300.5 says 24" for direct burial cable/conductors unless this is a residential, single circuit, GFCI at the source end, 120v, 20a max, which can be 12"
That loophole also eliminates the need for a ground rod.
Anything else puts you into 225 part II and 250.32 along with the deeper 300.5 requirements.
They put the single residential circuit loophole in there to compromise a safer alternative to running an orange cord through the trees or along the ground.
If digging is hard there, RNC at 18" becomes a very attractive option. Personally I would go that way anyhow.