Cork oaks: assessing winter’s damage

Winter has been long and fickle this year. Mainly, I’d say, it’s been drier and colder than usual, with fewer storms. But between the cold periods there have been unseasonably warm spells (up to 20C); between the dry spells there have been huge rain storms; and between the calm spells there have been some ferocious and freezing winds. Tipyn o bopeth (a bit of everything) really and quite challenging in the garden.

The young cork oaks (which are now two and three years old) have mainly suffered from the tortrix moth caterpillars, as per usual.

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Tortrix moth caterpillar damage on cork oak

But in addition, they have also been being eaten by some kind of leaf miner…

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Leaf miner and freeze damage on cork oak

and the younger trees have suffered a little from the lack of moisture and freezing winds – showing in the orangey-bronzey tints you can see  in the picture above, alongside the dull buff colour where the leaf miners have stripped the living cells.

However, I’m happy to say, that they all seem to have come through OK and have plenty of new buds for this year’s growth. Hopefully winter will soon go away and they can put on some strong new growth through the summer.

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Growing cork oaks from seed

In 2013 I was given three cork oak (Quercus suber) acorns to try growing.

The first cork oak seedling after one year's growth
The first cork oak seedling after one year’s growth

At the time, I was in the middle of bulb planting and I popped the three acorns into individual 9cm pots without much thought. That was at the start of December.

I was amazed when about a month later the compost in the pots started to heave, indicating that things were stirring underneath. I was more amazed when I went out one morning and found a hole where one of the acorns had disappeared completely from the pot. I don’t know who stole it – possibly a magpie. My lesson learnt, I covered the two remaining pots with pea netting to stop further thefts.

I then started reading about how to grow cork oaks from acorns: obviously, I should have done the reading first:) I learnt that I should have simply laid the acorn on the soil/compost and sprinkled a little more over the top – replicating what would have happened naturally. I hadn’t done that: I’d dibbed a hole and pushed the acorns quite deep (I blame it on the bulb planting frame of mind). And I should have used tall pots to allow long tap roots to grow.

So, I decided to tip out my two acorns and replant them with more soil beneath – for their long tap root – and less above – to let them reach the light quickly and easily. In so doing, I damaged the roots of one of the seedlings and it never recovered. Happily, despite all my maltreatment, the other one grew and grew. When it was about four inches tall, I planted it into the garden. Now, at just over a year old, it is just under a foot tall.

Last December, I decided to try again and to do things more correctly this time. I laid four acorns in tall pots (the kind usually used for raspberry canes and the like) containing a mixture of coir, multipurpose compost and garden soil, and I sprinkled a little more of the mixture over the top.

Cork oak shoots - this is about four months after planting
Cork oak shoots – this is about four months after planting

Initially I kept these pots out of doors, but with a propagator lid over them to prevent thieving birds and also to stop them getting too wet in our long damp winter.  Once the acorns started “moving” (germinating) I covered the pots with pea netting and got rid of the lid.

So far, two of the acorns have germinated properly and have sturdy young growth. Another one started to germinate, but I think it might have failed as it is a while since anything happened. The other seems to still be progressing, but very slowly: there are signs of root growth, but no shoot yet.

Clearly I can’t really draw any conclusions from such a little experiment, but it would seem that cork oaks are pretty easy to grow from acorns. The main things to watch out for are: thieving birds and rodents; to make sure that the pot you choose has sufficient space to let the acorn “move”; and to choose deep pots to let the tap roots grow. It’s also important that the acorns you use are fresh – plump and shiney, not crinkled and dull.

Cork oaks are relatively hardy, they can survive temperatures down to around -10C (depending on whose description you read – it may be a bit lower or higher than this) and they make beautiful trees as they mature.