I Can't use PVC where I'm at, (Galvy Only!)but out of town in other cities in my area... Direct burial romex, sleaved with PVC, or just PVC all the way. Then, I dig, ( Or, should I say, have some of the generals guys dig. )a post hole at each light location. Mount my box to a pressure treated 2x4, with a 45 at the top. Looks nice, like it was supposed to be there! Then have the general fill the post hole with concrete, around the 2x4, and the sleeve or conduit. Once the concrete is dry, it is a solid unit! Sounds like a bit too much, but practically in-destructable. You have to run over it with a truck!
But, to answer your question. You still need to support the box, and it depends on what is allowed in your area. Call your Inspector.
"Well - I oughta....." -Jackie Gleason