The majority of alliums are bulb-forming; however, there is a handful that grows from rhizomes, the way common chives (Allium schoenoprasum) do. These may never form any kind of bulb. Allium leaves tend to be long and strappy. Some—like the cork-screw allium—remain attractive all season, with a blue-green color that complements the flowers. Most early blooming alliums have foliage that tends to die-back early, as the plants go dormant for the summer.

The flowers form in clusters and are best known in the round pom-pom form, but they can be star-shaped, cup-shaped, semi-circular, or pendulous. There's a good amount of variety in allium plants. Drumstick alliums only grow about one foot tall with 1-inch flower heads, while giant “Globemaster” can top 4 feet in height and sport huge globes of 8- to 10-inch flower heads.

Most allium bulbs grow quickly, and they bloom in the spring or early summer after the earliest spring bulbs have faded. However, there are a few varieties that bloom later in the season, even well into fall.

The plump, round shape of the flowers looks charming poking through other plants, whether low-growing mats such as hardy geraniums or shrubby roses. The purple color is a great asset that complements most other late spring flowers, from peonies to iris to catmint. The shape also works well with other medium height plants, like foxglove or monarda.

There is some dispute over if ornamental onion is toxic to humans. It is safest for humans not to ingest allium, as it can cause upset stomach and digestion. However, the entire plant is very poisonous for cats and dogs if ingested because it contains organic sulfur compounds that convert to highly reactive oxidants in the intestinal tract.