Thank you for the comments.

It's the trunk of a tree.  The log is sitting on 2 limbs from the same tree.

I ground it smooth with a 9" grinder, then sanded it with 60 grit using an orbital sander and then went back over it with 150 grit using another orbital sander.

The planter boxes are 24" long and about 7" wide.  I chopped out the first opening with a circular saw and some chisels.  That took way too long and too much effort.  So I went back to my chain saw and carved out the rough openings with that and finished with a chisel.

Then I stained it with a semi-transparent stain and applied 2 coats of marine polyurethane.   I had it on saw horses and had to pick it up and turn it by hand.  I could only turn it about and sixth of a turn at a time and that was the most work, the thing weighs over 400 pounds.