Curly birch (or visa birch) is a genetic variety of the silver birch (Betula pendula). Its wood has curled grain, called "visa" formation in Finnish, an unusual growth form. At various places on the trunk and branches the bark is noticeably thicker than usual, and parts of the bark appear within the wood itself as brown spots or streaks. The wood is very decorative, at best resembling wooden marble. The visa formation is also visible in the tree’s outward appearance in the form of bumps or pits in the surface of the trunk, excessive localised branching, and exaggerated twisting or narrowing. The formation usually becomes apparent at about 5-6 years old. Similar formations can occasionally be seen in other genera, particularly alders (Alnus) and mountain ash (Sorbus).
The decorative and extremely hard wood of curly birch has been used for centuries by local craftsmen. In the Finnish pavilion at the Paris World Fair of 1900, Gallen-Kallela’s “Iris”-room with its visa furniture and other art treasures were a major attraction. Visa is both valued and valuable timber. Production of top-quality visa-timber requires expertise, and great attention to tree care and thinning.
These days commercial visa birch cultivars may well be micro-propagated but the seedling production was started with seeds from a tree still growing at Punkaharju, near Savonlinna. This was possible because visa formation had been noticed to be an inherited characteristic.