Allium: profile, winter hardiness & propagation

The imposing flowering umbels of the ornamental onion provide an impressive sight in the garden. Due to its ease of propagation, ornamental onion is perfect for novice gardeners.

The name ornamental onion refers to various species of the genus Allium. Its different flower colours are not only striking when in bloom, but also when dried. The dried flowerheads can be left in the garden or used as a pretty decoration in a vase. Here we explain the best way to dry Allium and how to propagate it.

Ornamental onion: flowering time, origin and characteristics

Under the genus Allium there are a number of very well-known bulbous plants in addition to ornamental onions. These include species such as cooking onion (Allium cepa), garlic (Allium sativum), and chives (Allium schoenophrasum), which are also used as ornamental onions. The genus Allium in turn belongs to the Amaryllis family (Amaryllidaceae) and grows almost worldwide, with most species found in the northern hemisphere. In our country, wild garlic (Allium ursinum) is the best-known native representative.

The useful Allium plants beautify many gardens with their magnificent balls of flowers. Typical of the amaryllis family, the Allium leaves are also mostly narrow, elongated and rather inconspicuous. The sturdy shoot grows almost gracefully from the rosette of leaves and ends with differently shaped flower umbels. Depending on the species, the flower umbels can reach a diameter of up to 25 cm. The individual flowers of ornamental onions are star-shaped and can be white, purple, blue or pink. The height of the flowerheads also varies depending on the species. There are Allium species that only grow to a manageable 15 cm, or those that reach a height of 150 cm.

Allium is used in rock gardens but also in herbaceous borders, where the umbels really come into their own.

Allium is fascinating not only above ground, but also underground. Most species form bulbs, or thickened shoot axes. Others have rhizomes, which usually run horizontally to the surface. Among other things, these serve to store nutrients that they take from the leaves during or after the flowering period. This allows the plants to survive and form species-specific reproductive organs such as daughter bulbs, which resprout in the spring.

When does Allium flower? Allium blooms from April to September, with the most popular species flowering from May to June. Ornamental onions really attract honeybees and wild bees and usually have good to very good pollen and nectar values as well.

Is ornamental onion perennial? No, the single Allium bulb is mostly annual. Allium species, however, form reproductive organs, such as daughter bulbs or bullets, with the help of which they can reproduce vegetatively, so it seems that the same bulb sprouts again every year.

Winter hardiness of ornamental onion

The sensitivity to frost varies between the different Allium species. Some ornamental onion species can withstand temperatures as low as – 40 °C, others no lower than – 1 °C. It is advisable to cultivate frost-sensitive species in a pot or tub and overwinter the plant in a protected place, for example, against the wall of the house. In our next article, we will introduce you to the most beautiful ornamental onion varieties and species and describe, among other things, their respective winter hardiness. In addition to temperature, soil conditions are important for successful overwintering. The soil should be dry to fresh; too much moisture can lead to rotting of the bulb.

Propagating Allium

Allium species can be propagated in several ways.

Propagation by daughter bulbs

• Bulb-forming species form one or more daughter bulbs to the side of the mother bulb.
• After the foliage has retreated, these can be carefully cut off and planted separately.

Propagation by bulbils

• Some species form bulbils, fleshy brood buds that resemble small bulbs, when they are in bloom.
• In late summer, they can be harvested and planted – the procedure is similar to daughter bulb propagation.

Propagation by division or chipping

• Certain species are slow to form daughter bulbs. This process can be encouraged by dividing the mother bulb.
• The bulb is divided vertically into individual pieces with a knife. Each section should have part of the base, i.e., of the rooted bulb.
• The sections can be planted directly separately.
• Between the bulb scales, new daughter bulbs now form, which will later produce new shoots.
• Rhizome-forming species can also be divided and planted separately. In this case, make sure that there is one bud per section.

Note: For frost-sensitive species, the propagules are removed from the soil in the autumn, cleaned and stored for planting in the spring.

Propagation from seed

• When the seed heads are almost ripe, they can be hung upside down above a cardboard box, for example. The capsules open and the seeds fall out.
• The seeds can be sown in the autumn.
• Storage at 5 °C over the winter is also possible. The seeds are sown in the spring from March.
• Allium plants are often cross-pollinated, which means that the harvested seeds may not be pure in variety.

Note: The germination and flowering ability of newly obtained seeds or propagules are highly species-specific, so it may take one to three years before the new bulbs form a flower. Germination can take between 12 weeks and 1 year.

Bulbous plants need a well-drained substrate to reduce the risk of rotting due to waterlogging, as mentioned above. A heavy garden soil must be mixed with an appropriate amount of sand; an extremely sandy soil, however, must be enriched with some humus-rich, nutrient-rich soil. A good basis for this mixture, for example, is our Plantura Organic Enriched Compost. This is ideal for nutrient-hungry plants and is also 100% peat-free. Be sure to use a healthy mix of one part sand and one part compost, especially for potted plantings.

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A drainage layer should be included in all cases so that watering or rainwater does not accumulate, and the bulb is spared from fungal diseases.  In our article that follows, we explain everything you need to know about successful planting and care of ornamental onions.

Drying Allium

Allium not only looks beautiful in the garden: Allium– species can also be beautifully displayed as dried flowers. To do this, cut off the flowerheads at the base when they are open and let them dry upside down in a shady place. After about 2 weeks, they are sufficiently dried and can be put in a vase, for example. If the seeds and thus the capsules have already formed, they can be equally dried and used decoratively.

Tip: Even as a fresh-cut flower, Allium will keep in water for 10 to 14 days, depending on the species.

Is ornamental onion poisonous?

Ornamental onions are not poisonous. In fact, all parts of the plant can be eaten, although a conventional edible onion or edible leek is more delicate and intense in taste. However, dogs should be kept away from Allium species as they are very sensitive to eating them.

Another rhizome-forming ornamental perennial and at the same time medicinal plant is the graceful black cohosh. Learn here everything about growing them in your garden and how to use them.