Betula pendula

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. 

Voted Finland’s national tree in 1988, the silver birch has a special position in the Finnish landscape owing to its pale greenness and in winter to its graphically sharp silhouette. It is also a part of the Finnish mindscape: the story “The birch and the star” by Topelius is familiar to most Finns, and local drama groups and summer festivals typically use silver birch as part of the scenery.

Silver birch is versatile in its uses: birch sap, bark, birch tar, wood for axe handles and as firewood have all been important. Leafy twigs were collected and tied in bundles for winter fodder for sheep and other animals, and the birch whisk is traditional in saunas, for both physical and spiritual purification.

In earlier times the traditional slash-and-burn method of farming helped the spread of birch woodland. Silver birch is a pioneer species which appears quickly in dense thickets after the ground has been broken during clear felling or by fire, and the areas burned over in slash-and-burn were quickly occupied after being abandoned as fields.

In open areas the silver birch is a large broad-crowned tree whose bark is white at first, but with age the butt of the tree darkens and becomes fissured. Some individuals develop drooping branches many metres long after they have ceased growing in height. The species is easily recognised, especially when younger, by the roughness of the twig bark due to resin glands on the surface; and the diamond shape of the leaves is another tip. In earlier times it was a popular garden tree and nowadays nurseries offer a wide range of forms and varieties.