Exodus 9:31

And the flax and the barley was smitten: for the barley was in the ear, and the flax was bolled.

This detail belongs to the account of the hail plague in Egypt, which was the seventh plague. Its timing was announced beforehand. The hail was very grievous and mingled with fire, striking throughout all Egypt everything in the field, people and animals and every plant and tree, while only the land of Goshen where Israel lived was spared.

Before the hail, a warning to shelter livestock tested those who feared God's word against those who hardened their hearts like Pharaoh. The flax and the barley were smitten because they were at a vulnerable stage of growth, the barley being in the ear and the flax being bolled with its seed pods formed, while the wheat and the rye were not smitten because they were not yet grown up. The hail demonstrated that thunder, hail and storm obey God's voice and that the whole earth is under the command of the God of the Hebrews.

After Pharaoh confessed his sin under terror and begged for the thunder and hail to cease, Moses prayed and the thunder, hail and rain stopped, but Pharaoh then hardened again in perversity and refused to let Israel go.