This was posted by one of our members in this forum:
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jim VH
Vancouver,Wa. zone 8b
March 22, 2018
Yes, My Sudachi and Yuzu easily survived 8F (-13.3C) in January 2017 in Vancouver Wa., just across the Columbia river from Portland Or., with only minor small twig damage and about 20% defoliation on each. The Sudachi appeared to have a higher percentage of small twig damage than the Yuzu. On the other hand, the Yuzu is a much larger tree, and size does matter.
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http://tropicalfruitforum.com/index.php?topic=26301.25Genetic studies have suggested that Sudachi originated form a cross between an unidentified parent and Yuzu. That unidentified parent probably was a cross between Kishu and a Koji-type citrus which had Tachibana in its ancestry. (Kunenbo is a Koji-type citrus, to give you some idea of what we are talking about here, indeed this "Koji-type" citrus may have been a Kunenbo, but not the same Kunenbo variety that Satsuma, Bloomsweet, and Keraji (as well as Kabosu also) originated from. I think the name "Kunenbo" in general was used to refer to a certain type of larger fruit aromatic tangor-like type of mandarins, which may have all appeared similar to each other but in some cases had heterogenous origins)
http://tropicalfruitforum.com/index.php?topic=27900.0I'm guessing Sudachi is nearly as, or possibly equal to, hardy as Yuzu and can survive down to zone 8.
It might not really thrive in the colder parts of zone 8, unless planted up against a house or in a large city near a large body of water.