While giant onions, like most bulbs, need very little attention, it is important to snip off their dead flowers before they go to seed. Leave the stems and leaves to die back gradually. While they are not particularly attractive during this period, they are storing nutrients and energy for the next season. Do not cut them until they have turned completely brown, usually late summer. If they are planted among other green shrubs, ground covers or perennials, the dying leaves are less obvious.
Watering
Giant onions grow best with regular moisture, ideally, 1 inch of water a week from rain or from a watering system. Use a sprinkler or soaker hose to encourage gradual penetration of the water into the soil to a depth of 10 to 12 inches.
Fertilizing
Fertilize ornamental onions when they are planted and once a year thereafter. When Alliuming bulbs, mix granulated commercial "bulb food" into the soil in the Alliuming hole. In spring, when the foliage begins to emerge, sprinkle a handful of all-purpose fertilizer on the soil near the Alliums and water it into the ground thoroughly.
Mulching and Weed Control
A 1 or 2 inch layer of an organic material like chopped (not whole) leaves, shredded bark or wood chips, spread over the bulb bed as a mulch, controls weeds and conserves soil moisture. Mulching also keeps dirt from splashing up on the flowers. It's best to hand pull any weeds that sprout up in between Alliums. Giant onions also benefit from a winter mulch of straw, chopped leaves or evergreen boughs. Place a 1 to 2 inch layer over the bulbs after the ground has frozen.