Here's an article that might help you - http://www.theiowagardener.com/Hydrangeas%20for%20Iowa.html
Old gardening rule"don't plant til after Mothers Day" maybe wait til end of month,especially where you live.Hopefully you've chosen one of these because some will not grow where you live https://hortnews.extension.iastate.edu/2018/08/hardy-hydrangeas-iowa
I don't plant until the soil reaches about fifty degrees unless is is a really cool weather tolerant plant. This allows the roots to not get too cold and acclimate themselves sooner.
I always plant after the danger of frost is over, usually Mother's Day weekend (although you can do the week before if the weather forecast says it'll be warm). I'm in St. Louis, so probably the same gardening zone as you.