John Gill Commentary 2 Samuel 14:30

"Therefore he said unto his servants, See, Joab`s field is near mine, and he hath barley there; go and set it on fire. And Absalom`s servants set the field on fire." — 2 Samuel 14:30 (ASV)

Therefore he said unto his servants
That did his business for him in the field, in keeping his flocks, and tilling his ground:

see Joab's field is near mine :
for great personages in those days attended to husbandry:

and he has barley there, go and set it on fire ;
it being ripe, and so capable of being fired, and therefore must be some time in March or April, when barley harvest began; he served Joab as Samson did the Philistines, (Judges 15:4) ; which shows him to be a bold, and revengeful, and ungrateful man, to use his friend, and the general of the king's army, after this manner:

and Absalom's servants set the field on fire ;
as their master had bid them, and which is no wonder; for as they murdered Ammon at his command, they would not stop at burning Joab's field, when he bid them do it; see (2 Samuel 13:28 2 Samuel 13:29) .