed side, otherwise it bursts uncontrollably. Then they go for a good 20 minutes into the hot oven at about 200 degrees or into a pan. They peel most easily while warm. Besides roasting they are lovely in soups, as a side to game and roasts, in stuffings, or sweet as marron purée and in cakes.

Diseases and pests to watch

The sweet chestnut is robust in principle, but you should know two problems. Chestnut blight, caused by the fungus Cryphonectria parasitica, is the most serious threat. It produces sunken, cracked patches of bark above which branches die off. Buy trees from healthy nurseries, keep the crown airy and cut infected branches well back into healthy wood.

The second frequent guest already sits inside the nut: the larva of the chestnut weevil. It is the reason for the float test and the brief soaking after harvest. You recognise infested nuts by small bore holes and by the fact that they float in water.