How long have they been in the ground? Hydrangeas do not wilt in the heat here, it's over 90 every day for months.
Looking at weather.com for Springfield, MO, looks like there's already more rain than in all of June, on average. No plants in ground since last year should need supplemental watering in those conditions. If the spot is muddy, not well-drained, the roots could be rotting/suffocating, like an 'overwatered' potted plant.