Common Name: Cork Oak

General Information

Quercus suber is an evergreen tree that usually grows up to 20 metres tall, occasionally reaching 25 metres. The bole can be up to 150cm in diameter. The trunk and larger branches of older trees are covered with a thick layer of corky bark[, ].
This tree is the main source of cork, a material that has been used by humans for more than 2,000 years and is the sixth-most important non-wood forest product produced globally. It is often cultivated, or managed, for its bark, especially in Iberia, but also in other areas incliding the Crimea, Caucasus, India and southwestern USA[]. The tree is also harvested from the wild for local use as a food, medicine and source of materials, and is also sometimes grown as an ornamental and shade tree.

Known Hazards

All parts of the plant contain tannins. Whilst tannins are found in many foods, and have a range of medicinal uses. They are usually only present in low concentrations. In some foods made from oaks (particularly the seeds), the tannin content can be quite high unless the food is treated to reduce tannin content.
Tannins are only of low toxicity and, because of their bitter taste and astringency, are unlikely to be eaten in large quantities. However, if they are taken in excess, they can cause stomach pains; constipation followed by bloody diarrhoea: excessive thirst; and excessive urination[].

Botanical References

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Range

Western Mediterranean - Portugal to the northern Balkans, Morocco, Algeria, Tunisia

Habitat

A dominant tree in oak and pine-oak forests and open woodlands, growing on siliceous hills on the littoral[, ].

Properties

Edibility Rating
Medicinal Rating
Other Uses Rating
Habit	Evergreen Tree
Height	18.00 m
Growth Rate	Slow
Pollinators	Wind
Cultivation Status	Cultivated, Ornamental, Semi-cultivated, Wild

Cultivation Details

Quercus suber is not very cold-hardy, tolerating temperatures down to around -8°c for short periods when dormant[, ]. It grows best in areas with hot summers, doing well in mediterranean climates. In areas with cooler summers (even if the winters are mild), such as the maritime regions of the temperate zone, it often grows poorly, failing to properly ripen its wood and suffering frost damage over the winter[, ]. Plants have a wide tolerance of rainfall - it is well adapted for growing in the drier Mediterranean climates, but also succeeds in parts of northern Portugal with an annual rainfall around 2,400 mm[]. The sub-species occidentalis is somewhat hardier than the type[] - there is at least one large healthy specimen of this subspecies at Kew Gardens in London (hardiness zone 7 - 8), it produced some seeds in the hot summer of 1989[]. Trees grow well in Cornwall where there are many large specimens[, , ].
Grows best in a sunny to partially shady position[], though young plants tolerate reasonable levels of side shade[]. Prefers a good, deep, fertile, well-drained loam, preferably somewhat sandy[]. Somewhat tolerant of alkaline soils according to one report[], whilst another says that it requires an acidic soil[]. Tolerates moderate exposure, surviving well but being somewhat stunted[]. Established plants are drought tolerant[].
Cultivated for its bark in Europe, it is the main commercial source of cork[, ]. The sub-species occidentalis has slightly thinner bark than the species[].
Trees have a tendency to form branches quite low down on the trunk - when grown as a source of cork these low branches are normally removed when small in order to maximize cork production[].
Trees are first harvested when they are 25 - 30 years old[], and then harvested every 6 - 12 years[]. The bark must be removed carefully so as not to harm the tree and to ensure future harvests.
A large tree can yield up to 1 tonne of cork.
Trees are slow-growing but relatively long lived, to about 200 years or more)[].
The seeds can take one or two growing seasons to mature[].
The number of acorns produced varies widely from year to year, with occasional very highly productive years followed by others with little or no production[].
Transplants badly unless it is moved regularly, it should only be moved in September or as growth commences in late spring[].
Does not fruit well in Britain.
Most of the trees grown in Britain as Quercus suber are in fact Quercus suber occidentalis[].
Hybridizes freely with other members of the genus[].
This species is notably resistant to honey fungus[, ].

