You probably have bigleaf hydrangeas (H. macrophylla) and these are vulnerable to winter cold. Typically, the recommendation is to wrap or otherwise protect them for winter in any zones colder than a USDA 7. In a zone equivalent to a US 5, you are pretty much guaranteed to get complete dieback in winter (although usually root hardy) and therefore no flowers.

Light frost will not hurt the current foliage. A hard freeze could do damage.

I'm in zone 5 and if I want blooms, I make circles around the hydrangeas out of green plastic fencing, then stuff them with chopped leaves. Mine are still covered, since the temps will be down in the 20's tonight and for a few more days.

Temps in the mid-high 20s spell danger. The stems leaves will be green in the morning but frozen solid. Soon as they thaw, they will turn brown and die. Lows around 30F are acceptable for the most part.

Ok thanks folks. Too late for this year then. They are all dead. I will have to cover them and stuff them with leaves or such next year. Appreciate the feedback:)

If you add more macs, here is a thought: a rebloomer macrophylla (like the ones in the Forever & Ever Series, Let's Dance Series, etc) would still loose the old wood flower buds but it would develop new buds later on during the current growing season. It would probably produce blooms later than if the flower buds had survived but you would still get some bloomage. Paniculatas, hardy to US Zone 3, would be best since they produce blooms on new wood. Hydrangea arborescens (like Annabelle) is also hardy to US Zone 4.

Thanks Luis. I believe 2 are of the endless summer series and are supposed to bloom on new stems but they didn't last year. They leafed out but no blooms. The other hydrangeas were there when I bought the house so not sure what they are exactly but I had thought they might be Annabelles. Perhaps if I can did up a few pictures from when they were in bloom I will post them.

Aaah, I see online they actually look a lot more like the macrophylla than the annabelle.

I did not mention Endless Summers Series because a lot of people in zones like yours have blooming problems. My first and only ES was a disaster. I killed the plant because the drip irrigation was watering away from the plant and it dried out when I left it unattended. I remember buying it when they were asking a lot of money for it. It was present so I took it and never replaced it. Now I would like to try one of those Forever Series plants that has double blooms. But there is no shaded space so, oh well. The panicle shaped blooms are either oakleaf hydrangeas or paniculatas. The round ones are usually macrophyllas (specially if they are pink/purple/blue). Annabelle blooms look like white semi-round macrophylla blooms although some of Anna's cousin plants come in pink (only) now.

Good to know Luis. Thanks so much for the information. I may have to rethink what I have in the garden now for Hydrangeas. I was particularly disappointed in the Wedding Gown Hydrangeas I put in last summer. They also lost their buds.

They were all doing so well the first five years in my house. Not so nice now with no flowers:( I'll have a look online at your suggestions. Good to know about the issues with ES. And yes that makes sense a lot of my Hydrangeas are likely macrophyllas then (not reblooming types though). They have that typical pink/blue thing going on depending on PH. I'll check out the Forever Series and the other lower zone tolerant varieties you mentioned.

The Star Gazer (macrophylla 'Kompeito') looks fantastic. It says it's good for zone 4. Might be worth trying. Says it's from the Double Delights series.

Check out 'Hydrangeas in the North' by Tim Boebel. The book is a good read but there's also good info on the web-site. Interesting stuff for us in zone 7 and above. I'm testing his techniques. I took my mulch off last weekend and just got in from throwing blankets and sheets on my 6 macrophyllas since frost is expected tonight.

I also live in Toronto - similar story. In the last 2 years I only had single bloom on each of my macrophyllas. Last fall I was determined that I will try my best - put a lot of pine and spruce boroughs and landscaping fabric around them as well. One was showing leaves with that heat weave a week ago, another was in deep shade and still brown. I had to put a big garden can over one with leaves in the last 2 nights and did nothing for one without leaves - see how it goes. Dropped a couple of buds in the process - alas. If I am not getting nice blooms this year I will consider replacing it with something else. Anabelle is fine, but it starting to be like a weed - too much suckers for a proper plant.