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.
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.