Any decent potting mix will work but I tend to shy away from peat based ones like Miracle Gro, as they collapse easily and rapidly and tend to hold too much water. And just a sufficient amount of drainage holes in the bottom of the barrel. Then just fill with the potting soil - adding rocks or gravel at the base actually slows good drainage rather than assists with it.

And a happy, well-cared for clematis will easily fill that size container with its roots!

Ok thank you but I think I'll have to put something in the bottom for drainage as it will be sitting on concrete.

All wooden wine or whiskey barrels have a sight lip. They do not sit directly flush to the ground. If anything, you can raise slightly by sitting them on bricks or flagstone.

But adding something to the bottom of the pot to "improve drainage" is a myth. It doesn't :-) And generally creates the opposite situation.

Ok! I'll place it on stones! I've got some

One more ? Could I use some of my home made compost in it I understand not to use it all for the barrel. Thank you!

Personally I wouldn't. And I do not use any with my containerized clematis (or any other containerized plants). Using compost in a potting soil mix generally creates the same unfavorable growing condtions (and promotes root rots) in the same manner as excessive peat does in a potting mix.

Ok Thanks.