The question of whether the groundcover plant Cerastium tomentosum, commonly known as Snow-in-Summer, is an invasive species is a source of debate among gardeners. Prized for its striking, silvery-gray foliage and abundant white flowers, this perennial is widely used in landscaping. Its vigorous growth habit often causes gardeners to mistake its garden aggression for true ecological invasiveness. Understanding the plant’s physical traits and its natural origins helps clarify the distinction between a difficult garden plant and a regulated invasive threat.
Identifying Snow-in-Summer (Cerastium tomentosum)
Snow-in-Summer is a low-growing, mat-forming herbaceous perennial that typically creates a dense carpet of foliage about six inches tall. Its common name comes from the profusion of small, star-shaped white flowers that emerge on wiry stems, rising eight to twelve inches above the mat in late spring and early summer. The plant’s specific epithet, tomentosum, refers to its most distinctive feature: the narrow, lance-shaped leaves are covered in soft, woolly white hairs, giving the foliage a silvery, felt-like texture.
This species is native to the alpine and Mediterranean regions of Europe, specifically areas like Italy, Sicily, and the Balkans. Due to its origins in harsh climates, it is tolerant of drought, full sun, and poor, dry, or rocky soils, making it a popular choice for rock gardens, dry slopes, and ground cover.
Defining Its Invasive Status
The term “invasive species” has a specific definition in ecology, referring to a non-native organism whose introduction causes or is likely to cause environmental or economic harm. Cerastium tomentosum is rarely listed on official state or federal invasive species registries, meaning it does not generally meet the criteria for causing widespread ecological damage outside of cultivated areas. Therefore, it is not officially considered invasive in most regions where it is grown.
Most of the concern surrounding this plant stems from its aggressive nature within a garden setting, leading it to be labeled as “weedy” or “rampant” by many home growers. Aggressive plants spread rapidly and can overwhelm neighboring species in a controlled landscape, but they do not typically colonize and disrupt natural ecosystems. Its status is primarily a management problem for gardeners rather than an ecological threat to wild habitats.
Mechanisms of Aggressive Spread
The reputation Snow-in-Summer has for rapid takeover is due to two highly effective reproductive strategies. The primary method of vegetative expansion is through the production of shallow, rooting stolons, often called runners, which creep along the soil surface. Wherever these lateral stems touch the ground, they root and form new, independent plantlets, rapidly expanding the original plant’s footprint. This continuous rooting action allows the plant to form dense mats that can overwhelm nearby perennials.
A secondary mechanism of spread is self-seeding, which occurs if the spent flowers are left on the plant after blooming. This prolific reseeding ability allows it to colonize new, disturbed areas of the garden quickly. Its preference for poor, dry soils and full sun means it successfully establishes itself in challenging spots where other garden plants struggle to take hold.
Managing and Restricting Growth
Controlling the aggressive spread of Snow-in-Summer requires proactive and consistent intervention from the gardener. The most effective cultural control measure is regular shearing or pruning immediately after the main flowering period in late spring or early summer. Removing the flower stems before they set seed prevents the plant from self-seeding into new areas and encourages a denser, more attractive mat of silver foliage.
For physical restriction, the shallow root system makes it relatively easy to manually control its boundaries by cutting away the encroaching sections of the mat. For long-term containment, installing a physical root barrier or edging beneath the soil surface can effectively limit the lateral spread of the rooting stolons.