When the temperatures rise into the 80's plants require extra water because the ground moisture evaporates quickly. Any type of flowering plant will quickly begin showing signs of stress in very warm conditions. If there is little to no rain, extra water is necessary to prevent the plantings from wilting. Specifically flowering plants like Viburnum, Hydrangea, Azalea, Spirea, and Lilac are the least forgiving in drought conditions. When temperatures start flirting with 90 degrees plant stress can be 3 times higher than 10 degrees less because ground evaporation multiplies as heat increases.
The simple solution to a hot, dry spell is to add an extra cycle to your daily watering schedule. Remember when temperatures cool down to cancel that extra cycle to avoid over watering.