Use sharp tools
Cut in the morning, and use a razor or sharp knife to cleanly slice, not crush, the stem. The cut ends of some alliums (especially Allium giganteum) drip a clear fluid that oxidizes blood red and permanently stains countertops; be sure to recut alliums outdoors or in the sink.
"The fresher the water, the better," says Mark Kintzel, a floral designer in Allentown, Pennsylvania. "Hollow stems need clean water, or they clog and the flowers wilt prematurely." Inserting them into floral foam will also clog the stems.
A half-teaspoon of bleach per quart of water keeps bacteria from growing in the vase. Don't use floral preservatives; the sugar in them can cause an overgrowth of bacteria.
Best for cutting
Koenders recommends 'Purple Sensation', 'Giganteum', 'Globemaster', and the white 'Mt. Everest.' He likes the dark purple A. atropurpureum for its fragrance.
A good source for more information is fact sheet #767, "Production of Alliums as Cut Flowers," from the Maryland Cooperative Extension.