Growing clematis in a pot
I have a pergola in a corner of the garden, but when it was put in. they didn't leave any open ground around the posts, so there is nothing to plant into.
I am thinking of planting clematis in a couple of pots at the base of the posts so it can climb up and over the pergola, but would like an idea of how large the pots should be to enable good root growth and ensure they don't dry out too quickly, and also not require regular repotting. Should I go for John Innes Number 3 for the compost?