Cork oak is a medium-sized tree that can be found along the western Mediterranean shore. This tree has several uses, but its bark stands out since it can be used to make stoppers for wine bottles, building materials for infrastructure, and even components for automobiles. Additionally, this tree is amiable to birds and squirrels.
I. Appearance and Characteristics
Quercus suber, commonly called the cork oak, is a medium-sized, evergreen oak tree in the section Quercus sect. It is native to southwest Europe and northwest Africa. In the Mediterranean basin the tree is an ancient species with fossil remnants dating back to the Tertiary period.
- General appearance and bark
The cork oak grows as an evergreen tree, reaching an average height of 10 to 15 meters (33 to 49 feet) or in rare cases up to 25 m and a trunk diameter (DBH) of 50 to 100 centimeters (20 to 39 inches). It forms a dense and asymmetrical crown that starts at a height of 2–3 m (6+1⁄2–10 ft) and spreads widely in free-standing trees. The crown can be divided into several separate, rounded partial crowns.
The young twigs are densely hairy light gray or whitish. Older branches are strong and knotty. Older trees only form short shoots between 7 and 15 cm (3 and 6 in) in length.
The thick, longitudinally cracked cork layers of the gray-brown trunk bark are characteristic of the cork oak. The cambium of the smooth bark of young trees forms a cork layer very early on, which can be 3 to 5 cm (1+1⁄4 to 2 in) thick. The light and spongy cork fabric shows vertical cracks and is white on the outside and red to red-brown on the inside. After the cork has been harvested, the trunk appears reddish brown, but later it is significantly darker. The wood is ring-pored, has a brown heartwood and a light reddish sapwood. The cork oak develops a taproot that reaches a depth of 1 to 2 m (3+1⁄4 to 6+1⁄2 ft) and from which several meters long, horizontally running side roots extend. The trees can live over 400 years, and harvested specimens can be 150 to 200 years old.
- Leaves
The leathery leaves are alternate and are 2.5 to 10 cm (1 to 4 in) long and 1.2 to 6.5 cm (1⁄2 to 2+1⁄2 in) wide. The shape varies between round, oval and lanceolate-oval. The leaf blade has five to seven sharp teeth on both edges and a pointed vegetation cone (apex). The midrib stands out clearly on the underside of the leaf, the first-order lateral nerves usually lead to the teeth of the leaf margin. The upper side of the leaf is light green, the underside of the leaf whitish and densely hairy. There is no hair on young trees.
The leaf stalks are 6 to 18 millimetres (1⁄4 to 3⁄4 in) long and are also hairy. At the base of the petiole are two narrow, lanceolate, 5 mm (1⁄4 in) long and bright red stipules that fall off in the first year. The new leaves appear in April and May, when older leaves are also shed. They usually stay on the tree for two to three years, less often only one year, the latter especially in severe environmental conditions and on the northern border of the distribution area. Extremely cold winters can also lead to complete defoliation.
- Inflorescence and flower
The cork oak is single sexed (monoecious), with both female and male flowers on one specimen. The female flowers form upright inflorescences in the leaf axils of young branches. These are formed from a hairy axis 5 to 30 mm (1⁄4 to 1+1⁄4 in) long with two to five separate flowers. The female flowers contain a small, hairy, four- to six-lobed flower envelope and three to four styles. The male catkins also arise on the leaf axils of young branches. They are bright red at the beginning and stand upright, older catkins are yellow and pendulous, 4 to 7 cm (1+1⁄2 to 2+3⁄4 in) long and have a whitish hairy axis. The single flowers are sessile and have a densely hairy flower cover that is colored red when opened. The four to six stamens are whitish with yellow, egg-shaped anthers. They are longer than the bracts.
- Infructescence, fruit and seed
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The fruit clusters are 0.5 to 4 cm (1⁄4 to 1+1⁄2 in) long and carry two to eight acorns. About half of the fruits are enclosed in the fruit cup (cupule); the fruit cups are 2 to 2.5 cm (3⁄4 to 1 in) in diameter. The upper scales of the cupula are gray and hairy, in the subspecies Quercus suber occidentalis the scales are close together or are fused. The size of the acorns varies between lengths of 2 to 4.5 cm (3⁄4 to 1+3⁄4 in) and diameters of 1 to 1.8 cm (1⁄2 to 3⁄4 in). The fruit casing (pericarp) is bare, smooth and shiny brownish red. The hilum (the starting point of the seed) is convex and has a diameter of 6 to 8 mm (1⁄4 to 3⁄8 in).
II. How to Grow and Care
Sunlight
Cork oak should be planted in a field with full sun. It gives shade, shady plants can be planted under it.
Temperature
Cork oak grows in a large range of temperatures. It is adaptable in hardiness zones 5-9.It prefers well drained soil with adequate ventilation in the tree canopy. Cork oak will not do well in standing water. Also, leaf molds pose serious threats if the leaves cannot dry out in the wind. So make sure they are not stuck against buildings where rainwater may drain.
