Wood preservative for a planter

[Tail End Charlie]

At the very least, keep the base of the planter off the surface that it sits on.

Absolutely. I use rubber feet for chairs, which raise up the planter by about 2cm, so it is not sitting in water.
@Chief Broom if you have an old pine bunk bed, presumably there are some thicker pieces which were the bed posts. I've no idea of your general handiness but these would make good corner posts for your planter with the slats filling in in between and I'd just clad a large square black plastic pot in the size you want. You can paint or stain with whatever you like (cheap - Screwfix water seal, pricy Osmo outdoor UV), but because the wood isn't in direct contact with the compost, it'll last a while.

 

[Gillstay]

Use the right wood. Larch will last fifteen years untreated.
Sweet chestnut will last thirty.
Slapping chemicals on wood to preserve it is just slapping chemicals around.

Correct, saved me saying. Just like fence posts. Shame there is less Elm about.

 

[Chief Broom]

Absolutely. I use rubber feet for chairs, which raise up the planter by about 2cm, so it is not sitting in water.
@Chief Broom if you have an old pine bunk bed, presumably there are some thicker pieces which were the bed posts. I've no idea of your general handiness but these would make good corner posts for your planter with the slats filling in in between and I'd just clad a large square black plastic pot in the size you want. You can paint or stain with whatever you like (cheap - Screwfix water seal, pricy Osmo outdoor UV), but because the wood isn't in direct contact with the compost, it'll last a while.

Good idea and thats what i was intending [corner posts] i shant be using a pot inside the planter though, i need an extra large planter with huge volume and depth for a climbing rose and ive never seen it done before. Potted miniature climbing roses maybe but not full size climbers. Really big plastic pots are about 60cms or so, i want even larger . The reason for this exercise is that i have a south facing wall crying out for a climber and the soil just isnt there....its foundations, rock and rumble....Going to give it a go