Bit to use on landscaping timbers? 30 1/2" holes.

I've used both 3/8" long length twist bits and 3/8" X 18" augers with a 1/2" corded drill. I pre-drill the holes for 12" galvanized spikes for 6" x 8" timbers and nail them in with a 3 lb sledge. After a while it got tiresome, but it helped the spikes go in much easier.
Like they say about clamps, "you can't have too many kinds of drill bits"

What he said ^.
I use a "dead man" about 3 ft or 4 ft long at right angles to the wall every 8 ft or so, depending.
It gets nailed into the wall making an 8" space when laying the others up tight to it, so I need to account for that added dimension.