Over /under Watering Hydrangeas

[wired]

I have 2 hydrangeas ,in large pots, some of the leaves are turning yellow so I suspect a watering problem, does anyone know of a way to check this please?
thanks, wired

[Chipchopper]

When you water the pots (moderately) the water should be seen to drain down fairly quickly, if thats not the case then they could be sitting in an unsuitable boggy condition.

do the pots have crocks in the botton to aidd good drainage ?

[wired]

Thanks Chips, yes there are crocks and added sand. Our "soil" is heavy clay which is unkind to most plants, despite my efforts to improve it, that is why I'm using pots- will watch when I water.Probably over watering as the garden dries out so quickly.
wired

[hammerman]

Might be worth giving them a feed with miracle grow or something every week until september. Hydrangeas prefer a moist soil and hate drying out.