Step by Step: from a chestnut picked in autumn to your first little tree: how to germinate a chestnut, grow it in a pot and eventually move it outdoors.
Have you ever picked up a chestnut and thought: I wonder if I could grow a tree from this? I have. And the short answer is: yes, you can.
The slightly longer answer is that a chestnut isn't exactly the easiest seed to experiment with. It doesn't like being left to dry out, it needs to go through a cold, moist period before it can germinate and, when it finally decides that spring has arrived, it often starts by doing something very sensible: putting down roots before showing us a single leaf.
But that's precisely what makes it so interesting. Because taking a chestnut, getting it to germinate and accompanying it as it grows into a little plant is one of those experiments that can turn a fruit picked up from the ground into something much bigger. Literally!
And since this is a series about trying to grow plants and trees starting from simple things, I think we can get started.
Let's clear up a small point first, because we often use these two words as if they were synonyms.
Chestnuts and marrons both come from the European chestnut tree (Castanea sativa), but marron refers to specific types and varieties, generally characterised by larger fruits and particular characteristics in terms of shape, flesh and ease of peeling. Italy has numerous varieties, as well as several chestnut and marron products with protected DOP and IGP status.
In other words: not all fruits of the chestnut tree are marrons, even though all marrons are, broadly speaking, chestnuts. For our experiment, however, the distinction doesn't make much difference: we can try to germinate both. There is another distinction, though, that matters quite a bit more.
The chestnut we're interested in is the fruit of the European chestnut tree, Castanea sativa. The horse chestnut (Aesculus hippocastanum) is a completely different plant, belonging to a different family, which produces seeds that look very similar to chestnuts. So if you find one in a park or along an avenue, don't assume you've discovered a giant chestnut: it's a different story.
And, most importantly, its seeds are not edible.
First rule: fresh.
Chestnut trees produce what are known as recalcitrant seeds. Translated from botanist-speak: these are seeds that don't like being dehydrated. If they lose too much water, their ability to germinate can quickly decrease. So a chestnut that has just fallen from the tree is very different, biologically speaking, from an old, dried-out chestnut that has been sitting in your kitchen for months.
So, if you want to give it a try, collect several chestnuts and choose ones that are:
I'd take at least four or five, because with this kind of experiment it's always a good idea to give statistics a helping hand.
If a chestnut floats when you put it in water, that may be a sign that it is hollow or has some other problem, but I wouldn't treat the float test as a definitive verdict. It's simply a practical way to make a first selection.
In nature, a chestnut falls in autumn. It doesn't germinate straight away. It spends the winter in the soil, in cool, moist conditions, and then germinates when conditions become more favourable.
This phase is called cold stratification.
Chestnut seeds have a period of dormancy and, in order to achieve faster and more uniform germination, they need a period of cold under moist conditions. Nursery guidelines generally recommend periods of one to three months; for spring sowing, around two to three months of stratification is commonly used.
In practice, we need to do something quite simple: make the chestnut believe that winter has passed. And for that, we can use the fridge.
Take some potting soil or, even more simply, a piece of kitchen paper. Moisten it thoroughly and then squeeze it out: it should be damp, but not dripping.
Wrap the chestnuts in the paper or place them in the soil, put everything into a resealable bag and pop our little artificial winter into the fridge.
Place the chestnuts in the moist material, seal the bag and put it in the fridge, ideally at around 1–4°C. Nursery practices generally use temperatures close to 1–3°C; for growing at home, the vegetable drawer of the fridge is a practical solution.
Leave them there for about 60–90 days, checking them periodically. Open the bag every now and then. If you find a chestnut that is completely rotten or covered in mould, remove it. Also check that the material hasn't dried out.
One of the most interesting things about chestnuts is that germination can actually begin during stratification. The first thing you may see emerging from the chestnut won't be a tiny green shoot, but a small white root.
