I am in 6a, and attempted to grow Yuzu Ichandrin (on Flying Dragon rootstock - available from One Green World) outside last winter, and it failed. It was a cold winter in my region (about -13F on the end, plus wind). I mulched mine heavily, but also made what I now consider a big mistake. A drain pipe from the roof led to the same bed the citrus was in, meaning that there were times where the roots were not just cold surface soil temps, but actually waterlogged and frozen - which I think it what really sealed this thing's fate.
I also didn't give it my absolute best microclimate, and I only do pretty half-assed insulation (ie. throw a big pile of straw on top). I'm trying to push the limits of my zone a bit, but not if it takes a large time investment, which I don't have right now. I lost pineapple guava this winter (not surprising), but my Chicago Hardy Fig made it. Some frost-kill on the branches to be sure, but that was true of many things this winter.
Yuzu Ichandrin is your best bet, as far as my research goes. I'd like to try again with an older specimen in a better place. I've got one in a pot to play with and clone from. It's said to be similar to lemon in taste, and used in Japanese cooking.
For a sweet citrus eaten off the tree - I'd agree that either a solar green house or a potted indoor/outdoor plant is the best way to go.
I'm excited to see what Paul does for his lemon tree outdoors in Montana - that would almost certainly work for Yuzu Ichandrin, because it's even more cold hardy.
I'm 6b, and I'm building walipini and planning on some dwarf citrus and maybe even a mango (10a or NO frost or warmer) in controlled conditions in this climate. Next few years after I get some permanent stuff finished here in the urban ranchette-ette I am looking to try a few palms in localized micro climate spots where I can fake 1/2 to 2 grow zones. (heck I'd be happy to be able to leave dahlia's in the ground and grow a jasmine (8a) ) At a 4a site I was able to push a small strawberry bed tucked into the right space between front of house and a concrete step to be able to get blooming pansies and strawberry fruitset from sometime mid March to first week of December and that was before I discovered floating row cover, and building a cold frame.
If you are willing to build greenhouse framing, double wall plastic insulation *aka two sheets of plastic with dead air space and use a small blower to keep the space inflated* you might be able to make them happy over the winter up there in Kansas. Meyer Lemon can briefly take a night or two of 28f but that's about it, and if they do, they will lose bloom or fresh set fruit. Small trees I was told to look at Washington (orange) and the Meyer Lemon...
I want to set-up an undergroundgreenhouse as well - was hoping for this fall, but have a lot of other things on the go. Besides citrus, and perennials that are not quite hardy for me, I'd like to try avocado. Some of the ones grown in the mountains in Mexico are hardy to Zone 7, and there are also dwarf rootstocks available - so I feel like that would be a fun little adventure. (I heard Mark Shepard say that his retirement project one day might be to try and get hardy coffee and cocoa going up in Wisconsin - though in his case, he's not talking greenhouses!)
Rob Read wrote:I want to set-up an underground greenhouse as well - was hoping for this fall, but have a lot of other things on the go. Besides citrus, and perennials that are not quite hardy for me, I'd like to try avocado. Some of the ones grown in the mountains in Mexico are hardy to Zone 7, and there are also dwarf rootstocks available - so I feel like that would be a fun little adventure. (I heard Mark Shepard say that his retirement project one day might be to try and get hardy coffee and cocoa going up in Wisconsin - though in his case, he's not talking greenhouses!)
Bro, I wanna do the exact same thing. I always buy cold hardy crops every year more and more although I have not purchased the avacado yet.
There are a few great books on underground greenhouses (including Mike Oehler's great Earth Sheltered Greenhouse, my original inspiration), but my fave so far is Solar Greenhouses: Underground by Daniel Geery. It's really beefy, with a lot of different threads of science woven together with some solid testing (though only over a couple of seasons). I really wish this book (from 1978, I think) would come back into print in a new edition, and report back on longer term prototypes. It's hard to track down, but I got it through inter-library loan. The insights in that book, combined with Oehler's major contribution of the 'cold sink,' plus some of the wolpati stuff that's been coming out online lately in forums like this one. There's a lot of information out there.