Why has my hydrangea stopped flowering?

It'll be the cutting back that's doing it - hydrangeas need to be pruned very lightly, literally just removing old flowerheads and any obviously dead wood, and tipping the tops back to where leaf growth is appearing, and this is done, here in England, in April and not before. If you're cutting off most of the topgrowth at the start of your winter, you're losing the flowering growths every time. I realise your winters there are very cold compared to ours, but would it not be possible to leave it in a large pot and just move it into a bright, coolish spot inside? Or erect some kind of shelter over it outside in a sheltered spot, wrapping the pot in bubble wrap or similar to stop the roots freezing?

Thanks for this - I think outside would be a problem as it can drop to minus 30 here - would even a wrapped plant survive this? But a light spot indoors might be ok - should it be watered throughout the winter then? And I take it not cut back till I return here at the end of April?

This happened to me until I put them in ericus compost and left them on the patio with the seed heads left on for protection. Now I do not touch them and just tidy in spring as required.

the important part is the roots, these need to be frost proof as it will penetrate the pot, and kill the roots as to pruning you don't really prune all over , if its too big reduce whole stems by a third annually if nessesary, if you do need to cut back again just a few at a time keeping the plant balanced-- hope this helps

Yes, worth a try - though hope its cool inside, needs to be no more than 55 to 60 degrees F, or 0-10 degC, if possible. Watering should be kept to a minimum, just enough to ensure that the compost isn't too dry - hopefully, very cool conditions and a little water might be enough to persuade it that its winter so that it drops its leaves and has a rest.

just thought -- if you are lucky and get flower heads don't cut them off as the old heads protect the new buds from frost and damage

The temp inside is 11C, lowest I can leave it....

I really don't think I can leave it outside unless it's actually in the ground. I did however bubble wrap the lavender pot last year and it was ok.....

You might get away with 11C - leave it near a window or door so its cooler (if you don't have double glazing there!). Have checked my container gardening book - hydrangea macrophylla will grow outside in a pot all year in Zones 6-9 - think you're in Czech republic and thats mostly Zone 6 with an area in Zone 4 (even colder, though my book is old and still shows it as Czechoslavakia). England is Zone 8. Lavender will tolerate (in a pot) temps in Zone 5, so seems a bit hardier anyway.

I'm afraid we have excellent double glazing! The Czechs are paranoid about draughts....:-)

Many thanks for all this....very helpful

Couldn't I just leave it in its pot, not cut it back, and put it in the frost free place where the fuchsias are which is fairly dark. Wouldn't it then think it's winter and drop its leaves and rest? Or does it need to be where it's light?

Well, normally it would be outside all winter, so it depends how dark a place it is - dim should be okay, but would you be able to arrange to move it to a lighter spot to trigger growth in the spring?