"Hard mud"sounds awful. It sounds like that would drown the roots, cut of any air from reaching them. Drown, then die, then rot. Not good. The roots need to be in soil that drains, so water can flow down and out. Then air can flow in. Roots need to breath.
One way to get drainage is to RAISE the root ball. Make a raised bed with some kind of walls (or even just mounded up soil). The soil in the bed should be somewhat loose, at least enough to drain and be aerated. For example, add compost and something coarse and gritty, to improve drainage, if you have clay now.
Another way, if you have some slope to work with, is to LOWER the "water table", or at least lower the level that water can rise to after a heavy rain.
Approach the bed from BELOW it, making a slit trench between your bed and some lower part of the yard. The floor of that trench has to go in a little lower than the floor of your bed. Now water can drain from the bed into the trench, until the water level in the bed is as low as the floor of the trench. That lets air into the bed.
