Is 12" deep enough for roses? :(

If you really wanted to plant a rose or two in the planters, and providing the planters will be a permanent fixture, then I'd either cut or knock out the base, rough up the base soil, then give the soil a good supply of fish, blood and bone. Fill the planter with compost, then plant the rose. Its roots can then go as deep as they like.

If the planters are not in a permanent place, why not put them on casters so you can move them about with relative ease. Planting them up with suggestions for plants as above.

Another thought, maybe plant the rose in a extra large and adequate pot and sink it in the planter as far as you can. No doubt the pot will be a bit proud off the planter, you could make a feature of it, or even hide it, by growing plants around the pot.

 

Planters in deep ugly horrible gravel Jenny, so permanent fixtures and canny use castors. Good ideas for roses thanks!