There are two kinds of willow in Palestine, the White Willow Salix alba being the northern one, and the Common Willow Salix acmophylla being the southern, more heat-tolerant one. In the Jordan Valley the willow gives way to the salt-tolerant Euphrates poplar as the river gets close to the Dead Sea. Some confusion of names arises from the fact that the Euphrates poplar has two kinds of leaves. The younger shoots produce a narrow leaf like the willow, and the mature shoots produce a wider, ovate leaf.
The willow is found along streams, where it can grow to a height of 6 meters (20 feet). It has long, narrow leaves which drop during the winter and the tiny green flowers appear around October.
If “willow” is indeed the correct translation of ‘aravah in Leviticus (23:40), then it is one of the four species recommended for use in building shelters for the Festival of Shelters. In Job it is the habitat of the mysterious Behemoth that no man can tame. In Isaiah and Ezekiel it is a metaphor for something that grows luxuriantly.
Over three hundred Salix species are found around the world in temperate regions. They are sometimes known as ossiers or sallows. At least one Salix species (Safsaf Willow Salix subserrata) is found in sub-Saharan Africa in addition to being found in Egypt, Libya, and Israel. Flora of West Tropical Africa lists three other species. People who make round, thatched houses use thin, flexible sticks from a shrub to form the concentric rings that hold the radial sticks (either bamboo or sorghum stalks) in place. Some of these may be willows, and if so, the local word would be appropriate in the biblical context where shelters are being constructed.
In Isaiah 44:4 and Ezekiel 17:5 the willow is symbolic of quick growth and may require an appropriate local equivalent of a thriving plant.

Source: Each According to its Kind: Plants and Trees in the Bible (UBS Helps for Translators)