For six years I volunteered at a NYC community garden called Green Dome at North 12th St./Union/ Driggs in Brooklyn. Although I don't miss the "gardening by committee" feeling, I do miss the tenacious people and plants. Green Dome is no vegetable-plotted garden but a great mix of perennials, trees, shrubs and hardscaping. It feels like a little oasis in McCarren Park, of which it is technically part. Because the garden is open daily to the public (unmonitored) and mostly unwatered, the plants need to be tough to survive. Comfrey (genus symphytum, I'm unsure of the species), one of the toughest, came along with me to New Jersey when we left Brooklyn in 2006. This is a plant with serious territorial ambition and it took me a year or two of moving it around to find the right spots for it. I planted it on both sides of the dominating and thirsty silver maple that towers over our garden. The soil near the root flare of this tree is full of roots, some major, but the comfrey has dug in and flourished. I always welcome its elegant downward-facing white bells in the spring but cherish its toughness in out-competing all the other weeds year round. That lean situation is probably what keeps it from taking over the whole garden.
Another plus is that comfrey foliage is especially high in nitrogen, phosphorous, and potassium. When I remember, I sometimes throw a few comfrey leaves into the planting hole of new plants. Supposedly they break down quickly and supply a nutritional boost to the new plant.
In the end, I know that if the comfrey gets out of hand in the garden, it can always enrich the compost pile.