"What are we doing wrong?"

Maybe nothing :-) And no, they are NOT biennial.

By your description, I assume these are macrophyllas or bigleaf hydrangeas. The majority of these are old wood bloomers, with an increasing variety of those that have an ability to bloom on new wood/growth as well. But regardless if only old growth or both new and old growth flowering, they are sensitive to winter cold damage and even more so to late winter/early spring temperature swings that can lead to to cold damage. Especially in USDA zones 7 and below, which seem to produce the late season fluctuations in temperatures that most often affect the flowering of these shrubs.

Really the only way around this is to provide winter protection. And to keep any pruning to a minimum - deadheading only and the removal of dead wood after the rest of the plant has fully leafed out.

dead wood is dead .. it should simply be removed ... especially if after the first cut you observe no green layer which would indicate it MIGHT grow ... i think this is a false flag .. meaning.. you are focused on the wrong thing ....

where are you .. big city name... do you get snow cover.. ever notice.. which you dont mention.. they usually die back to snow cover.. meaning the part above is not winter hardy in your area ...

if no snow cover.. and they die back.. well .. then gal is spot on ... and they need protection ...

a pile of chopped up dry fall leaves.. applied late in fall.. might do the trick.. many of us in z5 and colder ... hold bags of such until late fall.. to bury things like this.. that need a little extra help ... putting it on too early creates its own problems .... you want to do it AFTER they are dormant .. not prolong active growth too late ....

lastly.. a pic or two.. will get you the full latin name... so you can figure it out... hydrangea alone is too broad.. so you need to hone in on researching your actual plant ...

ken

Thank you for this advice! I'm in Canada in Zone 5 and I normally covered it with a pot over the winter, but I'll look to see what else I can do to protect it. The way it was blooming perfectly every other year had me convinced something was going on.

How far would you suggest cutting down on the dead wood for next year? And should it be done this fall, or wait till next year's spring?

Even reblooming macs will not bloom reliably in a zone 5 without winter protection. And reread my post......bare stems (without leaves) should remain in place until the rest of the shrub leafs out and you are certain that the bare stems are indeed dead. Generally, that would be around the beginning of May. And cut these back to the ground.

Ditch the mopheads (H. macrophylla) and instead get "Annabelle" (H. arborescens) --> they bloom exclusively on new wood and can be completely winter-killed above ground will still bloom beautifully reliably. Problem solved.

Blooming of mopheads is VERY iffy in zone 5. Before the climate got warmer, they were total failures in zone 5.

Yes, laceyvail, my mother was an avid gardener in Wisconsin for decades and then in her 60s moved to western NC. The first word out of her mouth when she closed on her new home: HYDRANGEAS!

I agree with mxk3. The Annabelle (or Smooth Hydrangea) is native to your region and needs no winter protection. It blooms all summer if you deadhead all spent flowers, and there are large varieties as well as mini ones. I have one of the smaller ones in my garden in NW Ohio and it does amazing and I don’t have to do a thing to it other than deadhead to get new flowers.

Hydrangea arborescens/Smooth Hydrangea is not native to Canada, although it can certainly grow well there. Southern New York is the northern extent of its wild distribution.

It‘s also important to note that “Annabelle” is not another name for the species, but rather, a cultivar. Although ‘Annabelle’ is one of the most popular and widely planted cultivars of H. arborescens, there are many others to choose from.

Personally, I’m a big fan of the lacecap varieties like ‘Haas Halo‘and ‘White Dome’, which mimic the look of the wild type, rather than the huge ”snowball” effect of ‘Annabelle.’ The lacecaps also have the added benefit of attracting numerous pollinators, whereas the sterile (but showy) flower heads of ‘Annabelle’ do not.

if they should grow in full sun you could try Hydrangea paniculata (same growth habit)

H arborescent does quite well near/ under trees competing with their roots,

paniculata hates that (near trees), flowers are visited by a number of hoverflies and others