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How to Care For Allium (Ornamental Onion)

Last Updated on August 15, 2022 by Plant Mom Care

Allium, a large genus of garlic- or onion-scented bulbous plants from the (amaryllis) Amaryllidaceae family, is found almost everywhere in the world except in the tropics, New Zealand and Australia. The number of species in the genus varies, from estimates as low as 260 and almost 979. 

The majority of the species are indigenous to various locations, from dry subtropics to the cold northern forests, primarily in Asia, with 138 species from China, in 5 subgenera. 

Some species are indigenous to Africa, Central America, and South America, with a single species from South Africa. They mostly grow in sun-lit locations, but several are also found in forests or swamps, or water.

Carl Linnaeus scientifically described the genus in 1753. Various species have been grown for several centuries, with around a dozen species still being economically important as garden vegetables and food crops such as A. cepa (onion), A. sativum (garlic), A. schoenoprasum (chive), and A. porrum (leek) with an increasing number of species being grown as ornamentals.

allium purple flower seeds grow garden spring care

These plants are described as producing 1 – 12 strong-smelling leaves (due to chemical compounds) and vary in height between 2 – 60 inches, with a spread of 3 – 10 inches. The leaves are either linear, flat, channeled, straight or coiled, but some have broad leaves, 

like A. tricoccum and A. victories. Most grow from rhizomes or bulbs and most are largely perennials. The flowers have 6 petals that often grow in spherical clusters from erect stalks, varying in form from round pom-poms, but can also be cup-shaped, semi-circular, star-shaped, or pendulous. 

They come in various colors of purple, pink, white, blue, yellow, and green and usually bloom in spring or early summer. The fruits are dry capsules containing small round black seeds. 

The bulbs can vary in size in the species, from small around 0.1 inches to large 3 – 4 inches in diameter. Some species such as A. fistulosum and A. ampeloprasum form thick leaf bases instead of bulbs as the other species. 

The bulbs can be solitary, clustered, and tunicate (have concentric layers) and perennialize with the bulbs re-growing annually from the bases of old bulbs or produced on rhizome ends or, in some species, at the end of stolon stems.

 A small number have tuberous roots. The smooth outer skin of the bulbs is usually brown or gray and fibrous or reticulated while the inner tissue is membranous.

The characteristic pungency depends on the sulfur content in the soil the plants grow in. In rare occurrences of growing in sulfur-free soil, all the species will completely lose their customary pungency.

Some bulbous species propagate from offsets around the main bulb, as well as from seeds. Many species can form small bulbs in the flower heads, like the A. × proliferum (Egyptian onion). 

Ornamental species and hybrids such as A. giganteum and A. cristophii are used in garden borders. These ornamental plants produce circular umbels on single flower stalks in a broad range of sizes and colors.

However, some species such as A. ursinum and A. triquetrum often become troublesome weeds.


Allium Light Requirements

Cultivate these plants where they can get a full day’s worth of sunlight to produce more flowers and stay healthy. They can grow under partial shade but, although they grow back every year, they usually have short seasons, so give them as much sun as possible.


Allium Watering

They need occasional watering and, if it rains frequently, that should be enough. If not, water them every 3 or 5 days.


Allium Humidity

They prefer average to moderate humidity levels. 


Allium Temperature

They prefer growing temperatures between 40 – and 70°F. Hardiness varies on the species and growing conditions, but most species do well in zones 4-10.


Allium Soil

They tolerate various soil conditions – from dry, well-draining soils to moist, organic soil. However, soil drainage capacity is more important to them than soil pH.

Don’t let the bulbs be in damp or soggy soil, particularly in their dormant period, as they will start to rot adding a decent amount of organic matter before planting will help improve drainage while still allowing water to get to the bulbs.


Allium Repotting

Ornamental species can be cultivated in large containers to display their remarkable appearance in any place under bright sunlight.

When selecting a container, make sure it has a good draining capacity. Fill it with rich, well-draining soil. If you’re planting bulbs, make sure the bulbs don’t touch each other when they outgrow the container, dig up the plant in autumn and divide the offsets.


Allium Propagation

The bulbous species have to be planted in autumn, about 2 – 3 times deeper than the diameter of the bulbs – for example, a 2-inch bulb would have to be planted 4 – 6 inches deep. 

Bulbous species are rather slow to multiply, when they eventually start forming offsets on the main bulb or even on flower heads, the bulbs can be lifted out and the offsets removed and planted.

This is usually done after the plants finish flowering. The offsets can be immediately replanted but might take a few years before they start flowering.

The rhizomatous species can be propagated anytime by lifting and dividing them once the clump starts becoming over-crowded. Don’t wait for the central portion of the plant to die – divide them as soon as they look overcrowded.

Additional Care

Regularly amending the soil is sufficient and you might not need to fertilize at all if the soil is poor, some balanced fertilizers can be applied when they start setting flowers to help them replace the energy used for blooming.

These plants will not bloom again, trimming down the flower stalks when flowering has finished directing the plants back to storing energy in their bulbs. Dried flower heads can be as attractive as live flowers and several gardeners prefer to leave them standing.

Allium Common Problems

These plants don’t attract many pests, even deer and rodents tend to avoid them.

They can suffer from a few fungal infections like downy mildew or rot, but these diseases are not a big problem in ornamentals as they would be for vegetable species. Don’t overhead water and remove infected bulbs.

Insects like leaf miners and slugs and snails can be a problem at times since their foliage doesn’t last for long, any cosmetic damage is not something you should worry about.

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