Why are they compared to grass?
Malbim: Grass does not last long. It dries before all herbs. Therefore, one need not compete with them or be angered at their success. It says they will be cut off soon!
What is the meaning of "Yimalu"?
Rashi: It is an expression of cutting off. Radak - the root is Namal; it is Po'el Omed (the action applies only to the subject; it is cut off). The same applies to "Yimal Ketziro" (Iyov 18:16).
Why does it say "Yerek Deshe"?
Radak: Yerek is Samuch to Deshe. We similarly find "Yerek Esev Le'achlah" (Bereishis 1:30). Animals do not eat the roots of grass and herbs, only what sprouts above the ground, the green, moist part. It dries and withers quickly. The root remains in the ground and sprouts afterwards. Something that is finished off quickly is compared to the Yerek in it.
Malbim: This corresponds to "Al Tekanei b'Osei Avlah" (verse 1). Their success is not success. They are like herbs that wither while they are attached, for they have no substance.
