Sweet Alyssum (Lobularia maritima) is a popular garden flower often confused with invasive species due to its vigorous spreading habit. Understanding the specific differences between invasive and aggressive plants is necessary to correctly categorize this fragrant, low-growing annual.
Distinguishing Invasive Species from Aggressive Growers
The distinction between an invasive species and an aggressive grower is defined by science and law. A plant is officially designated as invasive only if it is non-native and its introduction causes economic or environmental harm. This harm typically involves outcompeting native flora, disrupting food webs, or altering ecosystem processes. Invasive status is regulated by government bodies.
An aggressive grower, in contrast, spreads rapidly and can become dominant within a cultivated space, like a garden bed. This rapid growth is usually due to prolific self-seeding or fast vegetative spread. While aggressive plants can be difficult to manage, they do not escape cultivation to cause widespread ecological damage in natural habitats.
Origin and Growth Habits of Sweet Alyssum
Sweet Alyssum is native to the Mediterranean region, including parts of southern Europe, northern Africa, and the Azores. Since it did not evolve in North America, it is considered a non-native species. This status is the first condition for invasiveness, fueling the common confusion.
The plant’s reputation for being aggressive stems from its effective reproductive strategy. Sweet Alyssum is a mat-forming, low-growing plant, typically reaching 3 to 12 inches in height and spreading up to a foot wide. Its dense habit allows it to quickly cover bare soil, functioning as an effective living mulch that suppresses weeds.
The primary mechanism for its spread is prolific self-seeding in disturbed garden soil. Seeds readily germinate, often resulting in a dense carpet of new plants. Alyssum’s spread is contained and dependent on garden conditions, unlike true invasive species that spread via deep rhizomes. It thrives with minimal care and tolerates a range of soil types.
Assessing the Ecological Status
Despite its non-native origin and aggressive garden behavior, Lobularia maritima is generally not considered an environmental threat in the United States and Canada. It fails to meet the second criterion for invasive status: causing significant ecological harm. The plant typically lacks the traits necessary to outcompete established native flora in undisturbed natural ecosystems.
Its inability to penetrate and dominate dense wildlands means it rarely displaces native species on a landscape scale. Sweet Alyssum is often classified as a naturalized plant, meaning it is established without causing broad environmental destruction. This classification keeps it off most official state and national invasive species lists.
The plant’s classification is not uniform across all regions, demonstrating the localized nature of invasiveness. In California, it has been noted for escaping cultivation, especially in disturbed coastal areas like dunes and bluffs. The California Invasive Plant Council (Cal-IPC) gives it a “Limited” rating, indicating minor statewide ecological impacts. This concern is restricted to sensitive habitats where the plant can compete with rare native coastal species.
Practical Management of Alyssum in the Garden
Management is straightforward for gardeners concerned about Sweet Alyssum’s aggressive self-seeding. The most direct method to prevent unwanted spread is by deadheading the spent flower clusters. Removing faded blooms before they set seed significantly reduces the number of volunteer seedlings that appear the following season.
To maintain the plant without re-seeding, shear the plant back by one-third to one-half after the initial bloom flush. This action prevents seed formation and encourages a second, stronger flush of flowers later in the season. Unwanted volunteer plants are easy to remove manually because Sweet Alyssum is shallow-rooted.
Applying a layer of organic mulch around the plants can also help suppress seed germination. Despite its aggressive nature, the plant offers benefits, including excellent ground cover and a long blooming season. Its flowers attract beneficial insects and pollinators, and its vigorous growth can be utilized for quickly filling gaps or creating a fragrant border.