Adam Clarke on Psalms 129:6

As the grass upon the housetops - As in the east the roofs of the houses were flat, seeds of various kinds falling upon them would naturally vegetate, though in an imperfect way; and, because of the want of proper nourishment, would necessarily dry and wither away. If grass, the mower cannot make hay of it; if corn, the reaper cannot make a sheaf of it. Let the Babylonians be like such herbage - good for nothing, and come to nothing.

Withereth afore it groweth up - Before שלק shalak, it is unsheathed; i.e., before it ears, or comes to seed.