Rhizomes are a modified stem that grows underground. Kind of like if the bamboo decided instead of growing up it would grow down, really far down, and to the side, reealllyyyyty far off to the side.

Then, all along the way this rhizome is creeping around it's starting to grow out new roots and chutes every couple of inches. The bamboo just starts spreadig wherever it can by growing these things.

And rhizomes are pains to get out too, all dense and fiberous like rope. If you want to get out bamboo rhizomes you have to dig down a few feet sometimes and then get a sawsall to chop it up and /then/ remove every single inch of it or the cutting will just cause it to grow more bamboo.

Not all bamboos are this aggressive or resilient (infact some are rather delicate and princess-like with their environmental needs) but some are total monsters.

Quick aside and fun botany fact: Plants Move. They don't want you to know this. They move by growing but they move. If you did a super long time lapse you could see bamboo scooting around like a little mole that puts up flagpoles wherever it goes underground. They have shmovement.