Which Witch', Paperback/Eva Ibbotson

When Arriman the Awful, the handsome wizard of the North, announces a contest to choose his bride, every witch in town is a flutter. The meanest, most powerful witch will wed the wizard. But little Belladonna is dismayed, because as hard as she tries, her spells conjure up begonias and baby birds, and not a single viper or bloodshot eyeball. She just has to do something seriously sinister in time for the contest....