From: paghat@netscapeSPAM-ME-NOT.

This means mainly that you have to insert small
starts of things & not try to plant, say, a gallon-pot of epimediums
amidst roots. But a three-inch pot can be inserted safely, with care, & as
the underplanting grows, the roots will sort themselves out safely.

This is very good advice. I have a red maple under which the ground is like
rock; there's no digging around under there. But I have successfully inserted
tiny bits of hostas at the base of the trunk. Not much to look at at first, but
now three years later they have filled in nicely. I've done the same with tiny
fringed bleeding heart seedlings under my rhododendron. They have grown in
nice and full and no more digging around every year shoving annuals anymore.

~flick
L.I. zone 6