I guess you could but I would not. I think you would be better off using finger joint pine 3/4"x4 9/16".
there are the same quantities of both in 3 nearby stores I checked.
ONLY when the radiator is NOT in use and has NOT been used or expected to be used !
only if You want it for a year or less . if You paint it or pressure treat the wood it may well kill the plants ! IF I were building such a project I would use cedar wood !!!! cost more at first and would last many more years !
there are pieces that are pre-cut that may be 1" x 4" (or what ever the tag would say) , sometimes it is 3/4' x 3 1/2 ". when a piece tag would say 3/4" x 3 1/2' you could fairly much take that for the truth . what I do is keep a small ruler in my pocket and always measure what I am buying !! unless you go to a lumber yard which cuts boards from logs then you can order the "finished" boards to what you need, this does cost a bit more and takes some time due to the drying time ! if you think that is worth it good luck finding a true lumber yard !