Watering
Keep soil moist but well drained. Oaks form a taproot and will draw moisture up from below. So make sure there is humid soil if you dig down a few inches, but do not flood the topsoil. Reducing summer water use will make cork oak more healthy, they tolerate summer dry spells very well.
Soil
Ideal soils are well-drained loam, sandy-loam, or sandy-clay soils. Some alluvial fan areas and silts harbor good oaks stands. The key in all of these is that the soil is well-drained. Standing water cannot be tolerated by oaks.
Fertilizing
Cork oak can survive and thrive without supplemental fertilization. But if you wish to give them extra nutrients you can add some of 12-6-6 (N-P-K) fertilizer. This has more of a ratio of nitrogen than phosphorus and potassium. Also, consider the natural environment of oaks. They grow where there is lots of forest litter. This forest litter acts as natural mulch that breaks down into organic matter and humus. So one way to give some natural nutrition is to spread mulch by your oak trees. They will love the extra organic matter.
Planting Instructions
It may take a very long time if you want to get a mature oak tree from an acorn. But with patience, planting a small tree is still fun. Be sure to kill weevil larvae by soaking the acorns in 41 ℃ water for 30 minutes, stratify in moist sand in the refrigerator (not freezer), and plant in the springtime.
Pruning
Branches should be pruned to avoid moist pockets or where heavy branches may fall on people or buildings. Avoid having branches that grow with leaves tight together or pressed against buildings. If rain collects in these pockets then molds and fungi can attack. Cork oak love having their leaves dry out in a well ventilated breeze.
To ensure that no danger occurs from the brittle hardwood branches, make sure that heavy branches do not hang over walking paths or outbuildings. Also, do not let children play near oaks in a thunderstorm as they are susceptible to falling branches and lightning strikes.
Propagation
When it comes to propagation, you have a few different options. The main two methods to propagate the cork oak tree are from seed and through cuttings. Beginners will find it much easier to propagate their cork oaks through seeds, however, though the downside is that Quercus suber takes a very long time to grow. Gardening enthusiasts who have experience with propagating plants and trees through cuttings will likely want to choose that method.
If you would like to try to to propagate your cork oak from seed, this is how you should go about the process:
- First, you will need to grab a bunch of fallen acorns from around your mature cork oak tree (or from someone else’s mature Quercus suber, of course) during the fall. Collect many acorns, and select only the finest ones to start a new oak. To find out if your cork oak acorns are damaged, put them in a bucket of water. The ones that float to the top are damaged, while the ones that sink to the bottom are good to go.
- After collecting the perfect acorns, it’s time to plant them! All you’ll need to do is plant the acorns just below the surface of the soil in an area of your choosing. Make sure that the soil remains evenly moist, but do not overwater the acorns.
- You’re done! Now that you have done your part, you’ll need to give the acorn time to germinate. Don’t be worried if the first few don’t sprout, as not all will succeed.
If you would, instead, like to try to propagate your cork oak through cuttings, you’ll need to take cuttings during the late summer, just before the tree is about to go dormant for the winter season. Select young new shoots that have appeared during the previous growing season. To propagate a cork oak through cuttings:
- Pick a healthy and mature parent tree to take cuttings from, looking for healthy young shoots.
- Using a sharp knife or pruning tool, cut off about six inches (15 centimeters) at a slight angle, making sure to make the cut right under a leaf node. After you are done taking the cutting, remove any excess leaves around the top of your length.
- Plant your cutting as soon as possible in moist soil. As the cutting grows, keep the soil moist but don’t drown your cutting. Make sure to place it in an area that receives full sun.
- Cork oak trees started in planters should be transplanted to the garden as soon as possible, and within two years. Oak trees in general do not tolerate transplantation very well once they become established.
III. Uses and Benefits
- Culinary uses
The bark, which contains around twelve percent extractable tannin, is also used. In addition, the acorns are used as feed in extensive pig fattening (acorn fattening), such as for Iberian ham production; although the Holm Oak (Quercus ilex), is preferred for this due to its sweeter fruits. One cork oak tree can provide 15 to 30 kg (33 to 66 lb) of acorns per year.
- Other uses
The cork is mainly used for the production of stoppers and corks, as well as for heat and sound insulation, cork paper, badminton shuttlecocks, cricket balls, handles of fishing rods and hand tools, special devices for the space industry and for other technical applications (including composite materials, shoe soles, floor coverings). Bottle cork production accounts for around 70% of the added value in cork cultivation. Since natural corks are increasingly being replaced by plastic or sheet metal closures, there could be a significant decline in the cork oak population in southwestern Europe, which endangers the biodiversity in these areas.
IV. Harvesting and Storage
Cork harvesting is done entirely without machinery, being dependent solely on human labour. Usually five people are required to harvest the tree’s bark, using a small axe. The process mandates specialized training due to the skill required to harvest bark without inflicting too much damage to the tree.
Find Where to Buy the Best Cork Oak (Quercus suber)
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