Let me know if you are in the UK and have interest in growing hardy citrus outdoors and would like some seeds.
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Not in the UK, and I probably couldn’t grow citrus outdoors here, but I think this is an interesting topic. I’d like to hear how it goes for you or how your citrus is doing if you are already growing them.
I wonder if I could grow citrus outdoors if I moved a greenhouse over them in the winter.
How cold do your Winters get? There are nice (but not sweet) citrus that grow down to -12°C.
Niels Rodin in Borex near to me (CH) is breeding cold hardy citrus, contact him
@Justin, I thought of you when I read this thread, are you in this group?
Testing out the reply via email function first time…
We’ve already gotten to -18c/0f this year. That was only a few nights, but they say it will get colder than that. This is my first winter here so we’ll see.
Thanks @julia.dakin ! And yeah I am in that Facebook group but forgot - Facebook can be confusing! I wish we were able to choose what we see in our feeds rather than the damned algorithms which can end up permanently hiding stuff one really wants to see!
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I guess so long as your greenhouse is warm enough and your pots big enough for the trees you’d have in them then, yeah why not? You know in England they used to have special greenhouses for citruses - it would be called an orangery. It’s hard to even find an image of a functional one now, I guess there’s no need now with international trade. Even the one at Kew Gardens (London’s World Heritage Site botanic garden founded in 1840, with a herbarium with has over 8.5 million preserved plant and fungal specimens, a library containing more than 750,000 volumes, and the illustrations collection contains more than 175,000 prints and drawings of plants) has been turned into a restaurant! But here’s there’s to give you an idea of how it looks:
A bit big for personal use 
But nice, isn’t it! They also have quite a big greenhouse - wouldn’t it be lovely to have one of these for landracing in! What a place to be making all the crosses and producing F2 seeds ready for the outside!
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I was thinking of planting them in the ground then moving an Eliot Coleman style greenhouse over them just for the coldest part of the winter.
The greenhouse could be used for other things the rest of the year. I don’t want anything to be completely dependent on the greenhouse, but it would be pretty useful for season extension.
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I’ve had a couple years luck with this model. Only problem is that I have run out of southern walls to use.
I put the lights on a thermostat during the cold months and keep fingers crossed for power to stay on.
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I live in the northeast US, do you think your seeds have enough genetic variability to have a chance of getting a few that might survive outdoors? I would be willing to entertain building some sort of hoop house for aiding, but probably wouldnt be able to go in on a fullblown greenhouse and certainly wouldnt be heating it. How long would you estimate a generation would be for these plants?
David The Good posted this article recently, about Russia growing citrus in trenches to survive freezing temperatures:
Maarten
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@kdurivage I think there could be issues sending citrus seeds to the US but also since nobody asked me for seeds, the ones I didn’t manage to plant all dried up and apparently citrus seeds are not viable once dried, unfortunately. They also got a bit mouldy before drying - I regret not planting them all but I was really expecting people to want some but as I was waiting, more and more time passed by… At least I’ve learned now how easily they can mould!
But you could probably get yuzu seeds there. Generally people grow them using grafts (not me though) but if you could order the fruit, maybe online, you can get the seeds. It seems they may need winter protection for the first… maybe 4 years? (I am guessing from a few things I saw online). And then after that apparently ok down to -12 degrees C. Still they would probably not like to be that cold for very long. But I heard that is meant to be their maximum tolerance.
I’m interested also in the idea of a landrace of locally adapted cold-hardy edible citrus that could grow without artificial protection. I live near the Seattle area, a cool maritime climate not unlike England, so we face similar challenges.
I know I can grow Poncirus trifoliata, the trifoliate orange, or hardy orange, which is hardy in zone 6. The issue is mostly eating quality, as PT is bitter, sour, and resinous (sticky), so it needs a lot of improvement by crossing with edible Citrus to have a fruit worth growing for culinary purposes. Fortunately, some of that work has already been done. That is able to cross with Citrus, and hybrids exist between PT and various oranges, mandarins, and grapefruits which are worth trying outdoors in zone 7 and 8 climates.
I’m starting by gathering up seeds and some plants of various hybrids, mostly citrange, citrandarin, and citrumelo types, but also some Japanese varieties, because Japan has a similar climate. That includes Yuzu, Sudachi, Kabosu, Shekwasha, and Sanbokan, although I’m hesitant to put those out for winter, so they’ll go in the greenhouse for the coldest months. There are also hardy mandarins (Changsha for example) and the Ichang Lemon. These however are just on the edge of being hardy in my area, so although they can be used to add useful genetic combinations, they probably aren’t the end goal.
I also just finished a greenhouse where I can grow some citrus that wouldn’t survive outdoors here, but could still contribute to the gene pool. I guess that’s a double-edged sword, because anything that has to grow in a greenhouse instead of outside would not have hardy genetics, but with citrus, we still need the higher eating quality traits in the gene pool, and those are not traits that are necessarily in the cultivars I could grow unprotected outdoors.
Eventually by growing out enough trees in outdoor unprotected conditions, given enough time, space, and diligent efforts with seed planting, I could hope to improve the eating quality of the cold hardy varieties.
