(Editor's Note: This article was originally published on May 29, 2008. Your comments are welcome, but please be aware that authors of previously published articles may not be able to promptly respond to new questions or comments.)
The genus Allium is important both from both an agricultural and horticultural point of view. Agriculturally, we rely on Allium in the form of onions, garlic and chives. However, there are some 750 species of Allium in the world thus it is not surprising that there are many species that have merit for their flowers rather than as a culinary object. The ornamental onions had a slow beginning in the gardening world as most people thought "I don't want to grow stinky onions in my garden". In reality, other than the bulbs themselves, most of the ornamental onions do not have the typical onion fragrance and many have delicately, sweet-smelling flowers.
Other than a single species found in southern Africa, all the other onions hail from the northern hemisphere. In the wild, they inhabit a wide variety of habitats from deserts to grasslands, open forests and alpine regions. Some live where rainfall is steady all year. These species are summer blooming and retain their leaves well into the fall. Others live in regions where the soil bakes hot and hard for half the year. Onions from these areas bloom after the wet winter months then promptly go dormant and disappear during the hot, dry summers. So it seems, the ornamental onions fall into two mains groups; the spring blooming-summer dormant species and the summer blooming-summer ‘green' species.
The focus of this article will be the summer blooming ornamental onions. The fall planted-spring blooming species will be covered in part two, to featured later in the summer. The most common of the summer blooming Allium is our common garden chives, A. schoenoprassum. Chives may be a little weedy at times, but close inspection of their flowers will show them to be quite ornamental, albeit small flowered. This is the case with most of the summer blooming onions. Their flower-heads are relatively small compared to some of the spring blooming types. Of course, the plants themselves are also smaller, making them ideal candidates for the front of the border or rock garden settings. In the nursery trade, the summer bloomers are not as widely available as they cannot be shipped in a dormant state like the spring bloomers. Your best source are various seed exchanges and specialty bulb and/or alpine nurseries.
The two most well-known summer-blooming onions are chives (A. schoenoprassum) and garlic chives (A. tuberosum)
Culturally, these ornamental onions are care-free and oftentimes easier to accommodate than the spring bloomers. Provide full sun to part shade, well-drained, reasonably moist garden soil (although some are drought-tolerant) and the plants will take care of themselves. They have very few insect or disease problems. Growing from seed is generally easy. Provide them with 4 weeks of stratification and they often germinate with abandon. Many flower within 2-3 years from seed.
There are WAY too many species for me to describe in detail. I will stick to those that I have grown myself (which are probably the most readily available species). There are many ornamental onions with rounded heads of lilac, lavender or purplish-pink flowers. Chives is the most common of these. The others I've grown are a little more refined and include A. senescens, A. splendens, A. angulosum, A. nutans, A. lusitanicum and A. spirale. These all grow to about 30 cm and should be hardy to zone 4. Of these, A. senescens is the most variable in bloom-time and plant form. There are two particularly nice forms, one called ‘Glaucum' with blue foliage and the other ‘Blue Eddie' whose leaves are blue, low and twisted. The only summer bloomer I've grown with white, somewhat spherical flowers is garlic chives, A. tuberosum. There seems to be at least two strains on the market as one plant I have blooms consistently in late July while the other never blooms until September-October!
A collection of similar lavender-lilac species: A. senescens, A. spirale, A. lusitanicum and A. splendens
There are two very exquisite, dainty, blue-flowered summer onions; A. cyaneum and A. beesianum. These both stay under 15 cm with wiry leaves and unique dark-blue flowers. Allium cyaneum has rounder heads while A. bessianum is more nodding. I grow mine in alpine troughs as they are so small, they may get lost in the open garden. Another small blue-flowered species is A. sikkimense, but so far, I have not been able to track down the seeds or a plant. A white-flowered version I grown is called A. sessile. These are listed as hardy to zone 8 but I grown them with no problems in zone 5.
Among the smallest ornamental onions are A. beesianum, A. cyaneum and A. saxitile
The other group of summer-flowering onions I grow have pendant flowers. Perhaps the most well known is the nodding onion, A. cernuum, a native of the Rockies. The one with the largest individual flowers (again on fairly small heads) is A. insubricum (aka A. narcissiflorum), a reddish-pink flowered species. I tried to obtain this one for many years, ordering them on every seed exchange I could find. It took 10 years to get the real McCoy! Most times I ended up with A. cyathophorum var. farreri, itself an attractive, if somewhat prolific species, with nodding reddish-pink flowers but much smaller than A. insubricum. I also obtained A. senescens, A. cernuum and A. carinatum masquerading as A. insubricum. Now, at last, I have the real thing, which bloomed last year for the first time.
Common nodding onions include A. cernuum, A. insubricum and A. cyathophorum var. farreri
Allium flavum is another variable species. Some are quite small, under 15 cm while others may reach 45 cm. Some have plain green foliage, other striking blue. Typically the flowers are nodding and bright yellow but in the variety called tauricum, the flowers range from white through shades of yellow, orange and pink. A similar species which blooms about 4 weeks later is A. carinatum. This one reaches to 45 cm with thin, wiry foliage and nodding clusters of pink or white flowers. Yet another similar species is A. thunbergii, again with pendant reddish-purple flowers. This species claim to fame is that it is one of the latest-blooming species, often flowering into November!
Variation in floral colour of A. flavum var. tauricum
Some other notable alliums include A. flavum, A. pulchellum and A. thunbergii
This is just the tip of the proverbial iceberg in regards to ornamental summer-blooming onions that are out there. It is simply a manner of keeping your eyes open for new listings and giving them a try. In many cases, the hardiness is not even known or they are listed as being more tender than they really are. Being small, inexpensive plants, it is worth experimenting with them. Stay tuned for part 2, the spring-blooming ornamental onions, a far more popular group, several which are indeed large and imposing plants for the spring landscape.