I just have never been able to grow a clematis in a large pot
the roots get too hot for all plants here in lower zone 8
in alabama
I wish you have a raised area and half day sun to plant your clematis..
Mine are in half day sun...morning and have done well ..
I bought soil and mixed in a sandy soil with it for their holes
and the holes were 2 foot wide and 2 foot deep...
They liked to be watered with the water hose deeply
every 10 days here ...April through november...
I always just put my finger beside a plant or take
a spade and go down 2 inches...if it is dry then
I need to water it one inch deep or 2 inches down
with a bucket of water slowly...
never water at night or over head sprinklers
any perennial.. foliage ...
that was the hardest part when I was younger and learning
was when to water or not water
Here are my trellis to show you where they are planted
in half day sunshine and not in a low boggy area...
Good luck with your clematis..
Sheri in Alabama