Clematis like good drainage and the roots must be kept cool and moist during the hot weather. Quite often Clematis are killed by over-watering or bad drainage. If they are planted in a large pot be sure there is good drainage. Each year check the bottom hole of the pot to make sure that the hole is clear of dirt and rubbish.
During extreme hot dry conditions, Clematis will require continual observing as they must never dry out. During very hot dry conditions it may be necessary to water twice a day with 5 litres of water once they are established. Under normal summer conditions give a good deep soaking 2 or 3 times a week.
Around the beginning of June add a generous amount of slow release or timed release fertilizer and let mother nature do the rest! Keep in mind that your clematis will need regular watering the first year after planting, give each plant at least 3 gallons of water once a week, or more depending on the soil and weather conditions. Deep watering in this manner will make the roots grow deep down rather than staying close to the surface, where they could dry out easily.