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Tree of the Week: Cork Oak

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The Cork Oak -- Quercus suber

Cork grows as a thick protective layer of outer bark, much thicker in the cork oak than in any other tree. Strange as it is to see a stripped cork oak with its lower 12 or 15 feet of dark inner bark exposed, the cork can be safely hand-harvested every 10 years or so as long as the underlying cambium is not damaged.

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Cork, which is the oak’s way to protect itself from a harsh environment, has unique properties: it is very light, resilient, waterproof, fire retardant, abrasion resistant, insulating, buoyant, and a poor conductor. The cork oak is native to the Western Mediterranean and North Africa. Portugal produces half the world’s cork supply; it started protecting the trees in the 13th century and has developed a unique half wild production landscape.

A fairly fast grower, this evergreen tree initially grows taller than wide; eventually it becomes 30 to 60 feet tall and wide. It may live to 250-plus years old. Light gray, thickly fissured ridges cover the trunk and the heavy limbs. The cork oak tends to form co-dominant (i.e. equal thickness, competing) upright trunks. Three- inch-long leaves are toothed, oval, dark green above, gray green below. Inconspicuous flowers develop into short pointed acorns, three-quarters to 1 1/2 inches long, sitting in bowl shaped caps.

The tree tolerates many garden conditions, including drought and desert environment, but it wants full sun and good drainage.

Cork is an excellent bottle stopper material: its suberin, a natural waxy substance, makes it impermeable to gas and liquids and prevents it from rotting. Cork stoppers were found in Egyptian tombs. In 1688 Pierre Perignon found the material unsurpassed to seal champagne bottles with. But screw tops and plastic stoppers may soon make cork obsolete for this purpose; then the greater half of the cork industry and the way of life that depend on it may simply disappear.

--Pieter Severynen

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