Monkey grass, a common name for groundcover species in the genera Liriope and Ophiopogon, is highly valued in landscaping for its evergreen foliage and adaptability. These plants are frequently used for mass plantings, edging, and erosion control. The rate at which monkey grass spreads varies dramatically depending on the specific species planted and environmental conditions. Understanding the plant’s inherent biology determines if it will be a slow-moving border accent or an aggressively spreading groundcover.
The Two Primary Growth Habits
The speed at which monkey grass spreads is directly determined by its root structure, which falls into two distinct categories: clumping and running. The clumping varieties, such as Liriope muscari (Big Blue Lilyturf) and Ophiopogon japonicus (Mondo Grass), are the slowest spreaders. These plants expand outward from a central crown, gradually increasing the size of their individual tufts.
Clumping monkey grass spreads moderately by short underground stems called stolons or slowly expanding rhizomes that remain close to the parent plant. A mature clump of Liriope muscari may expand its width by only a few inches per year, eventually reaching 12 to 18 inches. This slow, predictable habit makes clumping types ideal for maintaining neat, defined borders or edges.
The more aggressive spread is characteristic of the running variety, Liriope spicata (Creeping Lilyturf), which rapidly colonizes an area. Unlike clumping types, Liriope spicata employs long, slender rhizomes that travel considerable distances beneath the soil surface. These rhizomes continuously send up new shoots, allowing the plant to spread indefinitely. Under favorable conditions, this variety can aggressively colonize several feet of new territory within a single growing season, quickly forming a dense, weed-suppressing mat.
Environmental Factors That Accelerate Growth
While the species’ growth habit dictates its potential for spread, external environmental factors significantly accelerate the rate at which it reaches that potential. Consistent moisture is a major factor, as the spread is most rapid in moist, highly organic soils. Regular watering, particularly during establishment, promotes vigorous root and rhizome development, pushing the plant to colonize an area more quickly.
Soil composition also affects the rate of travel; loose, well-draining soil allows rhizomes to navigate and expand more easily than dense, heavy clay. Although monkey grass tolerates a wide range of soil types, planting it in rich, humusy soil provides optimal conditions for maximum vegetative growth. Furthermore, applying fertilizer, especially a balanced formula in early spring, provides nutrients to boost the overall growth rate and encourage lateral spread.
Light conditions also influence vigor. While monkey grass is highly shade-tolerant, moderate sun exposure, such as partial shade, often results in the most robust and rapid spread. In deep shade, growth may be healthy, but the overall expansion rate is slower compared to areas receiving direct sunlight. The combination of ample moisture and moderate fertility in well-structured soil creates an environment where even clumping varieties expand at their fastest possible rate.
Strategies for Containing Aggressive Spread
For gardeners using the aggressive Liriope spicata or wishing to strictly maintain boundaries, physical barriers offer the most reliable control. Installing a root barrier or landscape edging deep into the soil prevents traveling rhizomes from escaping the planting area. The barrier should be sunk at least 18 inches deep, especially for running varieties, to ensure the deepest rhizomes cannot pass underneath.
Regular division and pruning of the outer edges are also effective cultural methods for containment. For clumping types, dividing the clumps every few years maintains plant vigor and prevents the gradual outward expansion from becoming unruly. For runners, manually digging out and removing the encroaching rhizomes at the border annually will keep the spread in check.
A strategic planting approach involves using running species only where aggressive coverage is desired, such as on slopes for erosion control or in large, contained beds. If complete removal or control of an established infestation is necessary, non-selective herbicides can be used as a last resort. The most effective chemical control involves cutting the foliage back in late winter and then spraying the fresh, new growth that emerges in the spring.