Cork Oak Tree Guide: How to Grow & Care for “Quercus Suber”

Quercus suber goes by the common name cork oak, because this huge and very drought-resistant oak has traditionally been used to produce cork bottle stoppers and flooring. It is also occasionally referred to as Sobreiro, a common Portuguese word for the cork oak. The cork oak tree belongs to the same family as beech and chestnut trees, the Fagaceae family. The oak (Quercus) genus counts over 500 known species — but while all oaks are beautiful in their own right, there is no doubt that the cork oak tree is one of the most interesting ones.

Cork oak trees are slow growers, but be careful before adding a young sapling to your garden — with time, Quercus suber will dwarf anything around it, eventually growing to be as tall as 70 feet (or over 21 meters). Because of this, these beautiful oaks are exclusively suited to large gardens.

If you have the space, Quercus suber is an absolutely wonderful tree to grow — cork oak trees are low-maintenance and they offer quite a few unique perks.

About the Cork Oak Tree

• Quercus suber is a broad-leaf evergreen oak tree with dense foliage. Its flowers include beautiful catkins, and once the bloom subsides, this oak will, like other oaks, produce acorns. The cork oak tree is, however, most famous for its unique peeling bark, which has a spongy texture.
• Cork oak trees are native to the Iberian peninsula, especially Portugal, as well as to western Africa. In the United States, these majestic oaks can be grown in USDA hardiness zones 8b through 10a, in other words in regions with warmer winters.
• Many people have popped cork bottle stoppers (indeed, often simply called corks) without ever realizing where this material comes from. The cork oak tree is the source of this spongy material, which comes from its bark. Quercus suber has a unique feature, in that the bark can be peeled away from the tree without causing any harm to the tree. Cork has been in use for millennia, at least since ancient Egypt. Cork can be harvested from trees that are at least 25 years old, and will begin regenerating the lost bark soon after it was stripped.
• The cork oak is an ancient tree — believe it or not, there is fossil evidence that Quercus suber has been around for at least two million years, and possibly for much longer. Sadly, the growth habitats of these amazing trees have started to come under threat as cork bottle stoppers are increasingly being replaced by plastic and other options. By adding a cork oak to your garden, you would support the continued existence of a prehistoric plant species.
• You may not have much use for the cork you could harvest from your own cork oak tree, but this plant of course also produces acorns — and these fruits aren’t just squirrel fodder! People can eat acorns, which are fruits, too. Add them to a nut bread to allow them to shine in their full glory.
• Numerous small mammals, including squirrels, call the cork oak tree a valuable food source. The cork oak tree supports many butterfly and moth species, and provides cover to birds. Do be careful if you have horses or dogs, however, because acorns are toxic to both of these animals.
• Quercus suber is a fairly disease-resistant tree, but it can develop leaf spot, powdery mildew, cankers, and other fungal infestations if it is growing in overly moist conditions. The tree can also be attacked by some moths and butterflies, whose larvae burrow into the bark.

Cork Oak Tree Features: An Overview

• Cork oak is a tall broadleaf evergreen tree that can live to be up to 300 years old!
• Quercus suber grows rather slowly. In its mature state, this tree can reach a height of around 70 feet (more than 21 meters), with an equally wide spread. Even considering this large size, the cork oak is considered to be a “medium-sized” oak tree.
• The external part of the cork oak tree’s bark has a light brown to burgundy red color, and features deep grooves and a rough texture. The inner bark has a spongy texture you will already be familiar with if you have ever handled a wet cork.
• Like other oak trees, the cork oak tree gives rise to insignificant green female flowers, which are just over an inch (around three centimeters) long, during the spring time. These are accompanied by the tree’s male flowers, which are stunning deep yellow catkins that hang down from the tree and can be up to six inches (eight centimeters) long.
• The fruits of the tree, the acorns, arrive during the fall. The cork oak tree’s acorns are especially large, with a length of one to three inches (three to eight centimeters), and can be around an inch wide. They have a beautiful copper brown color, and the outer shell can be peeled off to use the acorns in culinary applications. The acorns typically harbor a single seed, which can be used to propagate the cork oak.
• The cork oak tree’s dense foliage consists of dentate and lobed evergreen leaves, which have a silver or gray green color. These leaves feature small hairs and can be as long as six inches (15 centimeters).
• Cork oak tree branches arch, and the foliage has an arching shape that forms a shade-friendly canopy. If you have a mature cork oak in your garden, you can look forward to enjoying a lovely shady seating area underneath.
• Cork oak can also be grown as a bonsai tree indoors. Doing so instantly takes Quercus suber from a low-maintenance plant to an extremely high-maintenance one, but if you are willing to learn how to care for a cork oak bonsai, the experience can be very rewarding. If you love these trees but don’t live in the right climate or don’t have the space to grow one in your garden, growing cork oak as a bonsai is an option you will definitely want to explore.

Growing Cork Oak Trees

Cork oak trees are notoriously easy to care for once they are established — and they are also amazingly drought tolerant. Despite that, they can’t be grown just anywhere. Quercus suber, being native to Portugal and northwestern Africa, prefers slightly warmer climates. They do best in regions where the spring and fall temperatures hover around 55 to 63 °F (13 to 17 °C), and where the summer temperatures reach up to 104 °F (40 °C). While cork oak trees can tolerate some mild frost, they should not be planted in regions with extremely cold and snowy winters.

These oak trees should be grown in conditions where they can receive full sun — Quercus suber needs at least six hours of direct sunlight every day, and prefers even more. This makes them a perfect centerpiece in a lawn, without too many other tall trees in the vicinity.

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Since the cork oak tree often naturally grows in coastal regions, it is no surprise that this species prefers sandy, loamy, or clay soil. Quercus suber does best in slightly acidic to neutral soil, and does not tolerate alkaline soil types well. These are not moisture-loving trees! Cork oak trees’ native zones often experience prolonged periods of drought, and the soil they are planted in should be able to drain well, Quercus suber does not cope with soggy conditions at all well and could develop fungal diseases in such an environment.

Young saplings can benefit from a regular fertilizer regimen, which will help the cork oak develop the beautiful thick trunk you are probably aiming for. Choose a balanced liquid fertilizer, or a high-nitrogen fertilizer to encourage a wider trunk.

When your cork oak is young, or if you are growing Quercus suber as a bonsai, you will want to prune it to train your tree into a desired shape during the spring time. As your cork oak tree continues to grow, it is important to keep an eye out for dead or severed branches, which pose a hazard to people walking by.

Watering Cork Oak Trees

Because cork oak trees are native to warmer and drier areas, like North Africa (Tunisia, Algeria, and Egypt) and the Iberian peninsula, especially Portugal, they have evolved to adapt to droughts and to retain water. That means that established cork oak trees don’t really need much (if any, depending on your region) supplemental water to stay healthy. Cork oak trees do best in moist to occasionally dry water conditions and need soil with excellent drainage. Overwatering is a bigger risk than underwatering in this case.

Young saplings do, on the other hand, need more water. Don’t water young cork oak trees if the soil is still moist, but offer enough water to render the topsoil moist to the touch if you find the soil to be completely dry. If your young tree is growing outside in a warm climate, it is best to water them early in the morning, before the soil temperature rises.

Propagating Cork Oak Trees

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 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.

In Conclusion

Cork oak trees are absolutely wonderful to grow outside if you have ample space in your garden and live in the right climate. While it takes a hefty dose of patience to see these trees grow into mature specimens, the wait is worth it. People who don’t have large gardens can always opt to grow Quercus suber as a bonsai, but this is a much more involved process.