Albert Barnes Commentary 2 Kings 19:26

"Therefore their inhabitants were of small power, they were dismayed and confounded; they were as the grass of the field, and as the green herb, as the grass on the housetops, and as grain blasted before it is grown up." — 2 Kings 19:26 (ASV)

The weakness of the nations exposed to the Assyrian attacks was as much a result of the divine decrees as was the strength of the Assyrians themselves.

The grass on the housetops - Compare the marginal reference. The vegetation on the flat roofs of Eastern houses is the first to spring up and the first to fade away.