Comfrey is one of the easiest herbs to grow; its requirements are minimal, and its enthusiasm can sometimes be a bit overwhelming. All you really need to do is give it ample space to grow in, keep an eye on it's spreading tendency and wait. Comfrey is rich in nitrogen, so it makes a great addition to the compost pile.
Give it adequate space. Comfrey is a generously sized herb, and you can't squish it into a little 2-square-foot area. You can try, but you'll wish you hadn't when it starts squeezing out the other plants around. Plan on it growing to a mature 4-foot diameter.
Keep it contained. Comfrey grows so well, says Patrick Lima, author of "Herbs: The Complete Gardener's Guide," that it can become "one of a gardener's self-inflicted weeds." Conquer the problem by one of these methods: (1) put it in a secure, sunken border; (2) plant it in a large container or (3) place it in a raised bed. You'll still have to keep an eye on it, but you should be able to keep it in check.
Plant from seed in soil that is rich and well-drained. If you have a gardener friend with an established comfrey plant, you can also easily take cuttings at any time or divide roots in the fall, then plant and water them.