it depends how big you want them.
think more, they should be done around october/november.. so plant accordingly depending on what size you want them to be then.
i put mine out last year late may, but am holding off till probably around the end of june this season (i had way more than i needed for my personal stash last year
i also agree that you'd probably fare better stashing them amongst the local foliage than trying to do them in the back of a semi trailer (or at the very least, keep blocking them up so the tops are consistently level with the top of the trailer so they get some light. cause as mentioned, unless the sun is directly over them, the walls will cast shadows)