Bamboo is often viewed with suspicion by gardeners because of its reputation for taking over landscapes. While it is not classified as a weed in the traditional botanical sense, its ability to expand far beyond its intended planting site certainly makes it behave like one in a home garden setting. The key to understanding this perceived invasiveness lies in recognizing the plant’s true biological classification and its two distinct growth patterns.
Defining a Weed and the Botanical Status of Bamboo
A weed is generally defined in the horticultural context as any plant growing in a location where it is not wanted. Weeds are commonly characterized by rapid reproduction, unwanted colonization, and resilience against removal efforts.
Bamboo, however, belongs to the true grass family, Poaceae, specifically within the subfamily Bambusoideae. This classification places bamboo firmly outside the typical broadleaf plants often labeled as weeds. It is a perennial evergreen plant, and some varieties are among the fastest-growing plants in the world.
The perception of bamboo as a weed comes from its powerful underground spreading mechanism. The portion of the bamboo plant below the ground is a modified stem known as a rhizome, which is the source of its perennial growth. This rhizome system is responsible for the plant’s ability to quickly establish itself and expand its territory, leading to its aggressive reputation.
The Two Faces of Bamboo: Running and Clumping Growth Habits
The level of aggression in bamboo is entirely determined by its rhizome system, which separates all species into two main categories: running or clumping. These two categories are scientifically described by their distinct rhizome morphologies, known as leptomorph and pachymorph types.
Running Bamboo (Leptomorph)
Running bamboo, which utilizes the leptomorph rhizome system, is the type responsible for the plant’s invasive reputation. These rhizomes are long and thin, growing horizontally underground and away from the parent plant in unpredictable directions. Buds located along the nodes of these independent underground stems can sprout new culms or new rhizomes. This allows the plant to spread rapidly and widely, sometimes up to 20 feet in a single growing season.
Clumping Bamboo (Pachymorph)
Clumping bamboo is characterized by the pachymorph rhizome system. These rhizomes are generally short and thick, growing in a compact, contained cluster. Each new rhizome curves upward in close proximity to the main plant, with new culms only forming at the very tip of the rhizome. This growth pattern results in a non-invasive, slowly expanding clump that remains close to the original planting site.
Managing Aggressive Bamboo Growth
Managing bamboo requires a clear strategy based on its specific growth habit, particularly when dealing with the aggressive running varieties. For running bamboo (leptomorph species), control centers on physically blocking or pruning the spreading rhizomes. The most effective containment method is the installation of a high-density polyethylene (HDPE) rhizome barrier.
A sufficient barrier should be at least 24 inches deep and 40 mil thick to prevent rhizomes from penetrating or growing underneath. It is also important that the barrier protrudes about two to four inches above the soil line to prevent rhizomes from simply growing over the top. Alternatively, consistent bi-annual root pruning using a sharp spade around the perimeter of the grove can effectively sever the shallow, outward-seeking rhizomes.
Clumping bamboo (pachymorph species) requires significantly less intervention, though some maintenance is still beneficial. These varieties naturally stay in a tight area, but the dense root mass can exert strong pressure on nearby structures over time. Management typically involves root pruning around the edge of the clump every few years to keep the diameter in check. For both types, maintaining a shallow layer of mulch encourages the rhizomes to stay near the surface, making them easier to inspect and prune.