Cork oak is a medium-sized, evergreen oak tree. It is the primary source of cork for wine and olive oil bottle stoppers and other uses, such as cork flooring, insulation etc. It is native to southwest Europe, namely Portugal and Spain, and northwest Africa, Algeria and Morocco. The European cork industry produces 300,000 tons of cork a year, with a value of €1.5 billion and employing 30,000 people. Wine corks represent 15% of cork usage by weight but 66% of revenues.

Cork oak tree has a thick, insulating bark that may have been the cork oak's evolutionary answer to forest fires. After a fire, while many of the other tree species merely regenerate from seeds or resprout from the base of the tree, the cork oak branches, protected by cork, quickly resprout and recompose the tree canopy. The quick regeneration of the tree seems to be an advantage compared to other species that, after a fire, return to an initial stage of development.

The bark is produced as in other oak species and many other trees, by a special tissue called the cork cambium. The bark of cork oak is different from that of other oak species by having very low density, and thus fast growth. When the cork is harvested the generative tissue is left intact. Over time the cork cambium layer can develop new bark of considerable thickness and can be harvested every 9 to 12 years to produce cork. The harvesting of cork does not harm the tree. In fact, no trees are cut down during the harvesting process. The tree is cultivated in Spain, Portugal, France, Italy, Algeria, Morocco and Tunisia.

Cork oak is a slow-growing tree that grows to up to 20 m, and lives up to 250 years. The leaves are 4 to 7 cm long, weakly lobed or coarsely toothed, dark green above, with a pale cover of trichomes beneath. The leaf margins are often curved down.

Cork oak blooms in the spring and the flowers are similar to those of other oak species with the male flowers arranged in drooping catkins and female flowers singly or in pairs at leaf bases. It is wind pollinated. The acorns are 2 to 3 cm long, in a deep cup fringed with elongated scales ripen in the autumn.

Cork oak acorns can be dried, ground into a powder and used as a thickening in stews etc or mixed with cereals for making bread. The seed contains bitter tannins, these can be leached out by thoroughly washing the seed or ground powder in running water. A mulch of the leaves repels slugs, grubs etc, though fresh leaves should not be used as these can inhibit plant growth.

Cork oak is propagated in the autumn by planting the acorns in light soil, though the seeds’ viability decline quickly after ripening.

Written by Amram Eshel