I life in Australia in cool temperate climate and of course you can grow lemons. You can grow kimquat (I love them!) and you might be able to grow oranges but they might not get all that tasty and the grapefruits might remain a bit sour. The variety I think is'eureka'. We get -5 C. I would put some lemon trees in big pots because it looks great in the patio but some in the ground too (they are more frost resistant in the ground)
Friend lives in San Francisco Bay Area, about 9b. She had a Meyer Improved Lemon in a pot she would move in for the coldest part of her winter, about 6-8 weeks. Nights it might frost she would wrap it up if it was outside.
If you build a bit of a winter cloche of wood and cover with sheet plastic, and perhaps put a string of NON-LED Christmas lights inside (on the wood frame) to plug in on coldest nights plus an additional cover for night, you should be able to keep a Meyer. The cloche would have to be big enough to give the tree space (leaves can't touch the covering) but not much extra.
An established in the ground Meyer lemon in the space you describe, might make it. Keep it pruned to keep it smaller, hence easier to deal with, and it might happen. Good Luck.
My understanding is that one of the reasons people traditionally espalier trees, aside from keeping them from taking up as much space, is to keep tender trees alive in a situation like you described. Can you espalier your lemon tree against the wall in the north? If I'm understanding your post correctly, then that wall is presumably stone and would re-radiate heat which would help keep your lemon tree warm. I'd double-check this, since I'm not a gardener and I'm posting something I'm remembering seeing a while back. At least it gives you a line of inquiry to pursue.
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I live in zone 9b, SF Bay Area, and meyer lemons grow great here. We have always had them, 40+ years, and other than the malathion spraying in my childhood, it's always been a favorite. Now, I also have persian limes and kaffir limes in containers, both doing well. Eureka lemons tend to do fine, as well. Oranges tend to get very thick skins, so not really worth it.
I've always been surprised at how expensive lemons are given how easy it is to grow them here. I usually get 100+ lemons a year from one tree/bush, with really no extra input after initial establishment.
Okay, I'll add to it.
Survived two north east Arizona winter's.
The first year it was in a hydroponic (potponic) tray surrounded by two inch thick ice.
This will be the first year in the larger half barrel .
It's it a pit greenhouse so it is out of the wind.
The wire is to keep the bunnies off.
I am growing a meyer lemon in the U.K at the moment however it is in a pot I will bring in for winter. I will attempt to grow one outdoors one day but I need to buffer the wind and make an adequate heat trap first. As France and England are furthur from the equator than some of the other places mentioned I think one of the things to think about it (along with the winter temperatures) is ensuring the lemon tree has adequate drainage during the wet winter months. Also I think providing the tree some extra protection while it is young and still establishing would also be a good idea.
I have seen lemons and small satsuma sized 'ornamental' oranges succesfully grow here in heated greenhouses. Other citrus left outside here over winter in pots have either died or been set back quite a bit. However I think your courtyard sounds like it has a good chance of working.
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