Like most plants, clematis like to have even soil moisture without periods of excessive dryness or periods being waterlogged thru poor drainage. Clematis have proved themselves very adaptable in the garden, but being mindful of watering helps the plants establish and flourish.
Relying on mother nature can have its pitfalls as well, as some areas of the garden may not receive their fare share of rain due to tree branches or other vegetation blocking rain fall, or just the variance in how rain falls. Many gardeners use a simple rain gauge and check it after rain fall to see just how much rain the plants in their garden have gotten. Generally speaking, if the gauge has less than a half inch, the rain fall does not replace regular watering. Clematis that are well planted and receive an inch or more of water a week usually do well.
If you are making up the shortfall from rain, remember when daytime temps are in the low 70’s twice a week saturating the ground around the clematis is usually enough. When daytime temps increase to the high 70’s into the 80’s every other day watering is beneficial. Your good judgment as a gardener and knowledge of your climate is your best guide. When In doubt, a simple moisture meter can be used to see if your clematis plants are receiving adequate hydration.
Continue good watering practices throughout the growing season. Many times in late summers to early fall, we neglect the plants and this can cause early dormancy to onset and will deprive you of the long season that healthy clematis can provide