Brittle willow constitutes a hybrid of white willow and another
willow from Asia minor. White willow is native in Europe
and West Siberia. On most of its European range it hybridized with a
willow from Asia Minor that had been introduced to
Europe during the Middle Ages. As a result of introgressive
hybridization, white willow has lost its identity in West
Europe. The widespread hybrid for ages was taken by the
Europeans for a normal native species known as "brittle willow."
Its hybrid nature was first revealed
by A.K.Skvortsov only in 1973. Under the name "brittle willow,"
the hybrid was introduced to the US.
It then escaped from cultivation
and is now starting to show invasive tendency,
especially in western Massachusetts (own observations). The
crown of this tree is mostly rounded, unlike that
of white willow.