cover crops around the base?

Should comfrey and cover crops be planted right around the base of new trees?  Or leave the soil bare out to the drip line?  New 10' aspen trees for instance.

I have planted comfrey very close to the trees, and out to, or beyond the dripline.  I didn't notice a difference in their growth.

As far as cover crops or bare soil, you can plant cover crops or not, but you won't have bare soil :)  Cover crops just allow you to decide what is growing there.  If you don't, nature will decide for you.