Albert Barnes Commentary Job 8:12

"Whilst it is yet in its greenness, [and] not cut down, It withereth before any [other] herb." — Job 8:12 (ASV)

While it is yet in its greenness - That is, while it seems to be in its vigor.

And is not cut down - Even when it is not cut down, if allowed to stand by itself and if undisturbed, it will wither away.

The application of this is obvious and beautiful. Such plants have no self-sustaining power. They are dependent on moisture for their support. If that is withheld, they droop and die.

So with the prosperous sinner and the hypocrite. His piety, compared with that which is genuine, is like the spongy texture of the paper-reed compared with the solid oak.

He is sustained in his professed religion by outward prosperity, as the rush is nourished by moisture; and the moment his prosperity is withdrawn, his religion droops and dies like the flag without water.