Bamboo is a member of the grass family, often prized for its rapid growth and screening ability. Its behavior depends entirely on its underground structure, classifying it as either running or clumping. Running bamboo is infamous for its aggressive and invasive nature, spreading rapidly away from its original planting location. Understanding this difference is key to successful cultivation or effective control.
The Mechanism of Spread
The invasive nature of running bamboo is driven by its specialized root system, known as monopodial or leptomorph rhizomes. These rhizomes are long, slender underground stems that grow horizontally through the soil, sometimes traveling many feet from the parent plant in a single season. Unlike typical roots, these rhizomes feature nodes, each with the potential to sprout a new culm (the above-ground shoot) or another rhizome.
This structure allows the bamboo to colonize large areas quickly, with new shoots emerging in scattered locations. The rhizomes typically remain shallow, occupying the top 6 to 18 inches of soil, making them highly effective at traveling under lawns and garden beds. This indeterminate growth means the rhizomes continue to spread outward indefinitely until an obstruction is met, posing a threat to neighboring properties and hardscaping.
Running Versus Clumping Bamboo
The distinction between running and clumping bamboo is based on the structure of their underground rhizomes. Running bamboo, such as many species in the genus Phyllostachys, uses thin, linear leptomorph rhizomes, resulting in a diffuse growth pattern. The culms of running varieties are spaced widely apart, forming an open grove rather than a dense cluster. This scattered growth is a visual indicator of the aggressive spread occurring beneath the surface.
In contrast, clumping bamboo, primarily from the Fargesia genus, possesses sympodial or pachymorph rhizomes. These rhizomes are short and thick, and their tips curve upward almost immediately to form a new culm. This determinate growth pattern means new shoots always emerge very close to the parent plant, slowly expanding the diameter of the original clump in a tight, mound-like formation. Clumping bamboo generally expands only a few inches each year, making it a well-behaved choice for smaller gardens where containment is not a concern.
Managing and Containing Growth
Controlling running bamboo requires a consistent approach, with the most effective method being the installation of a physical rhizome barrier. This barrier must be made of a non-degradable, smooth material, with high-density polyethylene (HDPE) sheeting (60 mil or 80 mil thickness) being the preferred choice. To effectively deflect spreading rhizomes, the barrier must be installed at a depth of at least 24 to 36 inches and must protrude 2 to 4 inches above the soil line. This above-ground lip is necessary because rhizomes can grow upward and over a flush barrier, requiring annual inspection to trim any that attempt to cross.
The barrier must form a continuous, unbroken circle around the planting area to prevent escape points. It is most effective when installed with a slight outward slant to encourage rhizomes to hit the smooth surface and turn upward. For existing running bamboo that has already spread, management often involves energy depletion, which can take several years.
This technique, sometimes called the “mow and harvest” method, involves consistently cutting or mowing every new shoot that emerges outside the desired area during the spring shooting season. By immediately removing new culms, the plant is prevented from photosynthesizing, forcing it to use stored energy to produce subsequent shoots. Persistent removal of these new shoots will eventually exhaust the underground rhizome system, causing it to die back. A simpler containment method involves digging a narrow trench around the planting area and cutting any rhizomes that cross the open space once or twice a year. Complete removal of a well-established running bamboo grove is a difficult task that requires excavating the entire rhizome mass and continually monitoring for regrowth for several seasons.