Clermatis "Victoria"
 

I planted a Clematis "Victoria" on a south facing wall this spring. The wall
is a bit of a sun trap (that is, on the odd occasion when the sun shines).
It has grown very well and has flowered profusely. However, when the weather
is hot and sunny it droops very badly. However, when the sun moves off the
wall (about 4pm) it immediatley picks up and looks fine. Today it looked as
if it had wilted and looked quite pathetic.

It is kept well watered and the roots are protected with a large stone.

Is this normal or is there anything I can do to prevent this droop?