Running BamboosVirtually all of the frost hardy, timber and medium-size bamboos (30 feet and taller) have leptomorph rhizomes. These rhizomes grow laterally through the soil surface, and all have the potential to spread rapidly. In a mature grove, the culms arising from a bamboo with leptomorph rhizomes, usually have an open or diffuse habit. Because of their ability to spread rapidly and indefinitely, bamboos with leptomorph rhizomes are commonly referred to as "runners" or "running bamboos".
Clumping BamboosFrost hardy bamboos with pachymorph rhizomes usually spread much slower than leptomorph bamboos, and the culms are usually arranged in a tightly spaced clump, which is known as a caespitose habit. Because of their slower spread and clumping habit, bamboos with pachymorph rhizomes are commonly referred to as "clumpers" or "clumping bamboos". However, there are many exceptions that can be confusing to beginners.
Runners that Clump and Clumpers that Run, Oh My!
Several of the medium and timber-size bamboos that are notorious for "running" will remain in a slow-growing clump as a result of environmental stresses. Here in Shelton Washington, shade and cool summer temperatures are the most limiting factors on the big "running" bamboos I grow. For example, the Phyllostachys bambusoides 'Allgold' pictured below, remains in a clump of medium-size stems in the cool soils here, (a clumping runner, if you will) but grows much larger and spreads rapidly in warmer regions.Also, most of the shrub-size bamboos, and all of the dwarf, ground cover bamboos have leptomorph rhizomes with dense culm spacing. Out of the nursery container, they start as a dense clump, but can spread rapidly into the realm of garden thugs, as pictured below. On the other hand, a few bamboos with pachymorph rhizomes have longer rhizome necks, a more open culm spacing, and can spread quite rapidly. Pictured below is a Chusquea gigantea that has covered a 10' x 10' area in just a few years (a running clumper, if you will).