Why is lemon balm an annual if it's in a pot?

[SunshineAndSoil]

Hey everyone,
I have read that lemon balm is a perennial if it's in the ground, but that it's an annual in a pot. Why is this? Is it because the roots get too cold or something?

 

[Mrsclem]

It runs rampant in my yard so not an annual. I have had it in planters and it still comes back. Wish I could get rid of it!

 

[SunshineAndSoil]

Yup, it's in the mint family, and mint is so prolific it's like a weed. That's why I'm keeping it in a pot.