I have observed comfrey growing for many years. I agree with this research article that this plant is allelopathic and discourages the growth of other plants.

http://www.readcube.com/articles/10.3719/weed.50.Supplement_94

I received a comfrey plant from a swap recently and decided to exclude it from my herb garden and plant it near the woods in moist rich soil. I do not want comfrey to take over my garden. I have heard of the use of comfrey as a natural fertilizer, but am skeptical. I can see the need to compost this plant initially and then incorporate the rotted compost in the soil. A surface application of leaves will not be readily accessible to the roots.

Thanks for the information chervil2. I'll compost it first, if I decide to use it in the garden as a natural fertilizer.

You can make a massively stinky liquid manure from Comfrey if you steep the leaves in water. Personally I have noticed no allellopathic effects from Comfrey. But it does spread.

I had some leftover strawberry plants so I planted them in between my comfrey plants. Disaster. The main strawberry bed is thriving while the extras next to the comfrey are withering and not growing.

Comfrey is a big tough spreading deep rooted perennial. I would never try to grow anything less rumbustious with it for those reasons. So I am not surprised it didn't work with strawberries.

Mine lives amicably alongside rhubarb which is equally tough.