Ornamental onions (Allium spp.) aren't grown for food like their edible counterparts; instead they are prized for their tall stems topped with an orb of purple florets. They flower prolifically in late spring or early summer. Most ornamental onions grow in U.S. Department of Agriculture plant hardiness zones 4 through 10, although it depends on the variety. Later in the growing season, the plants can benefit from the removal of the old flowers, a process called deadheading.
Ornamental onions typically flower in late spring or early summer, producing large orbs of purple or pink florets. These plants reproduce from bulb division so it's not necessary to allow the flowers to form seeds, but allowing seed to form rarely weakens the bulb. The greatest benefit from deadheading an ornamental onion is realized in its appearance. Eventually the old, dry flowers fade and begin to look ragged. Deadheading quickly improves the plant's appearance.
Deadhead ornamental onions at any time after the blooms begin to dry. Onion flowers rarely whither and fall off. Instead they dry on the plant and often retain their color and basic form. Deadheading immediately after bloom shortens the flowering season, but it does allow fresh summer flowers a chance to take the spotlight in the garden. High winds or rainy weather may also cause the flowers to decline more quickly, necessitating an early summer trim. Otherwise, wait until midsummer to cut back the dead flowers so they can continue to grace the garden in their dried form.
Most ornamental onions have slender stems, but they are still too thick for easy pinching. Basic pruning shears or scissors cut through the stems without causing damage. Disinfecting the shears in a solution of one part bleach to nine parts water prevents the spread of plant diseases. Keep pruning shears stored out of the reach of children.
If you deadhead early in the summer, only cut off the dead flower. Leaving the healthy stem and foliage in place allows the ornamental onion to collect nutrients and energy to refuel the bulb for the next year's growth cycle. Cut back the entire plant to the ground only after the foliage begins to yellow or look tattered in midsummer. Ornamental onions like edible onions are toxic to cats, dogs and some other animals, so properly dispose of all deadheaded and pruned plant materials promptly after trimming.