The problem with mint is that it's so aggressive and invasive. I know this about the plant. I thought I was prepared for it. Ah, hubris.
This is my chocolate mint. I planted it in this half barrel on May 18, exactly one month ago. It was in a 3-inch pot and had 5 stems showing above ground, with all foliage completely contained within the diameter of its 3-inch pot.
Judging from the amount of growth it's shown in the past month, the $1 plant I bought was approximately 15 minutes old at the time of purchase.
The red arrows are pointing to the subterranean stealth runners that are spreading throughout the planter. The one in the top right quadrant is particularly horrifying. The stem of the runner is fully a quarter inch in diameter, and sprouting a new shoot every quarter inch or so. It's eight inches long, and was not there on Sunday.
The chocolate mint will be coming out of the half barrel this afternoon. It will go into its own pot, because children who do not play nicely with others have to be separated.
The peppermint in the planter next door is also spreading, but this one isn't quite as frightening.
It's spreading, but its runners are above ground and I can see where they are. This plant also started out as 3 inches in diameter a month ago, and is now approximately 14 inches in diameter.
Actually, come to think of it, this will be put into its own pot this afternoon, as well.