Hi Don
You didn't say how big the planter would be but I would suggest using one of the new deck stock boards(plastic man made boards) they are safe and cut just like real wood.
You can get it in 1x or 2x or 4x thick stock in many colors and it's great for planters, you can put the bear stock on the inside of the planter and not give it a 2nd. thought because it's plastic.
I would use the log house way, that's to say a lock slot on the bottom edge and slot on then top edge of the boards with some Phil..flat hd. stainless screws now and then to hold it as one. ( with out any sharp points hanging out anywhere just a nice sq.cut with a bit of a round over on the ends.)
If you don't want to use screws you can use 3/8" dowells and drive them in from the top as you go to lock it all in and keep the boards sq. (but drill the holes with a 1/2" bit) You can get them 12" long if you need one.
If you like the cedar look, you can buy paint that looks just like it but it's a once a year job to repaint it on the out side of the plater.
Just my cents, hope this helps and pls. post a picture after you have it up.
Bj