The flowers on the Clethra around here only began opening about two weeks ago, a couple of weeks later than usual. But it is always worth the wait. The scent is floral without being cloying, and the blooms attract pollinators of all kinds from miles around. (Have you ever noticed how butterflies seem to prefer clusters of flowers? Is it because they can get nectar from many blooms without having to fly to another plant?)
This deciduous shrub forms large clumps, and likes moist soil (think bogs and riverbanks) when it is getting established. Mature shrubs seem to do just fine in dry conditions, though. It is hardy in Zones 3 to 9, and can be aggressive as it spreads.
I encounter it sometimes when I am hiking, and at this time of year, I notice the scent first – very welcome when many flowering shrubs have ended their bloom time.