Edible Uses

Seed - cooked[]. A famine food[]. The narrow oval-oblong seed is 20 - 32mm long[, ].
The seed is usually cooked before eating, though it can also be eaten raw. It can be eaten whole, though it is more commonly dried, then ground into a powder and used as a thickening in stews etc or mixed with cereals for making bread.
In some species, especially many of those classified as 'white oaks', the seeds are low in tannins and have a more or less sweet and agreeable flavour. The seed of most species, however, have a very bitter flavour, due especially to the presence of tannins. In these species there are various processes that can remove or at least reduce the amount of these bitter substances (although other water-soluble substances, including some minerals, will also be removed).
Tannins are water-soluble and therefore the easiest way to remove or reduce tannin levels is by soaking in water. A few different methods are listed:-
A traditional method of preparing the seed was to bury it in boggy ground overwinter and allow the wet soil to gradually leach the tannins. The germinating seed was dug up in the spring when it would have lost most of its astringency and bitterness.
Another method was to wrap the seeds in a cloth bag and place them in a stream for several weeks.
Drying the seed and grinding it to a powder before soaking speeds up the process. The fastest method is to use hot water, by cooking the powder and changing the water several times until the cooking water is no longer bitter. Alternatively, you can use cold water (which is reported to produce the best quality flour). In this case, you soak the powdered seed in cold water for 12 - 24 hours then discard the water. Repeat this process for a number of times until the soak water is no longer bitter.

The roasted seed of many Quercus species has been used as a coffee substitute.

Medicinal

Quercus (oak) species are used in the traditional medicine of many cultures, being valued especially for their tannins. Various parts of the plant can be used, most frequently it is the leaves, bark, seeds, seed cups or the galls that are produced as a result of insect damage. A decoction or infusion is astringent, antibacterial, antifungal, antiseptic, styptic and haemostatic. It is taken internally to treat conditions such as acute diarrhea, dysentery and haemorrhages. Externally, it is used as a mouthwash to treat toothache or gum problems and is applied topically as a wash on cuts, burns, various skin problems, haemorrhoids and oral, genital and anal mucosa inflammation[]. Extracts of the plant can be added to ointments and used for the healing of cuts[, ].

Agroforestry Uses:

A fairly wind-tolerant tree, it can be used in shelterbelt plantings[].

Other Uses

The thick bark is the main source of cork. Cork is a remarkable material. Light in weight, waterproof, rot-proof, flexible, chemically stable and a natural fire-retardant. It has many uses, including flooring, heat and sound insulation, floats and various industrial products, although nearly 80% of the total value of the cork crop is used for making wine stoppers. Cork based agglomerates are an ideal core material for components of lightweight structures, and have even been used in aerospace application[, , ].

The leaves of most species in this genus are more or less rich in tannins. A mulch of the partially decayed leaves can be placed around vulnerable plants in order to repel slugs, snails, grubs etc, and these will in time break down to add humus and nutrients to the soil. Fresh leaves should be used with caution, however, since as these decay they utilize some of the nitrogen in the soil and thus can inhibit plant growth[, ].

Oak galls are excrescences that are sometimes produced in great numbers on the tree and are caused by the activity of the larvae of different insects. The insects live inside these galls, obtaining their nutrient therein. When the insect pupates and leaves, the gall can be used as a rich source of tannin, that can also be used as a dyestuff and is also used by many cultures to make ink[, ].
The bark is rich in tannins and can be used as a dyestuff and for waterproofing rope[, ]. On a 10% moisture basis, the bark contains 16.9% tannin[].

The wood is heavy with a specific gravity 0.8 - 1.0. It is not durable, warps and splits easily. It is mainly used for fuel and to make charcoal[].

Propagation

Seed - it quickly loses viability if it is allowed to dry out. It can be stored moist and cool overwinter but is best sown as soon as it is ripe in an outdoor seed bed, though it must be protected from mice, squirrels etc. Small quantities of seed can be sown in deep pots in a cold frame. Plants produce a deep taproot and need to be planted out into their permanent positions as soon as possible, in fact seed sown in situ will produce the best trees[]. Trees should not be left in a nursery bed for more than 2 growing seasons without being moved or they will transplant very badly.