How can I kill off bamboo that is taking over my whole backyard?

God Bless you. I made the mistake of planting this incredibly invasive plant. Sold the house and the people who bought it went through barrels and barrels of Roundup to get rid of it. Unless you don't want to do this, rent a backhoe and go deep. One small remaining root will multiply so fast. My heart goes out to you. I wouldn't even buy a house if bamboo is in the neighborhood.

You have to keep after it. It is very hard to completely eradicate. Pull up whatever you can, dig up the root balls where you can, and treat it chemically if it is safe to do so where it is. Unfortunately, it's usually a multi-year process.

Both Dmotan and Seth are right in that you have to kill deep roots. As you will never know where they might continue to pop up again, keeping them back will most likely have to be a part of your regular yard maintenance.

I used Ortho Weed B Gone Max multiple times. Put it on a strong ratio setting. It even killed a Sumac tree