The leaves look like those of a H. macrophylla (mophead, lacecap), A picture of blooms would help confirm it.

Luis

PS - the leaves look like they need iron. Amend the soil with either greensand, garden sulphur, aluminum sulfate or iron sulfate.

I have a several different hydrangea plants, some I would like to prune. Some are finishing their bloom right now. How do I know when to prune them? Is it that you generally prune them after they finish blooming? How do I know which are smooth hydrangeas, I would like to prune them down to increase their bloom size?

Hydrangea macrophylla. Prune at end of winter. However, if the cultivar you have planted is not budding for you there, cutting it back much will only make it certain you don't see any flowers.

Yellowing could just as well be something else, such as a nitrogen deficiency.

H. arborescens has different leaves.

The one pictured, I don't plan to prune, possibly after it blooms next year. I have some Anabelle that I will prune back to about 2 feet tall soon. But I have some much bigger plants, and I think my best option is to identify them first, then decide when to prune, and what. I was hoping to just have a general idea, but it seems there is too much variation. I also have oakleaf hydrangeas.

H. macrophylla develops invisible flower buds in July-August (late August maybe for you in Ontario) and these then open in the Spring. To get reliable bloomage in your zone, consider winter protecting.

In general, hydrangeas do not require pruning unless you have a problem such as it has been planted in a location where it grows too large and encroaches on other plants or walkways. However, you can deadhead any time after the blooms have gone brown or wait until the stems leaf out. Should you want to prune any of the hydrangeas, it is typically safe to do that several weeks after they have bloomed. Specifically, h. macrophyllas, oakleaves, climbing hydrangea and serratas are all tuned to produce invisible flower buds in July-August so, should you need to prune them for some reason, do it after they bloom but before the end of June-ish. Winter protect H. macrophylla to get blooms reliably.

Arborescens and paniculatas develop flower buds in late Spring or early summer. They can be pruned just about any time but I would be careful to prune when they are about to bloom in your area as you could accidentally cut off this season's blooms if you prune in the summer (but you could prune them when they go dormant or in the Spring before leaf out). Then I always go hunting around the Arborescens to see if they have some little plants growing away from the main shrub. Arbs can get out of control size-wise if I do not do this with some regularity.

Thus, I would not now prune the one in the picture, the oakleaf hydrangeas or any serrata or climbing hydrangeas (if you have any of those). If you must, wait after they bloom, enjoy the blooms for a little while and then prune by the end of June or thereabouts. This way, you will not cut off the 2017 flower buds that will be developed in July-August-ish of 2017 and which will open in Spring 2018.

Arborescens and paniculatas can be pruned at almost any time. Since they produce flower buds in late Spring 2017 to early-mid Summer 2017, it may be best to prune them as late as the mid Spring 2017 or thereabouts.