What is the comparison to roof grass?
Radak: It has no endurance.
Malbim: Grass does not last long, therefore they uproot it before all vegetation.
What is the meaning of "she'Kadmas Shalaf Yavesh"?
Rashi: Before they remove and uproot it from its place, it is dry. Radak - "Shalaf" is removal, like "Shalaf Ish Na'alo" (Rus 4:7), just there it is Po'el Yotzei (he removed his shoe) and here it is Po'el Omed (it itself is removed).
Radak: It is as if before it comes out and is seen, it dries in the heat of the sun. Surely it does not; this is an exaggeration. This is like "ba'Boker Yatzitz v'Chalaf la'Erev Yemolel v'Yavesh" (90:6).
Malbim: The Nimshal is, the Rasha will not live long. The sun burns roof grass before they can remove it with other grass. So Resha'im are like grass that is cut before its time due to punishment of Hashgachah. The sun represents burning judgment from above.
