The ornamental onions, called alliums, produce large puff-balls of small flowers in colors ranging from white to purple. Many varieties reach 2 feet high or taller, making them a striking addition to flower beds and borders. Alliums require only minimal maintenance to thrive. Deadheading is the only necessary pruning. This process removes old flowers before they have a chance to form seeds. Flower removal prevents self-seeding and sometimes helps prolong the flowering period.
Leave the flowers in place until after most of the petals have wilted and begun to drop off. As the petals drop, seed production begins.
Cut off the flower stem at the base of the plant, where it emerges from the foliage. Trim as low down on the stem as possible using sharp shears.
Prune out any yellowed and wilted leaves after removing the spent flower stem. Cut these out where they emerge from the main plant.
Dispose of or compost the spent flower stems and foliage. Leaving them in the flower bed is unattractive and may attract unwanted insect pests, which use the dead plant material as nesting material.