Plant bulbous kinds in mid fall, covering bulbs with twice their diameter of soil, 3 to 5 inches deep for small bulbs and 6 to 8 inches for large bulbs. Space bulbs from 3-4 inches apart for the smallest to 12 inches for the largest. Set out container-grown alliums anytime during the growing season. Prepare the garden bed by using a garden fork or tiller to loosen the soil to a depth of 12 to 15 inches, then mix in a 2- to 4-inch layer of compost.
Dead head ornamental onions to prevent self-sowing by clipping off the flower stems at their base. Feed bulbous ornamental onions in fall with a surface application of a complete organic fertilizer; even if foliage isn't present, the nutrients will be taken up by the roots, which are actively growing. If your soil is poor, you can make a second light fertilization in spring just as the bulb foliage is emerging. Feed rhizomatous alliums in early spring with a complete organic fertilizer. Apply a thin layer of compost each spring, followed by a 2-inch layer of mulch to retain moisture and control weeds. Water plants during the summer if rainfall is less than 1 inch per week. If flowering falls off, dig up and divide bulbous alliums just as their flowering finishes and foliage ripens. Divide rhizomatous types in early spring.