Giganteum, also known as ornamental onion (Allium giganteum), is a bulbous plant that typically grow to be 3 to 4 feet tall, but can grow up to 6 feet. Its full, thick and round blooms that fan out like stars grow to be 5 inches in diameter. Giganteum plants make excellent borders, or, when planted in groups of 5 to 7 bulbs,they add interest to a garden. Plant your giganteum bulbs in the fall in USDA plant hardiness zones 6 to 10.
Prepare an area of your yard in full sun to plant your giganteum. Till the soil to a depth of 12 inches and evenly mix in about 3 inches of compost. Wait a week for the soil to settle before planting your giganteum bulbs.
Dig holes that are about 8 inches deep. The space between the tip of the bulbs to the soil's surface should equal two to three times the bulb's height. Space bulbs about 9 to 12 inches apart.
Set one giganteum bulb in each hole with the tip facing up. Then, backfill the soil and gently pack it down as you go. When finished planting your bulbs, water them with about 1 inch of water. They will begin to grow in the spring.