I’ve recently moved into a new house that has a beautiful perennial garden. I’ve managed to get advice about most of the plants from friends and neighbours but hoping for help identifying what type of Hydrangea this is and when it’s best to prune.
Going by the leaf shape, size & margin and the flower shape, I think your shrub looks like a Wild Hydrangea, Hydrangea arborescens. They are native in the USA as far north as New York State and are an introduced species in Canada.
Wild Hydrangeas, also known as Smooth Hydrangea, bloom from June until August. If your flowers start green then turn white, I think we’re on the right track. These shrubs can grow to six feet tall, and sucker freely in the wild. They have fine stems, which look like the ones in your photo, so tend to die back each winter. In Canada they are treated like herbaceous perennials, and cut back to the ground every winter. This encourages strong stem growth in the spring, and ultimately, more flowers in summer.
Hydrangea arborescens occur near streams and in woodlands in its native habitat so it prefers well drained, moist, nutrient rich soils and dappled shade to full sun. It can get scorched if it is in an exposed location during hot weather.
Like all summer flowering hydrangeas, Wild Hydrangeas are pruned in late fall, when the plant becomes dormant. You can trim out dead or broken branches at any time, but if you prune to shape your shrub before winter, you could be removing flower buds and significantly reducing the chances of flowers this season.
See the attached link for further reading: http://www.wildflower.org/plants/result.php?id_plant=HYAR