Each trunk has to reach a circumference of 70 cm when measured at 1.5 meters from the ground. The first stripping, produces the so-called virgin cork which will be used for applications other than cork stoppers (flooring, insulation etc.), since its quality is far from that necessary to manufacture stoppers.
Nine years later, the second harvest produces material with a regular structure, less hard, but still not suitable for cork stoppers, this is known as secondary cork.
It’s from the third and subsequent harvests that the cork with the best properties is obtained, suitable for the production of quality corks, since its structure is regular and smooth outside and inside. This is the called the reproduction cork. From then on, the cork oak will supply good quality cork every nine years for around a century and a half, producing, on average, 15 to 16 bark stripping throughout its life.
After the harvest, the cork planks are stacked in piles either in the forest, remaining exposed to the natural elements. All these piles are constructed taking into account strict specific rules (ICCSMP), so as to allow the cork to stabilize. The piles should be stacked on materials that do not contaminate the cork and prevent contact with soil.
How to strip the cork from cork tree?
The stripping of the cork oak is an ancient process that can only (and should only) be done by specialists, the debarkers, since much manual skill and experience is required in order not to harm the tree.
The stripping process consists of five steps:
- Opening. Being performed a vertical cut in the cork, choosing the deepest crack in the cork bark. At the same time, the edge of the axe is twisted so as to separate the outer from the inner bark. The degree of difficulty of extraction can be gauged from the ‘feel’ of the axe. When the edge of the axe is applied to the strip, a hollow sound of tearing is heard if the cork is going to come off easily. If it is going to be difficult, the axe gives off a short, firm, dry sound.
- Separating. The plank is then priseoff the tree, by inserting the edge of the axe between the strip and the inner bark. The axe is twisted between the trunk and the cork strip to be extracted.
- Dividing.A horizontal cut defines the size of the cork plank to be removed and what is to remain on the tree. During dividing, the inner bark is frequently marked and these mutilations can sometimes alter the geometry of the trunk.
- Extracting.The plank is removed from the tree with care so that it does not split. The larger the planks extracted, the greater their commercial value. The removal of entire planks depends on the skill of the workers. After the first plank has been stripped, the operation is repeated over the whole trunk.
- After the stripping of the planks, some fragments of cork remain attached at the base of the trunk. To remove any parasites in these “wedges”, the debarker gives them a few taps with his axe.
Finally, the tree is marked, using the last number of the year in which the extraction took place.
Does harvesting cork kill the tree?
Cork is natural and all, but what about the trees? The good news is that harvesting cork does not damage the tree in any way. When harvesting the cork, skilled worker take great care to ensure the living part of the tree is not damaged. The cork trees are not cut down, but pried of their outside layer in a strangely satisfying way.
Cork trees undergo an initial 25 years of growth, after which they’re able to be harvested every nine years. The total lifetime of a tree can be up to a staggering 300 years, providing the harvesters with over 1.5 tonnes of cork (40kg – 60kg per harvest depending on the size of the tree).
As well as being regenerative, the trees also require minimal resources to grow. There’s no need for things like fertilizer, pesticides, or pruning. This results in the cork having a minimal carbon footprint compared to many other materials.
Once a cork oak tree has been stripped of it’s bark, it absorbs 3-5x more CO2 than usual in order to aid the regeneration process.
When a cork tree can be harvest?
A cork tree is ready for its first harvest when it is about 20 years old. The first harvest is of poor quality, and can only be used to make agglomerated cork products. Subsequent harvests occur at nine-year intervals, when the cork layer reaches a thickness of 1-2 inch (2-5 cm). The harvest from a young tree yields about 35 lb (16 kg) of cork, while the yield for an older tree may be 500 lb (225 kg). Each tree has a productive life of about 150 years.
How often can cork tree be harvested?
A cork tree gets its first cork bark at the age of 25 years! The first cork extraction yields very poor quality cork. After the first extraction, the tree needs to grow for another 9 years before getting the first good cork yield. After the first yield, cork can be harvested every 9 years. The cork tree can live around 160 years so, during the life of a cork oak tree, it can yield about 15 good harvests.
How do cork plantations track harvest dates?
If you go to a cork tree piantation, youll see that each tree has a large number painted on it. The number is the year of the last cork harvest from that tree. By looking that the number, the growers know when the tree is ready for the next harvest.
Cork harvesting takes a lot of skill and practice. The cork must be harvested to maximize the yield and size of the bark, while protecting the inner layers. If the inner layer is damaged, the tree may die or not produce any more cork. The cork trees in Portugal are protected and harvesting practices are regulated. As a result, Portuguese cork oak trees can live for 150-200 years and produce the best quality of cork.