Whilst it is yet in his greenness - That is, while it seems to be in its vigor.
And is not cut down - Even when it is not cut down. If suffered 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in his professed religion by outward prosperity, as the rush is nourished by moisture; and the moment his prosperity is withdrawn, his religion droops and dies like the flag without water.

Whilst it is yet in his greenness - We do not know enough of the natural history of this plant to be able to discern the strength of this allusion; but we learn from it that, although this plant be very succulent, and grow to a great size, yet it is short-lived, and speedily withers; and this we may suppose to be in the dry season, or on the retreat of the waters of the Nile. However, Soon Ripe, soon Rotten, is a maxim in horticulture.

Whilst it is yet in its greenness,.... Before it is come to its full height, or to a proper ripeness; when as yet it has not flowered, or is about it; before the time usual for it to turn and change; it being without moisture, water, or watery clay, will change:
and not cut down; by the scythe, or cropped by the hand of man:
it withereth before any other herb; of itself; rather sooner than such that do not require so much moisture; or in the sight and presence of them, they looking on as it were, and deriding it; a poetical representation, as Schultens observes: next follows the accommodation of these similes to wicked and hypocritical men.

not cut down--Before it has ripened for the scythe, it withers more suddenly than any herb, having no self-sustaining power, once that the moisture is gone, which other herbs do not need in the same degree. So ruin seizes on the godless in the zenith of prosperity, more suddenly than on others who appear less firmly seated in their possessions [UMBREIT] (Psalm 112:10).

Greenness - Whereby it promises long continuance. Tho' no man cut it down, it withers of itself, sooner than other herbs.