Growing Cork Oaks as Bonsai
A cork oak at the National Collection. Photo: Sisto B
I first started growing cork oaks (Quercus suber) in 1991, and found that picking up acorns in March yielded a high percentage of germination. The seedlings very quickly developed a very long and vigorous tap root. Cutting that root quickly did not appear to kill many, but pruning the top of the seedlings too late meant that frosts hit soft growth – and that proved fatal to many.
As it was impractical for me to put the seedlings in the ground to grow on, I often allowed the roots to grow through the pots, for a few years.
Digging them out was certainly stressful for the trees, but some did survive. I read that cork oaks need fungal mycorrhizae to thrive, so collected some soil from a few centimetres under established cork oak trees. This may help. (And I always use it now when disturbing roots.)
What I did find, however, is that if I waited for bud-swell to repot, my trees would often die: bud-swell in cork oaks tends to be early October, and waiting until then meant that the new growth was placing demands on the surviving roots just when our weather was becoming hot.
So I switched to repotting and root pruning (including digging well rooted pots out of the ground!) in the second week of August. I try very hard to avoid disturbing the central root mass of older trees too much (though I will remove large roots if necessary), and I do try to balance shoot with root. This works well for me.
Also, when pruning these vigorously growing trees, I try to time the last pruning in March to avoid fresh young growth being hit by frosts – which may knock them back a long way.
One last point on pruning – they seem to grow more vigorously from where they are pruned more and more often, which can create a problem with balancing growth.
After a while, preferably in contact with the ground, they start to develop cork around the base – and to live up to their potential as bonsai.
These are just my notes, and other people, including much more experienced bonsai artists, have different perspectives, but I hope my thoughts are helpful.
A last point – this year I have been unable to find many cork oak acorns at all – either on trees or on the ground. I am guessing that the severe spring and summer were not conducive to fruit production. Better luck next year!
Close-up of the thick, rugged bark. Photo: Sisto B
Neal H’s Quercus suber forest
Canberra Bonsai Society
Neal H