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How to Grow Alliums (Ornamental Onions)
Most of us don’t think of onions as beautiful plants, but onions have some very close cousins who definitely deserve a place in your flower garden. As with onions and garlic, the ornamental alliums have round flower heads composed of dozens of star-shaped flowers. However, these plants are not edible, although their leaves do have a slight onion-like scent when crushed.
The ornamental alliums won’t spice up your cooking, but their cheerful spherical flowers will enliven your garden. These are extremely tough plants that are both drought resistant and cold tolerant. They’re not even bothered by deer or rodents. And there are plenty to choose for any garden.
The majority of alliums are bulb-forming; however, there is a handful that grows from rhizomes, the way common chives (Allium schoenoprasum) do. These may never form any kind of bulb.
Leaves: Allium leaves tend to be long and strappy. Some–like the cork-screw allium–remain attractive all season, with a blue-green color that complements the flowers. Most early blooming alliums have foliage that tends to die-back early, as the plants go dormant for the summer.
Flowers: The flowers form in clusters and are best known in the round pom-pom form, but they can be star-shaped, cup-shaped, semi-circular, or pendulous.
Each species or cultivar has its own common name, but they are all known as "Ornamental Onions".
Hardiness depends on the variety being grown and the growing conditions, but most alliums will do well in USDA Hardiness Zones four to ten.
There's a good amount of variety in Allium plants. Drumstick alliums only grow about one foot tall with one-inch flower heads, while giant 'Globemaster' can top four feet in height and sport huge globes of eight-to-ten-inch flower heads. You will need to read the descriptions closely.
For the best flowering and healthiest plants, place your alliums in a site that gets a full day of sun. They will grow in partial shade, but since so many of them have short seasons, give them as much sun as you can.
Bloom Period or Days to Harvest
Most alliums bloom in spring or early summer after the earliest spring bulbs have faded. However, there are a few varieties that bloom later in the season, even well into fall.
Soil: Alliums prefer a soil pH that is slightly acidic, at around 5.5 - 6.5. However, how well the soil drains is far more important than soil pH. Do not let the bulbs sit in damp soil, especially during their dormant season. If they remain wet for too long, they will rot. Adding a good amount of organic matter to the soil before planting will improve draining while allowing enough water to reach the bulbs.
Planting: The bulb forming alliums will need to be planted in the fall. The planting depth should be two to three times the diameter of the bulbs. (So if you have a two-inch bulb, you would plant it four to six inches deep.) Water them well after planting. Then cross your fingers and wait for spring.
The rhizome forming alliums can be planted anytime. You may not find the fall-blooming varieties in the garden center until late summer. These should be planted at the same depth they were in their containers.
You can start alliums from seed, but many of the more popular varieties are hybrids and won't grow true. If you want to try your hand at growing from seed, harvest and sow them as soon as they are dried on the plant.
Alliums are very low maintenance. They will need regular water, especially while in flower, if rainfall is minimal.
If you regularly amend your soil, you may not need to feed them at all. However, if your soil is less than ideal, a little-balanced fertilizer as they start to set flowers will help them replenish all the energy they use blooming.
Alliums do not repeat bloom. You can the flower stalks down after flowering in order to send the plant's focus back into storing energy in the bulb. However, the dried flower heads are as attractive as the live flowers and many gardeners like to keep them standing. Some even spray paint them and surprise garden visitors.
Dividing: Bulb-forming alliums are very slow to multiply; however, they will eventually start forming small offsets on the original bulbs or perhaps even on the flower head. Once the plants have finished flowering, you can lift the bulbs and remove the offsets. These can be replanted immediately, but it may take a couple of years before they flower.
Pests and Problems
Ornamental onions, like their culinary cousins, don't attract too many pests. Deer and rodents avoid them.
They can get a few fungal diseases, like downy mildew and rots, but these are not as much of a problem in a flower border as they would be in a vegetable garden.
As far as insect pests, watch out for snails and slugs, as well as the allium leaf miner. However, since the foliage does not last very long, cosmetic damage to the leaves is not something to worry about.
The plump, round shape of the flowers looks charming poking through other plants, whether low growing mats such as hardy geraniums or shrubby roses. The purple color is a great asset that complements most other late spring flowers, from peonies to iris to catmint. The shape also works well with other medium height plants, like foxglove, that provides a form contrast or Monarda, which provides a form echo.
The big drawback to early-blooming alliums is how their leaves can start to go downhill, even before the plants have flowered. If at all possible, try to hide the foliage behind a denser plant. Daylilies work well for this.Continue to 2 of 2 below.
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Best Varieties of Ornamental Onions to Grow
Drumstick Allium (Allium Sphaerocephalon) are early summer bloomers. Their one-inch flower clusters start off greenish and eventually start to resemble red clover. They look best when allowed to waft their way throughout the garden so that they can surprise you by peeking out through other flowers. Once again, the foliage is rather tatty.
Corkscrew Allium (Allium senescens ssp. montanum var. glaucum) It’s the blue-green leaves that twist and turn and give this allium its common name. You may find this one in the perennial section of garden centers, since it grows from rhizomes rather than a bulb. The two-inch lavender flower heads are flattened balls that bloom in mid- to late summer.
Globemaster For the biggest impact, try one of the super-sized allium flowers. Globemaster flower stalks can reach three to four feet tall and be topped with flowers that form a ball that’s eight to ten inches in diameter. Mount Everest is another imposing, tall allium. It’s not quite the size of Globemaster and it blooms in a creamy white.
Japanese Onion (Allium thunbergii) To round out the season, these small, one-inch pink flower heads bloom in early fall. They form more of a mop than a round globe. Some cultivars to look for include 'Ozawa' with larger purple flower heads and ‘Alba' which has white cup-shaped florets. This allium grows from rhizomes, rather than forming bulbs, and may be available in containers.
Nodding Onion (Allium cernuum) Rather than a spherical flower head, the nodding onion produces flopping mop heads of flowers in pretty shades of pink and purple. The flower stems average two to three feet. This is one of the most widely adaptable alliums and can even be grown well in partial shade.
Purple Sensation Probably the most commonly grown and for good reason–they are incredibly reliable and very attractive. The flower stalks reach about two feet tall and are topped with a two-to-four-inch globe of bright purple flowers. Purple Sensation tends to be a long-lived bulb, but its leaves tend to get yellow or brown quite early, which can detract from the fabulous flowers. Try and hide the leave behind other spring emerging plants, like daylilies.
Schubert Allium (Allium schubertii) Dramatic and just plain fun, the flower heads of schubertii alliums look like a fireworks display. Even as they fade, they retain their explosive look. Another common name for schubertii is the Tumbleweed Onion, although you’ll seldom see these flower heads drifting by.