Thanks, Kate! The earliest blooming panicle hydrangeas are those in the Quick Fire series, often by the 4th of July here in Michigan. Incrediball and Invincibelle smooth hydrangeas are also usually starting to bloom by early July. Though the bloom times of all hydrangeas do vary a bit based on climate, opening earlier in warmer climates and later in cooler areas, I don’t think that any hydrangeas will be in bloom as early as May in St. Louis. Rather, I think what you are seeing are viburnums, and specifically, snowball viburnums, aka snowball bush. Though they are not botanically related to hydrangeas, they do look similar. For example, viburnum flowers can be mophead or lacecap, and their sterile florets look similar to hydrangea sterile florets. Plus, their foliage is arranged opposite like hydrangeas. Here’s a nice look at these spring-blooming viburnums; hopefully you have room for them in your yard as well as hydrangeas!