Charles Ellicott Commentary Exodus 9:31

"And the flax and the barley were smitten: for the barley was in the ear, and the flax was in bloom." — Exodus 9:31 (ASV)

The flax and the barley was smitten.

Flax was grown extensively in Egypt, as linen garments were commonly worn by the people and were the required attire of the priests (Herodotus 2.37). Mummies also were swathed in linen bandages (Herodotus 2.86); and soldiers wore linen corselets (Herodotus 2.182 and 3.47). Barley was grown as food for horses, as an element in the manufacture of beer, and as a material for an inferior kind of bread.

The flax is “bolled”—that is, forms its seed-vessel—toward the end of January or beginning of February, and the barley comes into ear about the same time. These facts fix the date of this plague and help to fix the dates of both the earlier and later ones.