Most types of alliums (also called "ornamental onions" or "flowering onions") grow from bulbs, as do many of your favorite spring bulb plants such as daffodils and tulips. Not all of these plants, however, bloom in spring (see below under Sequence of Bloom). I focus on the purple alliums on this page, because I find their flower heads the prettiest of the lot. But they do come in other colors, examples of which I also present below.
Plant the bulbs in fall. Measure the diameter of... the bulb, then multiply that number by 2 or 3; that will be the depth of your planting hole.
Grow ornamental onions in full sun and in soil that drains well. They need an adequate amount of moisture consistently, but their bulbs do not like to be waterlogged. Since they perform better in fertile ground (they like plenty of nitrogen), mix plenty of compost into the soil. After blooming, let the foliage die back of its own accord (and remember to water the plants during this period). They are deer-resistant, as well as being rabbit-proof flowers; even rodents leave them alone.
01 of 08
A Very Dark Purple Allium (A. atropurpureum)
Allium atropurpureum is the darkest purple allium of which I am aware. Its flowers have enough of a touch of red in them to be called "wine-purple" or "maroon-purple." Color is, in fact, the main draw in growing this type of flowering onion, as the blooms are small compared to most other entries on this page.
A. atropurpureum stands 32 inches tall when in bloom. The size of the flower head, which forms not a full globe (sphere) but a half globe, is just 2 1/2 inches.
Incidentally, the atropurpureum species has no common name that I know of, other than the generic "flowering onion" or "ornamental onion" used for all of the plants discussed here. Whenever you see the botanical name, atropurpureum, you know you are dealing with a plant that has something "dark purple" on it, because that is how the term translates from the Latin. In this case, it is the flowers. In the case of Bloodgood Japanese maple (Acer palmatum atropurpureum), it is the leaves.
Grow A. atropurpureum in planting zones 4-8.
02 of 08
From the semi-spherical flower head of A. atropurpureum, I move on to examples that boast more fully spherical flower heads (as you can see from the pictures). A. 'Ambassador' is a stately plant, achieving a height of about 46 inches. In terms of flower color, this is a mauve-purple allium.
When the flower buds are still tight, the globe perched atop the sturdy stalk measures about 5 1/2 inches. But soon, the individual flower stems start to peel off from the center, the buds fully open, and -- lo and behold -- the diameter of the flower head increases to an impressive 7 inches. It is fun to watch this transformation.
Grow A. 'Ambassador' in zones 5-8. Click the link above to learn more.
03 of 08
With a name like 'Globemaster,' this type of allium clearly claims to be the poster child for the ornamental onions that display spherical flower heads. The fact that its flowers are densely packed (thus forming a tight ball) lends some credence to this claim, since it gives the flower head the appearance (from a distance) of a solid ball, rather than a collection of smaller flowers (which is what it actually is).
Anna Pavord observes that Globemaster alliums will be their biggest in year one, bearing flower heads that are 8 inches across. After that first year, they downsize, both in terms of the flower heads and overall plant height. Pavord approves of this downsizing, preferring the 28 inches in height that the plants attain in subsequent years to the "almost too dominant" size they achieve in year one, when they can stand 3-4 feet tall.
I can neither confirm nor deny her observation, because mine did not live long enough for me to make such comparisons. Some sources list them as being cold-hardy to planting zone 6, others to zone 5. I tend to agree with the former, since mine died so soon (I garden in zone 5). But these large, lavender-purple alliums are well worth growing even if it is for only one year.
04 of 08
Allium 'Purple Sensation'
If the name of 'Globemaster' suggests a plant with big ideas about its standing amongst alliums with globe-shaped flower heads, then A. hollandicum 'Purple Sensation' would seem to bound and determined to be at head of the class in terms of displaying a purple color. Is it the best of the purple alliums? That is subjective. I prefer A. atropurpureum, personally.
Flower-head size is intermediate (4 inches) and the plants stand 24-32 inches tall. Purple Sensation is listed for growing zones 4-10.Continue to 5 of 8 below.
