Frost 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.