We can overwinter them outdoors in flower beds here - we have some in their 3rd season, which have flowered well this year. I would imagine that a cold greenhouse would be suitable for over wintering - you won't know unless you try janebal.
Thank you Guerney Donkey2 and B3. I will try to save them in my unheated greenhouse. They have been beautiful this summer with red Beedance Bidens in pots. Lots of insects on them.
They don't survive outdoors in my windy garden and if I had any sense, I would throw them away because they are not expensive or difficult to find. However, I grow very fond of them and pop them into my frost-free greenhouse with other tender plants. They repay me for this care by growing enormous and flowering most of the winter. Very cheering in the depth of winter.
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https://forum.gardenersworld.com/discussion/174459/overwintering-gazanias