I would suggest using UF type wire AND putting it in some
PVC pipe, probably 3/4 dia, so that if you should dig
into it while planting a shrub someday, you will hit the PVC
and avoid nicking the UF wire, regardless of the depth....
You don't have to make joints and connectors with the
PVC since it is being used only as an open conduit
and that makes it cheap and easy. The UF will be
resistand to anything that is going to happen...

Just a suggestion, which works well for me, who is
always digging around with a shovel somewhere....