Gardening has been a happy place for so many of us over the past few years, even those of us with no outdoor space. Undeterred, we’ve grown whatever we can indoors.
Among the most intriguing projects I’ve come across is growing an oak tree indoors. If you’ve never tried to germinate acorns, you must give it a go.
It’s fairly easy to grow an oak tree from an acorn indoors. You’ll need to move it outside eventually (unless you bonsai), but can grow an oak tree to sapling stage right in your home.
Here’s how to grow foraged acorns into a mighty oak.
How to grow an oak tree indoors – step by step
What you need:
- Resealable freezer (ziplock) bag
- Small 5-8cm pots with drainage holes
- General potting compost
- A moisture-holding medium (vermiculite, oak leaves, sawdust, compost, etc)
- Gloves (optional)
1. Gather your acorns
First, you need to find yourself some acorns to germinate. No shop-bought seeds here! This project is a little more hands on, but all the more rewarding for it.
Who doesn’t love a woodland walk, crunching through fallen leaves in the crisp autumn air (sorry, I know it’s crisp fall air for my American and Canadian friends).
It’s great fun to pull on your walking boots and go a-foraging. On your walk, don’t pick up the first acorns you see – you’re on a quest to find the best.
The fatter the acorns, the better
Bring home a handful of the freshest, biggest, plumpest acorns you can find. You should be able to find plenty to choose from.
A handy way to check the freshness is to try and remove the cap (with fresh acorns, it should come away easily). Discard any that are damaged, green, dark, soft, or nibbled.
It’s best to bring a few acorns home so you can start off several and choose the strongest seedling. This will give you the best chance of growing a healthy oak tree indoors.
Once you’ve got them home, do another quick freshness test. With their caps taken off, drop them in a bucket of water.
Leave them for a couple of hours. Any bad acorns will be floating on the surface, so throw those ones away. Dry off the good ones.
2. Prepare (stratify) your acorns
Given how they naturally grow, falling in September/October and sitting dormant over winter, acorns need a cold snap to prime them for growth.
To simulate that indoors, you can refrigerate your acorns to keep them cool and moist enough to germinate. This process is called stratification.
Fill your freezer bag with a moist (but not wet) medium like vermiculite, oak leaf litter or compost, push in the acorns and seal the bag.
Pop it in the fridge and leave it for about a month and a half, checking every so often to make sure it doesn’t dry out. You can add a little moisture if needed, but not too much.
3. Plant them up
After 40-45 days in the fridge, your acorns may have started to sprout – hoorah! But even if they haven’t just yet, you can still get them planted.
Fill your pots with compost to about an inch from the top. Plant an acorn into each one, just beneath the surface with the narrow pointy end down.
Water well, label them and place the pots in a brightly lit spot (a south-facing windowsill is ideal). Over the coming weeks, keep the compost moist. Never let it dry out completely.
Germination should happen 2-4 weeks after planting. Growth will be slow initially as the acorns put most of their energy into developing roots.
By 8-10 weeks after planting, you should see some green shoots appearing. At this stage, try and remember to turn the pots regularly (a half turn a day is perfect).
This will make sure your seedlings get exposure to light on both sides, which should keep them growing nice and straight.
All being well, by late spring you should be the proud owner of some sweet oak saplings standing around 6-8 inches tall. Congratulations!
You might also like: a guide to growing ivy indoors.
4. Harden off your oak saplings outside
Once they have their second set of leaves, you can start to introduce your oak saplings to the outside world.
Oak trees aren’t able to stay indoors forever as they need periods of dormancy, and the warmer temperatures in our homes don’t allow it.
Moving outdoors into harsher conditions is a shock for indoor-grown baby oak trees, so you need to go gently.
Take them outside during the day, making sure they’re safe from animals and other potential perils. Bring them back indoors overnight.
Keep doing this until all risk of frost has passed, which is usually June.
5. Pot them on and move outside permanently
Around midsummer, the trees will be ready to pot on into larger planters and make the move outside permanently.
At this point, you can choose the strongest to pot on and discard the others. Rather than seeing them go to waste, you could always gift them to friends/family or community projects.
Decide if you will be planting your tree in the ground eventually, or keeping it in a pot to limit its size. If you plan to plant it in the ground, a 30cm pot filled with compost will be fine until then.
Keep it well watered and wait until the tree is dormant (between October and March) before planting it out into its forever spot.
Choose a much larger planter (around 60cm in diameter with good drainage) if you’ll be keeping your oak tree potted permanently.
Inspired to grow your own oak tree?
Growing an oak tree indoors is super rewarding and gives you a front row seat to the development from acorn to sapling. It takes about a year to get to ‘planting out’ stage, but is well worth it. Imagine the sense of achievement when you’re sitting in the shade of your majestic oak tree in 30 years time!