Or HaChaim on Exodus 9:31/אור החיים על שמות ט׳:ל״א

1 והפשתה והשעורה נכתה, both the flax and the barley were smitten, etc. Why did the Torah tell us of this detail before describing Moses as concluding his prayer? According to normal practice the Torah should have reported that Moses did as he had said, namely that he prayed to G'd for the plague to stop.

2Perhaps the Torah wanted to provide us with a reason why the Egyptians had not really become penitent. Whereas the hail had destroyed the flax crop and the barley crop, it had not caused more than minimal damage to the wheat and the spelt which ripened at a later date. According to Midrash Tanchuma the reason that the Torah uses the letter ו in front of the word הפשתה where it could not possibly be understood as a conjunctive is that a miracle within a miracle happened so that only these two crops were smitten. Had nature taken its course, both the wheat and the spelt should also have succumbed to the hail. Basing themselves on the strange phenomenon that the wheat and spelt had escaped unharmed, Pharaoh and his servants concluded that G'd's authority did not include the wheat and the spelt. This encouraged them to continue in their obstinate ways. This is why Moses was able to say that he realised that Pharaoh had not yet come to fear G'd truly. He cited as the reason what happened to the wheat, or rather, what did not happen to the wheat. G'd had spared the wheat in order to encourage Pharaoh to remain obstinate.