In your zone, hydrangeas need a little more protection from the afternoon sun, such as shade starting around lunch time or close to 12pm. Less sun may help the hydrangeas wilt less although they tend to wilt often, especially on their first year and usually during summer days and/or on windy days.

I would change the watering schedule as it sounds like too much. Try the finger method to help you out: insert a finger to a depth of 4" and water only if the soil feels almost dry or dry. Test the soil daily for two weeks, hopefully at abouty the same time every day. If you need to water, add 1 gallon of water and then write down a note on your wall calendar indicating that you had to water on that day. After two weeks, go back to the calendar and determine how often you had to water (every 3-4-5-etc days). Then set the sprinkler or drip irrigation to water 1 gallon of water on the same frequency. If you allow the soil to get wet for too long, the roots may rot.

To reduce the frequency of your waterings and to make the soil stay moist longer, add more mulch than what you have. Something like 3-4" up to drip line; past the drip line if it is windy.

Stems on new plants sometimes may not be able to support the weight of dry/wet blooms. If you leave the stems alone (ie, do not prune them), in time they will get older and able to hold the weight better. But there are some varieties that always will have that problem (Annabelle for example is famous for having problems when the blooms get wet).

Thanks! The more compact ones aren't having problems staying upright, so it sounds like it's just a topheavy issue. The wilting I had a better idea how to deal with :) I'll probably end up building a pergola or maybe a temporary canopy over where the two in the back yard are planted, as they get full sun for the middle of the day and afternoon. Our yard is fairly bare right now.