(Editor's Note: This article was originally published on September 13, 2008. Your comments are welcome, but please be aware that authors of previously published articles may not be able to respond to your questions.)
Several months back I discussed the merits of the summer blooming ornamental onions. Among the 750 species or so of Allium, there are many which have great attraction as garden plants. From a horticultural view point, the fall planted-spring blooming onions are by far more important than the summer blooming types. The summer bloomers are still attractive plants and help to extend the bloom season of ornamental onions, but they are not as ‘economically' easy to grow as the spring blooming types. In recent years, these spring blooming onions are becoming a main cash-crop in the Netherlands in the same way as tulips and daffodils.
The fall planted alliums hail mostly from the Mediterranean region and Asia Minor, areas that are typically cool and wet in winter but hot and dry in summer. This same area is also home to many tulip species. So in the garden, these allium are typically planted in the fall, sprout and bloom in the spring months, then promptly go dormant for the summer. Some of these allium are easy to grow while others are more difficult as they need to bake in the summer months, a difficult prospect for northern gardeners (we often have the same problem with some species tulips). However, the price of the bulbs is relatively cheap so these difficult ones may simply be grown as annuals in climates where they do not prove to be long-lived.
While the summer blooming onions are typically under 45 cm, the spring blooming types can vary dramatically from under 15 cm to over 120 cm. The flower heads are often much larger than their summer cousins and the colours can be more vibrant. In the garden, provide these spring bloomers with full sun, well-drained soil and protect them with winter mulch in areas colder than zone 5. I use this group of ornamental onions in rock garden settings and in the back of the border. The one drawback with these spring bloomers is that many have dying foliage at the same time they are blooming. This is especially true for the large drumstick types. Hosta are very effective companions for this group. Diseases and pests are few and they are even deer resistant.
Among the largest of the spring bloomers are 'Purple Sensation', A. giganteum and A. nigrum
I'll start with the common, smaller-stature species; A. moly (bright yellow) and A. oreophilum (aka. A. ostrowskianum, bright pink). Both of these allium are prolific growers and can be used for naturalizing. The bulbs are so cheap, that you can afford to plant them in great drifts for maximal display. Both species grow about 15 cm and are great candidates for the rock garden. They are hardy to about zone 4. Growing a little taller is A. neopolitanum (aka A. cowanii), a dainty white-flowered species which, unfortunately, is more tender, best for zones 6 and warmer. Growing 30-45 cm tall but with flowers in the size range of the previous three is A. roseum.
Among the smaller spring-bloomers are A. moly, A. oreophilum, A. neopolitanum and A. roseum
The next group are the taller growers (50-80 cm) with relatively small, but still showy, flower heads. By far the best and most stunning is the blue-flowered A. caeruleum (aka A. azureum). Blue is a rare colour among ornamental onions and A. caeruleum is the only fall-planted, summer dormant species; the other blue ones are summer-green types with quite small flowers, including A. cyaneum, A. beesianum and A. sikkimense. I grow my A. caeruleum in groups of a dozen or more for best impact. They have held their own for me, returning yearly but not increasing much. This species blooms late spring-early summer. Another popular species is A. sphaerocephalum, which doesn't bloom until August in my area (probably July elsewhere). It has very dense, albeit small, purplish-red flower heads and makes an attractive cut-flower. Another species which is starting to crop up in the trade is A. atropurpureum. This one reaches 45 cm with fair-sized hemispherical heads of dark reddish-purple. While attractive, this one did not survive for me, suggesting it might need the summer baking to do well.
Mid-height, mid-sized allium include the beautiful A. caeruleum and the tight-headed A. sphaerocephalum
The last group are the large drumstick types which fall into two main groups; the low growers (under 45 cm) with relatively broad leaves and the tall types (over 100 cm) with narrower leaves. Among the lower growers, the most spectacular is A. christophii, which has huge heads of starry silvery-lilac blooms atop 30-50 cm stems. This species also has attractive seedheads so don't necessarily rush to cut this one back. Of similar growth habit is the unusual A. schubertii, again with huge heads of lilac-pink flowers but in this case, the flower heads are more reminiscent of a fireworks display as the individual flower stems vary in their length. The shortest of the drumstick types is A. karataviense, whose baseball-sized white or lilac-tinted flowers sit just above the broad, blue-green, heavily ribbed leaves. ‘Ivory Queen' is the most popular cultivar.
Large flowers on shorter plants are provided by A. karataviense, A. christophii and A. schubertii
Among the tall drumsticks, the most popular and least expensive is a hybrid called ‘Purple Sensation'. It is sometimes listed as a selection of A. aflatuense. This one grows to 150 cm with dense, large 15 cm diameter purple-pink heads. Of slightly paler colour is A. giganteum. The bulbs are indeed gigantic but the flowers are really no larger than ‘Purple Sensation' and considering the cost of A. giganteum bulbs, you might be better off to stick to ‘Purple Sensation'. At any rate, ‘Purple Sensation' is faster to multiply. ‘Goliath', ‘Globemaster' and A. rosenbachianum are all attractive look-alike hybrids and/or species. For a white-flowered alternative, there is ‘Mount Everest' or A. nigrum (don't be fooled by this last species as the flowers are white, not black, despite what the species epithet might suggest).
Side by side are 'Gladiator', A. rosenbachianum, A. giganteum and 'Purple Sensation'
Many other hybrid and species ornamental onions are in the making in Europe, using species not well known in the trade. Some of these are similar to A. karataviense while others are more like the ‘Purple Sensation' group. Still others are unlike anything in the current trade. I recently saw a chartreuse, small-flowered drumstick type that will drive flower arrangers to distraction! So keep you eyes open...it's only a matter of time before they will be available on this side of the pond!
I would like to thank ladyrowan for the use of her picture of Allium neopolitanum.