How do I select a planting hidden under another one?

Right-click the overlapping bars, no need to move anything out of the way. Under Select Planting, pick the one you want: it comes to the front for a few seconds, long enough to drag it to another bed or open it.

The same menu has Unassign Planting to take a planting off its bed; it stays in your Game Plan.

The steps are in Move, edit or unassign a planting.