Made up of 850 species, the allium genus includes onions, chives, leeks, garlic and scallion plants. Within the allium genus also exists many varieties of ornamental onions, such as the Allium giganteum, grown purely for its shear beauty. All varieties exist in the northern hemisphere except a couple of species.
Alliums either grow from rhizomes or bulbs. The species with rhizome-based root systems also develop tiny bulbs on the root ends. Allium plants grown from bulbs produce flowers in the spring and early summer. Once they flower, the plant dies back. Allium species that grow from rhizomes flower in the late summer and into the fall.
Virtually all the allium species produce fragrant flowers. Bees and butterflies are attracted to the blossoms. Many produce large, spherical flower heads in shades of purple, pink or white. The stems normally offer extreme strength and rarely bend after a rainstorm or watering. Windstorms often pose a problem for the large-stemmed varieties, but a sheltered location can offer protection. Cut allium flowers offer long vase life.
Allium species enjoy a location with full sunlight. Soil should contain abundant organic matter. Both the bulbs and rhizomes require well-draining soil. The plants require at least 1/2 inch of water per week to maintain their vigor.
The root system needs to be planted 4 to 8 inches deep. The larger species with bulbs require deeper planting to offer more support to the heavy top growth. If the bulbs are too shallow in the ground, the entire plant can tip over once abundant top growth is produced. Most bulbs benefit from fall planting.
Virtually all species have a general onion or garlic odor that permeates the area around the plant. Many people find the aroma unpleasant and prefer to place plants away from areas such as patios or porches. The odor is an excellent deterrent to deer, rabbits and other pests that enjoy consuming plants.