The willow flute, also known as sallow flute (Norwegian: seljefløyte, Swedish: sälgflöjt or sälgpipa, Finnish: pitkähuilu or pajupilli), is a Scandinavian folk flute, or whistle, consisting of a simple tube with a transverse fipple mouthpiece and no finger holes. The mouthpiece is typically constructed by inserting a grooved plug into one end of the tube, and cutting an edged opening in the tube a short distance away from the plug.
Similar, however not the same instruments were made by peasants in Poland, usually using a different method described in sources as "kręcenie" (that nowadays means literally "rolling", at that time possibly also "drilling-gouging"), "ukręcanie", "ulinianie" (nowadays literally meaning: "making moulted"). Such instruments are mentioned in folk poems or songs.
The willow flute is a type of overtone flute. It is played by varying the force of the air blown into the mouthpiece, with the end of the tube being covered by the finger or left open. The tones produced are based on the harmonic series. Playing the instrument with the end of the tube covered produces one fundamental and its overtones, playing it with the end of the tube left open produces another fundamental and series of overtones. Willow flutes cannot be tuned to an equal tempered scale.
Modern willow flutes are typically made of plastic (PVC tubing is often used), but the original willow flutes were made from sections of bark cut from green willow branches. Willow flutes could only be made this way during the spring, and became unplayable when the bark dried out.
There is also a Karelian variant of the willow flute that is made in Finnish Karelia and the Russian Republic of Karelia. It is made the same way as the willow flute, however instead of using willow bark, it is made out of birch bark. The musical group Karelian Folk Music Ensemble based out of Petrozavodsk in the Russian Karelia, uses this instrument in their music.