Exodus 9:31 - Ellicott's Commentary On The Whole Bible

The flax and the barley was smitten. — Flax was grown largely in Egypt, since linen garments were very generally worn by the people, and were the necessary attire of the priests (Herod. ii. 37). Mummies also were swathed in linen bandages (Herod. ii. 86); and soldiers wore linen corselets (Herod. ii. 182, 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” — i.e., forms its seed-vessel — towards 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 both of the earlier and the later ones.