There are three criteria that an arava must meet: 1) The leaves are elongated, like a brook, but not symmetrical; 2) the edges of the leaves must be smooth; 3) the stem must be reddish – even if it is green when young, it must be of a species that reddens later. The poplar is similar to the arava, but it lacks those features. Its leaves are symmetrically elongated, its leaves are serrated, and its stem is green. True, there is a type of arava whose leaf edges are not smooth, but its serrations are gentler than those of the poplar (Sukka 33b; SA 647:1).
Since most aravot grow alongside streams, they are known as “willows of the brook.” Still, this is not a necessary condition for a kosher arava; rather, any type of willow, even one that grows in the mountains or deserts, is absolutely kosher, even for the most meticulous.
One must take two aravot together with the lulav. Each arava must be at least 3 tefaḥim long (c. 24 cm, or 19 cm under pressing circumstances; see notes 4 and 6). There is no limit as to their length; they are kosher even if very long. However, when bundling the aravot with the lulav, one should make certain that the lulav extends at least a tefaḥ above the aravot (SA 650:1-2; below, 5:2).
The primary characteristic of the willow tree is that it is full of vitality and growth potential, so naturally it grows near water. When an arava is deprived of water, it quickly dries out. If most of its leaves dry out, to the point where it pales and loses its greenness, it is invalid. If the leaves are withered but not completely dried out, it is kosher be-di’avad (SA 647:2). Since aravot dry out quickly, those who are meticulous replace their aravot several times during the course of the festival. Sometimes, if the aravot are kept in a sealed plastic case and removed only to be used for the mitzva, their beauty is preserved for the entire festival.
If most of the leaves fall off an arava, it is invalid. One must watch out for this, because sometimes leaves get pulled off when inserting the aravot into the lulav bundle (SA 647:10).
If the top of an arava was truncated, the arava is invalid because it lacks hadar. However, if the top leaf falls off but the stem remains whole, it is kosher (MB 647:10).