05 of 08
A. schubertii bears small, pink flowers. But the attraction here is the size and shape of the flower head, not its color. The irregular shape of the 18-inch flower head gives it the appearance of fireworks that have just exploded. It shares this quality with A. cristophii.
As I relate in my full article on A. schubertii (click the link above), it was with this particular allium that I learned -- the hard way -- that alliums are poisonous plants for cats (and for dogs, as well). The plants grow to be 18-24 inches tall at maturity and are listed for zones 5-8.
06 of 08
The blue allium has been called by either of these botanical names:
- Allium caeruleum
- Allium azureum
But you get an idea of the flower color either way, since both "azureum" and "caeruleum" indicate "blue" (as in the English words "azure" and "caerulean"). Flower heads are even smaller than on A. atropurpureum (1 1/2 inches), but, as with the latter, the color is so nice that you will forgive the relatively small size. Plant height at maturity varies greatly, ranging from 14 to 20 inches.
You can grow this kind of allium in zones 4-10. Pavord rightly recommends plants with silver leaves such as (Artemisia spp.) as good companions for blue allium, to create a wonderful silver-blue color combination.
But in the case of alliums, the importance of using companion plants goes beyond color combinations. Since the leaves of flowering onions become less and less attractive as the season wears on, it is advisable to hide them. Companion plants can do just that. Use bigger companion plants to achieve this end when you are dealing with the bigger alliums.
Here is another factor to consider when deciding upon suitable companions: plant texture. The coarse flower head of an allium such as Ambassador will contrast nicely with the more delicate flowers of, say, 6 Hills Giant catmint (the two bloom at the same time in my garden).
07 of 08
Other Alliums (Including White, Yellow, and Edible Relatives)
As nice as the above pink and blue ornamental onions are, I think that the purple alliums are the most striking. Other ornamental purple alliums with spherical flower heads include:
- A. giganteum
- A. 'Gladiator'
- A. 'Early Emperor'
Although giganteum means "giant," it is to plant height (namely, 4-5 feet), not to flower-head diameter (which, at 5 inches, is smaller than that for both Globemaster and Ambassador) that the moniker refers.
There are also white alliums (photo) and yellow alliums (for example, A. flavum). Types with white flowers include such aptly named cultivars as 'Mt. Everest' (because the mountain by that name is snowy white) and 'Ivory Queen.' Like purple alliums, the white kinds, too are represented by some "giants," such as A. 'White Giant.'
In addition to all this variety of color, note that not all allium flower heads take the form indicated by the pictures that I have provided. For example, A. cernuum has a nodding flower head, while A. sphaerocephalon is pear-shaped.
Moreover, the ornamental branch of this genus tells only part of the story. Even non-gardeners are familiar with these edible alliums:
- Onions (Allium cepa)
- Garlic (Allium sativum)
- Leeks (Allium ampeloprasum)
- So-called "scallions" such as the Welsh onion (Allium fistulosum)
- Chives (Allium schoenoprasum)
08 of 08
Sequence of bloom is not something that those new to gardening and landscaping think very much about. But the longer you grow plants, the more the concept grows on you. The ideal is to have one plant pass the torch of blooms on to the next, in an unbroken chain of blossoming. To find out more about sequence of bloom, access my full article on the subject by clicking the link above.
Since the various types of alliums bloom at different times of the year, they can help you plan the sequence of bloom in your garden. Here are some examples:
- A. 'Ambassador': blooms late May
- A. atropurpureum: blooms early June
- A. angulosum: blooms mid-summer
- A. tuberosum: blooms late summer
- A. thunbergii: blooms in mid-fall
One of the benefits of growing flowering onions is that so many of them leave behind something attractive even after they are done blooming: namely, their seedheads (picture). Crafts enthusiasts insert the dried seedheads in arrangements. For this use, some people will spray them, while others will leave them their natural color. Sprayed gold, they would be striking and festive accents in a Christmas kissing ball.