Would allow a plant (at any stage) to be transferred to another empty tile in the garden.
The cost could be one tick for every plant to be transferred, effectively pausing that plant's growth for one tick.
I don't know how to handle this with plants that are within one tick of decay. Maybe this would be an exception to the above rule: plants within one tick of decay are non-transferrable. I don't know how this would be implemented either - maybe shft+click or ctrl+click?