Sweet Alyssum (Lobularia maritima) is a popular garden annual known for its low-lying, fragrant carpet of tiny flowers. Gardeners often wonder if this plant spreads beyond its original location; the answer is yes, primarily through self-seeding. While its spreading habit is not aggressively invasive in most temperate regions, the plant readily establishes new generations from year to year. Understanding how Alyssum grows allows a gardener to either embrace its naturalizing tendency or manage its spread effectively.
Understanding Alyssum’s Growth Habit
The physical expansion of an individual Sweet Alyssum plant is horizontal and low to the ground. This species is characterized by a mat-forming habit, creating a dense cover of foliage and flowers. The plant’s structure varies between two main forms: mounding and trailing.
Mounding varieties are typically compact, reaching a height of 6 to 9 inches while spreading to a similar width. Trailing varieties are bred for cascading over edges, like in window boxes or hanging baskets. These can exhibit a greater horizontal reach, with some cultivars spreading up to 24 to 48 inches. This growth pattern creates a lush groundcover, but this physical spread is distinct from its ability to colonize new areas.
Alyssum has a shallow root system, which supports its low-profile growth. This shallow rooting ensures the plant acts more like a living mulch, covering the soil surface without developing deep, tenacious root structures. The physical spread of a single plant is confined to a manageable area, with its true colonization relying on seed dispersal.
The Mechanism of Spreading: Self-Seeding
The primary method by which Sweet Alyssum spreads across a landscape is through prolific self-seeding. Following the bloom of its numerous small flowers, the plant develops tiny, flat, round seedpods known as silicles. These silicles mature and eventually split open, scattering their minuscule seeds onto the surrounding soil.
The seeds are extremely small and are often dispersed by wind, water runoff, or tracked on shoes or gardening tools. This scattering leads to the appearance of volunteer plants in the following season, often far from the original parent plant. Successful germination requires light exposure, which is why they sprout readily in bare soil or small crevices between pavers.
Older, non-hybrid varieties of Sweet Alyssum are the most reliable self-seeders, returning year after year in climates with mild winters. A single plant can produce thousands of seeds throughout a growing season, ensuring a dense population in subsequent years. Many modern, vegetatively propagated hybrids, such as the popular ‘Snow Princess’ series, are bred to be sterile. This means they produce few or no viable seeds and will not self-sow, so the extent of spreading depends entirely on the specific variety planted.
Managing and Controlling Volunteer Plants
Gardeners who wish to limit the spread of Sweet Alyssum must intervene before the plant sets seed. The most effective control method is shearing or deadheading the spent flower heads before the seedpods fully mature. Rather than clipping individual flowers, a gardener can use hedge shears to cut the plant back by one-third to one-half, which rejuvenates the plant and removes the potential seed source.
For gardeners who want to prevent all self-seeding in a specific area, applying a layer of organic mulch is highly effective. A 2-inch layer of mulch will cover the tiny, surface-germinating seeds, blocking the light necessary for them to sprout. This physical barrier suppresses the appearance of unwanted volunteer seedlings in mulched beds.
If volunteer plants do appear in undesirable locations, they are simple to manage due to their shallow root system. They can be easily removed by hand-pulling, especially when the soil is moist. Conversely, for gardeners who appreciate the natural, cottage-garden aesthetic, allowing some self-seeding is a low-maintenance way to fill in gaps and create a continuous, fragrant carpet of blooms throughout the garden.