I always suggest that is important to retry growing the same species from multiple sources. Too often our knowledge of any plant species is from a single mass-produced source, certainly true with bulbs, depriving our knowledge of the true breadth of variability of a plant species. Even something as common as nodding onion, Allium cernuum, can be had in amazingly diverse and beautiful forms (and nondescript ugly forms) from such efforts.
So it is true with Allium caeruleum, widely cultivated and surely mass-produced from a single clone for many decades. The problem is, the plant widely available in fall bulb bins at local nursery centers is an inferior form; flowers are indeed a good dark blue, but there's a tendency to produce an odd bulbil or two or three in the inflorescence, the flower heads often with amusingly weird aberrant florets.... multi-petaled ones, fused florets, or situations where a stamen morphs into a pedicel and sprouts one or more flowers from within a flower . Invariably the widely cultivated type is short-lived, only flowering well the first year after planting, dying out quickly in subsequent years. They're cheap enough to buy, but I was tired of these bad habits and replanting bulbs every couple of years.
Then one day, Panayoti Kelaidis sent me a photo of this blue allium growing at Denver Botanic Garden (DBG), and I couldn't believe my eyes... a gorgeous form with brilliant azure blue flowers in heads larger than normal, and taller too. Afterwards, he sent me a good crop of bulbs (thanks Panayoti!). Determined to get a good form established, I planted bulbs out in 7-8 spots around the garden, hoping to find just the right spot to the plant's liking. And sure enough, the bulbs only really prospered in one location, the successful one now forming basal offsets and bulblets to try again in other locations. Here are some photos of what I have dubbed the 'DBG Form'.
1. Allium caeruleum in a mixed planting at Denver Botanic Garden, beautiful!
2. close-up of the same planting
3. In my garden, a close-up of a single flower head in 2008, 3" (7.5 cm) in diameter.
4. In my garden, several azure flower heads in 2009, at early anthesis with intense color of young buds.
5. In late summer, bulblets can be found at the stem bases bulging friom the basal leaf sheaths, harvest and replant.
6. Spring growth 2010 shows much dividing and increasing, some of the bulblets replanted near mother plant are sprouting.
(Aster pilosus growing nearby, see: http://nargs.org/smf/index.php?topic=159.0 )