Catmint, a common name for plants in the genus Nepeta, is a perennial prized by gardeners for its extended bloom time and low maintenance requirements. This aromatic herb is often used as a border plant or groundcover, providing spikes of lavender-blue flowers that attract numerous pollinators. A frequent concern is its capacity for rapid expansion, but the degree of spread depends heavily on the specific species or cultivar planted.
The Mechanisms of Catmint Spreading
Catmint expands its territory through two primary methods: vegetative growth and sexual reproduction via seeds.
The first method is the widening of the original clump through underground stems, known as rhizomes. As the plant matures, these rhizomes send up new shoots around the perimeter, causing the overall diameter to increase each season. This results in a slow but steady expansion, forming a dense, larger mound.
The second method is self-seeding, which leads to new plants appearing unexpectedly in other parts of the garden. If spent flowers are not removed, they produce viable seeds that drop onto the soil and germinate. This is responsible for new plants establishing themselves far from the original planting location.
Comparing Growth Habits Across Common Cultivars
The aggressive reputation of catmint is largely attributable to the true catnip species, Nepeta cataria, which is generally not used in ornamental garden beds. N. cataria is an aggressive spreader, relying heavily on vigorous rhizomatous growth and prolific self-seeding to colonize large areas quickly. This species requires strict control if not grown in an isolated area.
The most common ornamental varieties, such as ‘Walker’s Low’ and ‘Six Hills Giant,’ belong to the hybrid Nepeta x faassenii. These cultivars are sterile hybrids, meaning they produce flowers incapable of forming viable seeds. The lack of seed viability eliminates the most aggressive form of spread, making them tamer garden residents.
Ornamental catmints like ‘Walker’s Low’ are robust clump-formers rather than aggressive spreaders. They increase their size steadily, often reaching a final spread of two to three feet, but do not send out runners far from the main crown. This growth pattern results in a dense, mounding habit that is easy to manage. Other varieties, such as Nepeta mussinii, are more prone to self-seeding than the sterile hybrids.
Strategies for Managing Unwanted Expansion
Controlling catmint spread requires applying techniques based on the variety planted. For varieties that produce viable seeds, such as Nepeta cataria or Nepeta mussinii, preventing self-seeding is the most effective control measure. This is accomplished by deadheading, where spent flower heads are removed immediately after the first bloom before seeds mature and drop.
If dealing with a vigorously rhizomatous species, or if a sterile hybrid is outgrowing its space, expansion must be controlled physically. For aggressive types, planting them in a sunk container provides a physical barrier to restrict root movement. Regular division is also an effective maintenance method, where the clump is dug up every three to four years and sliced into smaller sections before replanting. Shearing the entire plant back by about one-third after the first bloom helps maintain a compact shape and encourages a second round of flowers.