Before worrying about producing leaves, the new plant needs to develop a system for absorbing water and nutrients and anchoring itself in the soil. So ideally, after a reasonable amount of time, you should find a small white root emerging from the chestnut.
At this point, it's best not to wait too long. The root is delicate and shouldn't be bent, broken or handled more than necessary.
You can also avoid buying a pot specifically for the occasion. A transparent 1.5- or 2-litre plastic bottle, cut near the top and with holes made in the bottom, can work perfectly well to get started. It won't be our chestnut tree's permanent home, of course. But for the first few weeks or months, it will do the job very well.
Fill it with a well-structured, well-draining growing medium. Chestnut trees grow well in deep, well-drained soils, generally from acidic to neutral, and they don't like waterlogging.
Plant the chestnut at a depth of 2–4 cm. Position the root pointing downwards and gently cover the seed. Don't press down on the soil as if you were laying the foundations for a concrete pavement. It needs to remain loose and well aerated.
Chestnut trees need water, especially while they're growing, but their roots also need oxygen. A substrate that remains constantly saturated with water loses its air spaces and can create conditions favourable to root problems. Chestnut trees are also sensitive to waterlogging and to certain diseases affecting the root system.
So the rule is simple:
Moist soil, yes. Soggy soil, no.
Water when the growing medium begins to dry out, and don't leave standing water in the saucer.
If you're the kind of person who looks at a plant and thinks “I'm not sure if it needs water, so I'd better give it a little more just to be safe,” know that I completely understand. But that's exactly how many plants end up drowning in a great deal of love.
When the shoot emerges, it's time to give it light.
Adult chestnut trees grow well in full sun and, once the most delicate stage of germination is over, the young plant will need plenty of light to develop properly.
If our chestnut was born indoors, though, that doesn't mean we should put it straight out into the midday sun the next day.
A plant that has grown in protected conditions may need some time to adjust to life outdoors. Direct sunlight, wind and changes in temperature are very different conditions from those inside a room.
So take it gradually.
A chestnut tree can be started indoors. It isn't a houseplant.
Castanea sativa is a large tree: the Royal Horticultural Society lists it as growing to more than 12 metres tall, with a crown that can exceed 8 metres in width. It is a vigorous, long-lived species, not a natural bonsai for your living room.
So the pot is a temporary nursery.
The goal of the experiment shouldn't be to keep a chestnut tree on the windowsill forever, but to get to the point where the young plant is ready to find its place outdoors.
There is no single correct date. It depends on the climate where you live, how the winter has gone and the condition of the plant. In general, planting out should be done once the risk of significant frost has passed and the soil is in suitable condition for planting. Nursery guidelines for chestnuts include both autumn and spring planting and recommend protecting young plants during their early stages.
Before moving our little chestnut outdoors, though, let's give it a little training. If it has grown indoors, gradually get it used to life outside.
This is something that applies to many plants: don't confuse the fact that a plant needs light with the idea that it is ready to move suddenly from a window to full sun, wind and cold nights.
Chestnut trees grow well in full sun, in deep, cool, well-drained soils, generally acidic to neutral. Waterlogging is one of the conditions to avoid.
And above all, they need space. Before planting, ask yourself:
We're not planting a little plant. We're planting a tree.
If we take a chestnut from a particularly beautiful, vigorous or productive tree, the plant that grows from that seed will not be a genetic copy of the parent tree.
Growing from seed introduces the genetic variation associated with sexual reproduction.
If, instead, we want to preserve exactly the characteristics of a selected variety, chestnut growers use vegetative propagation, for example through grafting. The RHS also lists both seed propagation and grafting, noting that cultivars do not necessarily retain their characteristics when propagated from seed.
So:
Want to grow a chestnut tree? Start with a chestnut.
Want to reproduce exactly that particular marron you like so much? That's a different job.
And since we're here, one important thing: just because the chestnut is huge and delicious doesn't mean that the tree that grows from it will necessarily produce chestnuts that are just as large and delicious.
Genetics, every now and then, reminds us that there are no shortcuts.