Ornamental onion is only one of over 500 known species of allium, or onions. This group covers plants that are strictly ornamental as well as edible onions, garlic, and chives.
Description of ornamental onion: The ornamental types range in size from 3 inches to 4 feet tall. The flowers are rounded and vary in color, including purple, pink, white, mauve, and greenish. Most have straplike leaves and seed heads that remain attractive after plants finish blooming. Many of the ornamental onions have a pungent odor when crushed.
How to grow ornamental onion: Plant in full sun in well-drained, alkaline soil that is rich in organic matter. Provide 1/2 inch of water per week during the growing season. Allium is drought-tolerant at other times. Most alliums self-seed and some species such as garlic chives (A. tuberosum) and drumstick allium (A. sphaerocephalum) can become invasive. If this is not desirable, remove old flower heads before they go to seed. Some species of allium go dormant after flowering. It is normal for their leaves to turn yellow and die back to the ground during or immediately after flowering. They will reemerge the following spring. If you are growing types that go dormant after flowering, place taller allium varieties at the back of the garden behind or between other plants. As the foliage dies down, the surrounding plants will hide the bare spots. Remove the leaves after they turn brown.
Propagating ornamental onion: By division of bulbs that form from the mother bulbs.
Uses for ornamental onion: Plant in groups of three or more in combination with other perennials.
Ornamental onion related species and varieties: Allium 'Globemaster' is a vigorous plant with stout 21/2-foot stems supporting deep lavender flowers that get 4 to 6 inches across. Plants bloom for many weeks with newer blossoms replacing those that fade. It is an excellent cut flower and the seed heads are ornamental. Allium sphaerocephalum, drumstick chives, grow 18 to 36 inches tall and bear 2-inch purple flower heads that open in June and last for three weeks. This variety grows very well in hot climates (through growing Zone 8). Cut flowers last for around ten days.
Scientific name for ornamental onion: Allium species