I see an obvious drawback here - you can use tanalized (these days they call it pressure treated) wood to clad, but unfortunately, you'll find the containers rot from the inside out if you did not use pressure treated wood to construct them with. Marine ply is probably the most water resistant, but is very expensive.

Have you considered uPVC cladding? It comes in various shades of wood effects... No need to paint it and it will last for almost forever.

My wooden planters did exactly as Bamboo is describing - they still look reasonably good from the outside but the insides are crumbling away. As I found out only recently when I tried to move one! The base and bottom few inches has almost rotted through. I had introduced polythene liners at time of planting but the moisture soon got through that it seems.
It might be worth looking at it from a different angle and have some kind of decorative front/sides that won't rot and could be removed as and when the wood does eventually rot.

What are the beds actually made of Jimbob - are they wooden or plastic? If they are plastic I wonder whether something like the boards used for laminated flooring would appeal to you? Probably a bit too shiny but they wouldn't rot.

Can I ask what you made these frames out of and are you going to put the cladding on the outside of the posts with a 3" gap between your frame and the raised bed. You could put a topping on over the gap, which would keep the outside a bit drier. OH suggests you line the insides of your boxes before you fill with soil. Use the kind of sheet used as a moisture barrier when new roads are being laid or new homes are being built. It will prolong the life of them. We made our raised beds from Patio boards which as Bamboo says is tanalized. I still coat them on the outside with oil each year but I suspect they will eventually rot away. We will look forward to seeing them planted up. Just noticed you only joined goy recently so welcome to the site.

The plastic sheeting is called DPM (Damp Proof Membrane), it's usually a blue colour, cheap to buy and is ideal for lining planters.

If you put it across the base as well you will lose the benefit of larger climbers being able to root into the ground though, so do decide what your final plan is before making any decisions that are difficult to undo. And if you do put membrane over the whole area make sure you put some drainage holes in it.

You are correct Steragram. Sorry I did not make it clear that only the sides of your boxes should be lined. Thank you Myron that info is very helpful.

Be careful that soil doesn't seep out through the gap under the boarding or you could bridge the damp-course of your house and cause problems. You'll also need to regularly check the space between the planters and the wall for debris such as fallen leaves.

Thanks for the comments.

The structure is made from 3" posts which are treated and concreted into the ground then painted with bitumen. The frame is made from scaffold planks which have also been painted in bitumen (pre-build). I then painted the inside which will hold the soil again with bitumen. All the joints, corners and screw heads have been tape over with denso tape (grease water proofing tape). I then plan to line the inside walls with a damp proof course liner as suggested in previous comments. Hopefully this will hold of the wood rotting for as long as possible. (See new photos).

The wood cladding will sit on the front side of the posts, 3 inches from the inner and there will also be a top to bridge the gap. I'm not sure what type of wood to use for the cladding to give maximum.

The wall in the picture is a garden wall so there will be no issue of bridging the damp course.

Thank you for that information. We used patio decking boards from Wickes. We needed quite a lot. They have two different facing options. The boards were kiln dried to stop them buckling and tanalised. We shopped around and found Wickes our cheapest source The same boards could be used as a top.

Those deck boards are pretty tough - therefore I think Scotsgran has given you the best advice.
I've had a few lengths of deck boards sitting in water, for weeks on end, down the side of the shed for around 6 years and other than dirt, they are still sound.
It will be a matter of taste whether you treat them or not but I could suggest not to use the deck paint stuff they sell in the DIY stores. I've just given a friend a borrow of my pressure washer to take it off his deck as it started peeling after 1 year. Cuprinol garden shades isn't much cope either. I spent a fortune doing my shed etc giving them all 3 coats and it didn't even last out the year! I'll be getting the pressure washer out for that job too!
I think the suggestion of oil by Scotsgran would the better solution.

Deck boards are also pressure treated so they will last a very long time. I built an arbour with seating 10 years ago using pressure treated timber. It's stood up to the weather well, I haven't painted or treated it in any way and the wood is as good as it was when I built it.

I don't treat my deck - I clean it twice a year with an appropriate cleaner.