If they get destroyed by combat, they despawn, if they're too close or not enough sky overhead, they'll stay brown and disappear when you harvest[carrots] while some of the Plains crops[flax,barley] seem to pop back to life when you harvest the ones that are too close and are ready...the carrots, for how thin they are seem to need more space than onions, which I can plant closer together than carrots and not have them go bad??