Allium, commonly known as ornamental onion, is a genus encompassing hundreds of species, including familiar edible varieties like garlic and chives, alongside flowering bulbs. They do spread, but the rate and method of multiplication vary significantly depending on the cultivar and growing conditions. Many popular ornamental alliums are not aggressively invasive, while certain smaller types can be prolific. Their increase is divided into two processes: vegetative reproduction through the bulb and sexual reproduction via seed.
Vegetative Spread Through Bulb Offsets
The primary method by which most ornamental alliums multiply is through the formation of bulb offsets, also known as bulblets. These small offsets develop naturally at the base of the parent bulb underground. This process is asexual reproduction, meaning the offspring are clones of the original plant.
The production of these offsets leads to a localized “spread” where the plant forms dense clumps rather than migrating across the garden. A single mature bulb slowly generates several daughter bulbs over years, causing the planting site to become overcrowded. A single planted bulb often turns into a tight cluster of several plants after three to five seasons.
The offsets remain attached to the mother bulb but can develop into new, independent flowering plants. While this method ensures the new plants are true to type, the spread is constrained to the immediate vicinity. To maintain plant vigor and encourage better flowering, these dense clumps require periodic division, which assists the natural separation.
Multiplication by Seed Dispersion
Beyond vegetative multiplication, alliums spread through sexual reproduction via seeds, known as self-seeding or naturalizing. After the flowers fade, they develop into dry seed heads containing seeds. Once these heads mature, wind or wildlife can scatter the seeds away from the original location.
The propensity for self-seeding differs widely among varieties. For instance, some large cultivars like ‘Globemaster’ are often sterile and produce no viable seed. Conversely, smaller, species-based alliums, such as Allium sphaerocephalon (Drumstick Allium) or certain chive varieties, readily produce fertile seeds that germinate easily. When these seeds sprout, they often appear as fine, grass-like seedlings.
A significant difference between seed-grown and offset-grown alliums is the time required to reach maturity. While offsets often flower within one or two seasons, plants grown from dispersed seeds require two to four years to develop a bulb large enough to support a flower stalk. This delay means the spread from self-seeding may not be noticeable until several seasons after the original flowers drop their seeds.
Managing and Controlling Allium Growth
Managing allium growth involves applying techniques based on the method of spread, allowing gardeners to either contain or encourage multiplication. To prevent unwanted spread through self-seeding, the most effective action is deadheading. This involves removing the spent flower heads before the seeds mature. Deadheading must be done while the flower head is still somewhat green, before the seed capsules open.
To manage the localized clumping caused by bulb offsets, periodic division is the recommended practice. Gardeners should lift the entire clump after the foliage has died back and the bulbs have entered dormancy, typically in late summer or fall. The smaller offsets can then be separated from the parent bulb and replanted elsewhere to establish new plants.
If the goal is to encourage a mass planting or a naturalized look, gardeners can select varieties known for prolific self-seeding or offset production. Leaving the flower heads intact to scatter seeds and allowing clumps to remain undivided will maximize the number of new plants. For highly vigorous varieties, like certain wild garlic species, consistent digging to remove all bulblets is necessary for effective control.