Willows were waterside trees that grew throughout Middle-earth. The most famous of these was the ancient tree known as Old Man Willow, on the banks of the Withywindle in the Old Forest.
In Beleriand was the region Nan-tathren, a wooded vale which took its name from the willow trees that grew there.
The Sindarin name for "willow" is tathar.
Willow at Wikipedia