The willow, or osier
Archimandrite Nikifor’s Biblical Encyclopedia
(Lev. 23:40; Ps. 137 [136]:2; Exod. [Ezek.] 17:5, and elsewhere) — a well-known tree, growing on moist soil and by springs of water. The beautiful-looking plant known to us under the name of the weeping willow is otherwise called the Babylonian willow, and it is this kind of willow that is mentioned in the 137th [136th] Psalm: “We hanged our harps upon the willows in the midst of it [Babylon]” (v. 2). In another place of Holy Scripture the willow branches are called “willows of the brook,” that is, river willows (Lev. 23:40).
Willows were used by the Israelites at the Feast of Tabernacles. The banks of the Jordan are still to this day shaded by spreading willows (Muravyov and Norov), though not in such abundance as in Biblical times. Since the willow does not grow on the banks of the Euphrates, it is supposed that the tree mentioned under this name in Psalm 137 [136] was of the poplar or aspen family, which grow abundantly along the banks of that river and are known among the Arabs under the name al-gharab.