Quercus suber (Cork Oak)

Quercus suber, commonly called the cork oak, is a slow-growing, medium-sized evergreen tree. The thick grey-brown bark has deep cracks and is used to produce natural cork products. The stiff, leathery leaves are dark green on top and light grey underneath. It has female and male flowers (monoecious), and the fruit is called an acorn.

It is tolerant of hot summers with prolonged droughts and mild, wet winters. It is native to the Mediterranean basin but can be grown in any region with a Mediterranean climate.

Quercus suber is the National Tree of Portugal, the largest cork producer in the world.


Quick Overview

TYPE

Type tree

HEIGHT & WIDTH

Quercus suber (Cork Oak) height and width

BLOOM TIME

Quercus suber (cork oak) bloom time

SUNLIGHT

full sun / semi shade

HARDINESS

hardiness (-10ºC / 14º F)

DROUGHT TOLERANCE

drought tolerance aprox 5 months

ORIGIN

origin mediterranean basin

Cork Oak scientific name

  • Botanical name:  Quercus suber (KWER-kus SU-ber)
  • Family: Ericaceae Fagaceae (Fa-GA-see-ee)
  • Common name: Cork Oak
NameMeaning
QuercusThe Latin name for Oak.
suberThe Latin name for Cork

How to identify Quercus suber

Quercus suber tree

Tree

The Cork Oak tree is a medium evergreen tree with the trunk and main branches covered in thick, deeply ridged, soft bark that is greyish-brown. Beneath the bark, the trunk is red, which can be seen when the bark is harvested for natural cork. The tree can regenerate its bark.

The crown is dense and irregular and starts growing in an oval shape but later becomes rounded and spreads widely. In the older trees, it is subdivided into separate rounded crowns.

The tree can grow up and spread from 10 to 15m  (30 to 50 ft).

.

Quercus suber stem (cork oak stem)

Stem

The young shoots are densely-haired, which gives them a greyish-white colour. Older shoots are strong and knotty.
The stem has alternate, simple leaves. Growing from the leaves’ axils, we can see either drooping catkins of male flowers or clusters of female flowers, which will later produce the acorns.

Quercus suber leaf (front and back view)

Leaf

Leaves are hard and leathery with sharp teeth on the edges. The front of the leaf is a shiny greyish-green and the back is whitish and densely haired.

The leaves have a round to oval shape of 2.5 -10 cm ( 0.98 – 3.94 in) long and 1.2 – 6.5 cm (0.47- 2.56 in) wide. 

.

quercus suber female flower
Quercus suber male flowers

Flower

Quercus suber is a monoecious plant, which means it has both female and male flowers on the same tree. It is wind-pollinated.

Female flowers grow on the axils of young leaves of new branches in clusters of 1 to 5 individual flowers. Each flower has 3 to 4 stigmas and 4 to 6 sepals (0 petals), inserted in a cupule of overlapping bracts. The cupules will become the future scaly cup that holds the acorn.

These flowers appear in spring and, if pollinated, will eventually turn into acorns by autumn.

The female flower size is around  5 – 30 mm (0.20 to 1.18 in) long.

Male flowers also arise on the leaf axils of the previous year’s branches. They grow in catkins that are bright red and upright at the beginning but become yellow and droop when fully grown. The catkin has a whitish hairy axis with 15-25 staminate flowers radially set around it. Each flower has 4-6 stamens with yellow egg-shaped anthers. 

The male flowers release huge quantities of pollen into the air, which the wind will blow towards the female flowers. These flowers appear in spring and also in autumn.


The size of the male flower catkin is around 4 to 7 centimetres (1.6 to 2.8 in) long.

Quercus suber acorn

Fruit

The fruit of the cork oak is called an acorn, and it grows out of the female flowers in clusters at the leaves’ axils.

The acorn has smooth brownish-red skin with a green mark at the top. About half of the fruit is enclosed in a scaly greyish-cream cupule.

The acorns appear in autumn, and the number varies greatly from year to year. 

The acorn size is 2 – 4 cm (0.79 – 1.5 in).

Quick tips to identify Cork oak

How to identify Quercus suber (cork Oak) - Drought-tolerant tree

Quercus suber Usage

Quercus suber is best known for its thick, corky bark, which can be harvested for usage in multiple products.

Its bark can be sustainably used because it has the ability to regenerate every 9-12 years.

Ornamental

Although it is a slow-growing tree, it grows into a very attractive form after some years. So one or two trees will look great as a garden structure.

Commercial Cork Products

The main use of Quercus suber is as a source of cork. Cork is obtained by peeling the bark from the trunk every 9 to 12 years. The first harvest is made when the tree is around 25 years old.

Cork is an amazing material, light, waterproof, rot-proof and flexible. There is a huge variety of products that can be made from cork.

The main product is wine stoppers, but the cork left after producing the stoppers can be used for insulation panels, floor and wall tiles, soundproofing material, sports equipment, shoes, cloth, handicrafts, artistic items and even devices for the space industry.

How to plant Quercus suber

The best time to plant a cork oak tree is in early spring. Studies have shown the highest percentage of rooting and survival with cuttings planted in April.

It can be planted in a sunny or partially shaded area of your garden. It prefers acidic soil but also tolerates alkaline soils. Whatever the PH the soil must absolutely be well-drained.

You can find some useful tips in this article for more details on how to plant a cork oak tree.

If you choose to sow the acorn, it is best done in the autumn to mimic the natural process. Autumn is the time of the year when acorns naturally fall to the ground and are “sown” by rodents.

Sow them 2.5 cm (1 in) deep in pots of compost-mix sand, and lay them horizontally.

How to water Quercus suber

Quercus suber is a drought-tolerant plant. So it typically requires no water besides the winter rain once it is established. It does not do well in humid climates.

During the first two years after planting, you will need to water the young cork oak tree every two to three weeks during the summer. 

When watering, you need to do it abundantly, giving the soil a generous soak so the water can penetrate deeply into the soil to allow the roots to grow deeply. 

Additionally, you should add mulch around your cork oak tree (but not too close to the base) to help it control weeds and conserve moisture. Wood chips are good for mulching your cork oak tree.

How to prune Quercus suber

Quercus suber tends to develop many low branches, so it may require formative pruning when it is still young. You may also need to remove dead or damaged branches.

Pruning should be done in late autumn through winter, but try to avoid the frosts.

How to propagate Quercus suber

The cork oak may be sown or planted.

Sowing is best done during the autumn to mimic the natural process. The acorns must be sown fresh because they deteriorate quickly.

It can also be propagated by cuttings with higher success rates when planted in early spring.

You may also like

Sources

Sources of information used for this article

Article from Euforgen

Article from frontiers in plant science.

Article from San Luis Obispo Botanical Garden

Article from European Atlas of Forest Tree Species

Article from greenidiom