Rhizomes are modified stems running underground horizontally (as opposed to an arbolic structure, which runs more vertically). and they form by creating new roots and shoots through their nodes. What makes these roots so great (but also a pain if we're trying to get rid of certain plants) is that the act of separating the roots oftentimes gives rise to new ones.
Currently we've been taking out the Oriental Bittersweet Vine. Although they are beautiful, we have decided it's best for the rest of the courtyard to get rid of this plant, since it reproduces at such a rapid rate and often chokes out most plants it comes into contact with.