Sweet Alyssum (Lobularia maritima) is an annual flowering plant known for its dense, fragrant blooms and low-growing habit. It is a member of the mustard family, Brassicaceae, and is used as a ground cover or border plant. Alyssum self-seeds readily, which is why gardeners often find new volunteer plants returning each season. This natural tendency allows the plant to persist, even in regions where it is not hardy enough to survive winter as a perennial.
The Process of Self-Sowing
Sweet Alyssum transitions to seed production when the blooms fade. The small, four-petaled flowers are replaced by tiny, inconspicuous seed pods, which are characteristic of the Brassicaceae family. These pods mature and dry out, typically during late summer and fall, eventually splitting open to release their minute seeds onto the soil surface below.
The seeds are designed to be surface-sown, meaning they require exposure to light for successful germination. This light requirement signals to the seed that it is near the soil surface and ready to sprout. Because of this, the seeds only need to be lightly pressed into the soil or remain on the surface, rather than being buried deep.
Environmental conditions must be suitable for these dropped seeds to germinate the following spring. Optimal germination temperatures range from 65–75°F (18–24°C). The seeds often benefit from a period of cool, moist conditions over winter. This cold stratification helps break seed dormancy and ensures a strong flush of seedlings when the weather warms.
Once conditions are right, the seeds typically germinate in a short window of 7 to 15 days, providing a carpet of new plants. The lack of deep soil disturbance from tilling helps maintain the seeds on the soil surface where they can access the necessary light. The resulting seedlings, known as volunteers, appear naturally in the spring, often following the exact footprint of the previous year’s growth.
Encouraging and Controlling Volunteer Seedlings
Managing the self-seeding tendency of Sweet Alyssum depends on whether the gardener wants the plants to return. To encourage a new generation of volunteers, stop deadheading the flowers in late summer. Allowing the final flush of flowers to remain intact permits the plant to complete its reproductive cycle and form seed pods.
Gardeners should also avoid applying heavy layers of mulch in areas where self-seeding is desired, as a thick covering can block the light required for the seeds to germinate. Similarly, deep tilling or vigorous raking of the soil surface in the fall or early spring can bury the tiny seeds too deep, preventing them from sprouting. Minimal soil disturbance and allowing the spent plants to decompose in place will maximize the number of returning volunteers.
If controlling the spread of Alyssum is the goal, the opposite approach is necessary. Consistent deadheading throughout the growing season, or shearing the plants back by about a third after a flush of bloom, will minimize seed set. This shearing also promotes a bushier habit and encourages a second wave of flowers.
To prevent unwanted volunteers from sprouting, pull up the plants entirely before their seed pods open and scatter their contents. If a large number of volunteers appear in the spring, thin them out to prevent overcrowding, which can lead to poor air circulation and disease. Seedlings should be spaced about 6 to 8 inches apart to allow mature plants to spread and thrive.