Loosen the soil around an existing allium once the foliage dies back naturally. Use a handheld gardening fork and take care not to cut through the roots. Lift the plants from the loosened soil.
Shake the dirt from the roots. Allium roots have a thickened central root, or bulb, with several smaller roots growing from each.
Tease apart the bulbs and stems with your fingers. Divide each plant into two or three new plants, depending on the size of the root system.
Inspect the bulbs for damage or soft spots indicating rot. Dispose of any damaged or diseased bulbs.
Replant the bulbs into a full-sun, well-drained garden bed immediately after dividing. Plant each bulb so the top is 4 inches beneath the soil surface. Space bulbs 6 to 8 inches apart in all directions.