Posted: 19/08/2015 at 11:13
Definitely lack of moisture. Plants in pots are entirely dependent in you for food and water. Compost nutrients are all used up after 100 days so you need to give a top dressing or a well balance fertiliser such as blood, fish and bone or pelleted chicken manure every spring and keep it watered. It is better to give one thorough soaking a week than daily dribbles. Rainfall will never be enough whilst it is in leaf.
Keep it sheltered from heavy frosts once it goes dormant as this too can dry out the end of the stems and make for unsightly dead bits. Remove these in spring, cutting back to just above a healthy bud and then keep it moist till leaf fall.
If the pot is small, you can also re-pot in spring to a larger pot and using good John Innes no 3 compost.