I am excited about some cuttings I received last summer that I managed to root successfully, that are the results of a breeding project in Pennsylvania (zone 6) - so they are all known to be hardy enough to handle my zone 8 climate for sure. These are citranges (Poncirus x orange) and would be a great base to build from. Other than those, the most promising ones seem to be the progeny of the citrandarin known as US-852, which has proven to be hardy to 5 F. There’s some question about whether they have too much bitterness (inherited from Poncirus), but in a batch of seedlings, the taste can vary, so growing out a population and selecting the best could be done, if you have enough space to devote to the project.
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Yes, I have heard about quite a few that sound really good! I’m not sure how easy it would be crossing with them - I think it would be worth doing, but I guess would require quite a lot of space, I expect there would be a lot of variation and probably a lot of bad tasting ones, plus should take quite a bit of time since they take a while before fruiting! But a great idea I think, to head towards a stable - stable in terms of being reliably delicious at least - landrace.
It sounds like you’ve built up an excellent collection! How hot are your summers? Here it’s not just the cold winters that are the problem, but also the cool and often cloudy summers. I heard that that’s not suitable for sweet citruses, they never get sweet apparently, which is why I was ok to stick to yuzu. Otherwise I might have tried getting those other great crosses. Though I’d be really interested to know if anyone has been successfully growing them in places with summers similar to hear! There is a history of growing oranges in England, but only indoors, in orangeries! I guess they appreciate the warmth. Though, come to think of it, that does challenge what I had thought about demand for sun. Hmm… perhaps it’s just the temperature then, or, could it be a false assumption? If so, I’d be really interested in some of those new hybrids!
To me that sounds excellent and I don’t see a downfall really. If some of your material is not cold hardy enough, but you’ll cross it with the more cold hardy ones, sounds to me perfect to have the lesser hardy ones under cover. I take the same attitude, protect everything as best I can while making the hybrid swarm, then move to selection outside for the seeds that are sufficiently diverse. Also growing under cover means getting to that essential hybrid swarm faster. So I would do everything I can to make them safe, and fast, until getting F2 seeds, or for me in some cases a step further to triple or quadruple crosses (3 or 4 parents).
By the way how fast do you think you can go from seed to seed? (Germination to new fruit production I mean).
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They can be hand-pollinated, which is a bit of work if done carefully enough to ensure the intended cross.
The summers are not super-hot here, but we do have a hot July and August with highs in the 70-100 F (25 to 38 or so C). Nights most often stay cool, though, which makes a difference. So the key observation is that hardiness is not the only consideration. We also need fruits capable of ripening before the onset of cold weather. That narrows down the range of genetics considerably, since Citrus tends to be ripening in the winter months, not fall. Poncirus trifoliata, however, ripens in the fall, and some Citrus, like mandarins and many Japanese citrus, mature in the fall, hopefully before heavy frost.
I have been reflecting on the game plan and I think it involves growing citrandarins (Poncirus x mandarin), citranges (Poncirus x orange), and citrumelos (Poncirus x grapefruit) as well as the hardiest and earliest-ripening Citrus available. I love the idea of open-pollination, but it is the case that with open-pollination, you might get a lot of self-pollination, which might be OK for some purposes, but less OK if you want to rapidly explore the hybrids. With limited space, you would not want to spend too much space, time, effort, etc. on trees that are actually not hybrids at all, but duplicates of the parent trees. (This is also true for some Citrus varieties, that they come “true to seed” even if pollinated by another variety; it is a type of apomixis).
There are more extensive discussions going on at various hardy Citrus forums.
As for how long. 5-7 years to generate fruit? Yeah, that is where dreams hit reality. That, and the amount of suitable space for the project. And the fact that one of the parents (Poncirus) is more or less inedible. It’s not as easy as we’d like it to be. I have heard about fast-flowering Poncirus, which is a variety that takes less time to mature, although those can be hard to track down. Using Ichang papeda (a fairly hardy species) or its descendants can help because it develops its first flowers on lower branches, which I’ve heard generally means earlier flowering.
Are citrus seedlings amenable to having their scions grafted onto older trees? If so, that would probably help speed up fruiting. (It seems to work with apples, in any case.)
Granted, grafting a scion onto a rootstock might change some of the fruit’s culinary qualities, making it harder to evaluate. Still, if you’re grafting a dozen seedling scions onto a trifoliate orange, you can figure out which ones seem the most promising. That way, you can eliminate the seedling trees that seem the least promising in order to clear up more space for the best. (That’s what I’m planning to do with apples, so it seems like it may work for other species.)
In North America, Prickly ash (Zanthoxylum americanum) ranges up into Canada. It is in the same family as citrus, so perhaps might be usable as a grafting rootstock. However I do not think that a citrus scion would automatically become cold hardy just because it is on a hardy rootstock.
I think you’re probably right, grafting should be done to see if it speeds up the flowering process. My understanding is that the flowering is triggered when a set number of leaves are produced, which is genetically determined. So grafting onto a vigorous rootstock that promotes vigorous growth of branches and leaves should result in getting to the required leaf count more quickly.
I don’t know if Zanthoxylum is compatible with Citrus as a rootstock, but Poncirus / hardy orange we’ve been discussing is compatible and hardy to zone 6. Other hardy rootstocks that are hybrids with Poncirus could also be used, depending on the zone.
Ooh, that’s an awesome piece of information to know about the genus! If that’s how to speed up flowering with citrus, I think you’re absolutely right that grafting to a rootstock that makes scions grow fast would be helpful. Terrific! 
I could see prickly ash being useful as a rootstock if it already grows natively, and would allow grafting onto an existing huge wild tree. That would probably require less space dedicated to the project.