The Potato Mine is a single-use explosive plant. While it's one of the cheaper plants, it takes approximately 15 seconds (about the time it takes a regular zombie or any other slow zombie to walk three spaces, or half the time it takes to recharge and use it again) to arm themselves before surfacing from the ground. After that, it will detonate the first time a zombie enters their square. It may kill several zombies, if they are perfectly stacked.
Some folks say Potato Mine is lazy, that he leaves everything to the last minute. Potato Mine says nothing. He's too busy thinking about his investment strategy.