Running bamboos can exhibit “take-over-the-world” growth habit that leads to the fear (and occasional hysteria) about this group of plants. A newly planted running bamboo usually behaves rather tamely for the first couple of years and sends up new culms relatively close to the original planting. With time, the new culms grow progressively larger in diameter and height than the previous ones, and begin to emerge many feet away. The increase in size and spread corresponds to the increase in strength and vigor. This transformation in our perception of the plant from “benign” to “aggressive” is naturally alarming even in one’s own garden—all the more so when the invasion begins over the property line in the neighbor’s yard.
What follows are methods and strategies to contain running bamboos so that their beauty may be enjoyed in any landscape without anxiety.
Rhizome barriers are commercially available, heavy, flexible plastic underground “walls” used to surround and define the parameters of stands of running bamboo. When buried in soil, this material is longer lasting than either wood or concrete, and when correctly installed, it will completely stop rhizome spread and is therefore the best method to use along property lines. Rhizome barrier should be installed so that 2 inches remain above soil level with the walls tilted outwards. That way, rhizome growth is directed up over the edge of the barrier where it can be seen and eliminated. The genus Phyllostachys (Black Bamboo and its cousins) have relatively shallow root systems in our heavy clay, typically occurring in the top 1 foot of soil. Pleioblastus, Pseudosasa and Sasa have deeper and faster growing rhizome systems. When planting these genera of bamboo or when planting any bamboo in loose, loamy, or sandy soil, it is wise to use a barrier that is 3 feet deep.
Raised beds basically behave as an aboveground rhizome barrier. A concrete block or poured concrete wall surrounding running bamboo is an expensive solution, but when incorporated into an overall landscape design, it can be a dramatic way to display a specimen bamboo. On a smaller scale, pots and containers also act as aboveground methods to restrict spread. Rhizomes may grow out through the drainage holes and care should be taken to check and cut them annually. A saucer under the pot will also prevent rhizomes from entering the soil.
Natural barriers can also be used. Bamboo rhizomes do most of their spreading in warm summer months when we in the Bay Area have little or no rainfall. We can use the natural soil dryness to our advantage by watering a running bamboo only in the area where it is desired and leaving a 10 to 20-foot perimeter of unirrigated dry soil around the plant to block its expansion. Unamended clay soil is a more effective barrier than soil loosened with organic matter. Preexisting structures such as shallow concrete sidewalks, patios and asphalt driveways do NOT serve as effective barriers. Do not rely on them.