While it's true that comfrey isn't native to the United States (it originated in Europe or western Asia), it's not considered invasive. Rather, it's been adapted for agricultural and medicinal uses in the US, and is considered to be beneficial instead of harmful.

The variety that we sell (Russian Bocking 14) is particularly non-invasive because it is a sterile variety. While it will spread through the roots, it won't spread by seed, making it easy to contain.