Looking for information for someone in Hydrangeas, found this among search Ken forwarded; comes up frequently enough here seems worth sharing:

from Ego45 in 2008

Ok, right or wrong, but 3 years ago I did exactly what old gardener told me to do and it seems to be working fine, so far.
First, he told me NOT to dig in a maple root zone since DISTURBED maple roots will regrow anyway and will do that with much more vigor than undisturbed ones. So, I built that bed/border right on top of the existing soil.
Second, he said to limb it up as high as possible in order to reduce a green mass roots need to support/feed, thus reducing maple's needs to grow more roots than it needs. Mature tree already have a sufficient root system to support itself and in no need to grow more roots than it is already have.
Third, if you are building raised bed around maples, leave a circle with a radius of 2-3' around the trunk 'as is' (not raised, not filled) in order not to smother the trunk.
Fourth, plant shallow rooted or adaptable shrubs in a fall, as most of the roots growth occures in a cold weather. Try to chose low feeder plants that in ordinary beds wouldn't need fertilization. Plant as large shrubs as you can afford. Your new plants will have about 2 years of no or minimal competition from the maple. By that time they will be established enough to sustain on its own or with some help from you in form of ocassional feeding.
Fifth, be aware, that doesn't matter how established your plants will be, you'll be obligated to water them forewer as if they would be planted in a full sun. Maple will absorb only the certain amount of water it needs for itself thus your plants will not be thirsty if you proved adequate watering.
I followed his directions to the tee and that how the portion of the bed/border that was built around 3 mature maples look three years after, as of today: