Lulav leaves grow from both sides of the lulav and cover the spine. A lulav is invalid if it has leaves on only one side of the spine, while the second side is bare (SA 645:3). Generally, the lulav’s leaves grow one atop the other, covering the entire spine. If its leaves are so short that one leaf does not reach the one above it, it is invalid (SA 645:4).
The branches of the palm tree begin as lulavim, branches whose leaves are all tight to the spine. As the branch continues to grow, the leaves open, forming the fan-like branches that people associate with date trees. At this point they are referred to as ḥariyot.
Le-khatḥila it is best if the leaves of the lulav are tight to the spine, such that if the lulav is left on a table, the leaves still cling tightly to it without assistance. If the leaves of the lulav have started to open, the lulav is kosher as long as the leaves can be bound together and pulled tight to the spine. This is referred to as “a lulav whose leaves have separated.” If the opening leaves have hardened so that it is impossible to bind them together and draw them flat to the spine of the lulav, then the lulav is invalid. This is referred to as “a lulav whose leaves have broken free” (Sukka 29b; SA 645:1-2).
If the lulav has become so crooked that it is semicircular, like a scythe, it is invalid. If it is not so crooked, it is kosher but not mehudar (especially beautiful), as straightness is a feature that makes a lulav beautiful. If it is bent like a scythe, but the curve is toward the spine, this is somewhat common natural curvature, so the lulav is kosher (SA 645:8).
If the spine is bent over at an angle, it is invalid (SA 645:9). Some say that even if just the leaves are bent at an angle, the lulav is invalid (Taz; MB ad loc. 40-41). However, if just the tips of the uppermost leaves are bent like the letter vav, the lulav is kosher, as some lulavim grow this way.
Some say that if the tips of the uppermost leaves are bent over like the letter peh, resembling a button (a “knepel”), the lulav is invalid (Ran and Ritva). Others maintain that such a lulav is actually mehudar, as this ensures that the tiyomet of the upper leaf will not open (Rosh). In practice, this lulav is kosher, as many lulavim grow this way. However, since some are stringent, it is not considered mehudar (SA 645:9 and MB ad loc. 42).