Thyme is a popular perennial herb and ground cover used in many gardens. When considering its place in a landscape, gardeners often ask if it will expand beyond its initial planting spot. The straightforward answer is yes, thyme does spread, but the extent and speed depend heavily on the specific variety chosen. This distinction determines if the plant will form a contained clump or a wide, dense mat.
Growth Habits: Creeping Versus Upright Thyme
Thyme varieties are separated into two broad categories based on growth habit, which directly affects their spreading potential. Creeping thyme, including varieties like Thymus serpyllum or woolly thyme, is valued for its ground-covering ability. These plants form a low, dense mat, typically staying 2 to 4 inches high while spreading horizontally. Creeping varieties are often used as living mulch, between stepping stones, or as a lawn alternative.
In contrast, upright thyme varieties, such as French thyme and common English thyme (Thymus vulgaris), are cultivated mainly for culinary use. These types grow as small, semi-woody shrubs, typically reaching 6 to 12 inches in height. Upright thyme is clump-forming, expanding slowly outward while maintaining a contained, bushy structure.
How Thyme Spreads and Its Growth Rate
Thyme expands its footprint through two primary biological mechanisms. The most rapid and reliable method of spread, particularly for creeping varieties, is by rooting stems. Creeping thyme stems sprawl horizontally, and when a node makes contact with the ground, it develops new roots. This process, similar to natural layering, establishes an independent, rooted section attached to the parent plant, allowing the plant to cover ground efficiently.
The second method of expansion is self-seeding, where both upright and creeping varieties drop seeds after flowering. This method is slower and less predictable, depending on soil disturbance and environmental factors for successful germination. Thyme is considered a moderate grower. The initial establishment phase is slow as the plant focuses on developing a strong root system in its first year. Once established, creeping thyme can spread outward by 6 to 18 inches per year under favorable conditions. It may take two to three years to form a dense, weed-suppressing mat.
Controlling or Encouraging Thyme’s Expansion
Gardeners have practical options for managing thyme’s spread, whether the goal is containment or maximizing coverage. To control the horizontal expansion of creeping thyme, routine pruning of the outer edges provides an effective barrier. Trimming back the sprawling stems prevents them from rooting into unwanted areas.
For upright, woody varieties that become too large or sparse, rejuvenation is achieved by cutting them back by about one-third in the spring, avoiding the hard, old wood. If the aim is to encourage faster mat formation, especially with creeping varieties, use a technique called top-dressing. This involves lightly sprinkling a thin layer of soil or compost directly over the trailing stems. This practice covers the stem nodes, encouraging quicker rooting and accelerating the development of a dense ground cover. Established plants can also be easily divided by separating a rooted clump from the main plant and transplanting it.