However, you should plant only once per tile. Each planting is of a fistful of seeds, and it replaces the results of the previous planting, so the only reason to plant the same tile over and over is to train the agriculture skill (apart from trying to repair the damage caused by animals eating your sprouted plants).
The game will tell you if it's too early to plant the particular kind of seed you're trying to plant. It will also block your attempt if the ground is frozen (even if the time is right).