The Magic Sapling is the only item being plantable, and after you've planted it, you must wait for a while before you can check and see how much harvest you've got. Do note that when harvesting (or checking that is), the sapling is consumed. Therefore waiting for a while before checking it is critical to get more harvest.
#!plant set magic sapling
If using Dynobot, I suggest to set a reminder somewhere in the future to remind you to check the plant, like in
?remindme Do #!plant check in 4 hours or however long you'll want to wait.