Plantable crops need an amount of space (about 1m radius; more for trees) to be healthy. After planting, it takes several seconds for the game to determine if crops are healthy (have sufficient space). Unhealthy plants will die and disappear if left alone.

Hostile creatures, notably greylings and greydwarves, will wander into your fields. Either by throwing rocks or scratching in melee, they will attack crops. You can ward off these intruders by building fences/walls around your fields. Spiked barricades can do a lot of good to repel these pests; keep them repaired to prevent losing them.