Cork is extracted only from early May to late August, when the cork can be separated from the tree without causing permanent damage. When the tree reaches 25–30 years of age and about 24in (60 cm) in circumference, the cork can be removed for the first time. However, this first harvest almost always produces poor quality or "male" cork (Portuguese cortiça virgem; Spanish corcho bornizo or corcho virgen). Bark from initial harvests can be used to make flooring, shoes, insulation and other industrial products. Subsequent extractions usually occur at intervals of 9 years, though it can take up to 13 for the cork to reach an acceptable size. If the product is of high quality it is known as "gentle" cork (Portuguese cortiça amadia, but also cortiça secundeira only if it is the second time; Spanish corcho segundero, also restricted to the "second time"), and, ideally, is used to make stoppers for wine and champagne bottles.
The workers who specialize in removing the cork are known as extractors. Extractors use a very sharp axe to make two types of cuts on the tree: one horizontal cut around the plant, called a crown or necklace, at a height of about 2-3 times the circumference of the tree, and several vertical cuts called rulers or openings. This is the most delicate phase of the work because, even though cutting the cork requires quite a bit of strength, the extractor must not damage the underlying phellogen or the tree will be harmed.
To free the cork from the tree, the extractor pushes the handle of the axe into the rulers. A good extractor needs to use a firm but precise touch in order to free a large amount of cork without damaging the product or tree.
These freed portions of the cork are called planks. The planks are usually carried off by hand since cork forests are rarely accessible to vehicles. The cork is stacked in piles in the forest or in yards at a factory, and traditionally, left to dry, after which it can be loaded onto a truck and shipped to a processor.
The cork is then boiled stripping away any bacteria, dirt or contaminants, cut into smaller more workable pieces, flattened and sorted into quality assortments and then processed further into various products.
https://www.youtube.com/watch?v=ztr-RP0XYd8 Informative on Harvesting