My argument to the question of "why didn't they test its hardiness" was that they already can grow it to 38 degrees north, and if you go any further north, you start to run out of Japan. Sendai is already pretty far up Honshu, so unless you have determined to grow oranges in Aomori or in Hokkaido, there is little point to pushing farther north with Japanese citrus.
To be totally fair, I think Satsuma is going to be difficult to actually grow in Sendai.
I found one video with the title: "Global warming brings changes to agriculture: Japan's northernmost mikan [mandarin orange] cultivation in Yamamoto Town, Miyagi".
That implies that it was not known that Satsuma mandarins could grow there or that they did not traditionally grow there. Yamamoto Town is 40 km south of Sendai, and is right on the coast.
https://www.youtube.com/watch?v=hryq8vkBo9gIf you look at those trees and their leaves, it does not look as healthy as they look further south. This is exactly what one would expect from a colder, cooler oceanic climate that is right on the edge of what these trees can tolerate. Also notice all the greenhouse coverings in the background. That is not a coincidence.
As I stated before, Japan's climate very rapidly becomes colder after going north of Tokyo.
In this area, it's probably as much the lack of heat throughout much of the year (other than summer) as it is the absolute cold temperatures in winter.
Asia has a strange climate. There are very cold winds that blow from Siberia to the west. These cold winds flow over the Sea of Japan, and are only warmed a little bit. The mountains of Japan help block this wind a little. The fact that Japan is surrounded by ocean helps prevent temperatures from dropping lower than it would otherwise be, given Japan's close location to the continent of Asia. But, especially north of around Tokyo, the ocean also means that temperatures are going to be kept cooler throughout the year. So the moderation of temperatures works both ways. It will not get as cold but it will also not get as warm. The temperature of the ocean water off the coast of Sendai in mid-March is around 42 to 51 degrees Fahrenheit (5.6 to 10.6 C). Citrus cannot really grow in those type of cool to cold marine temperatures. Even by late April the ocean temperature is still only 48 to 58 F.
Some might counter by trying to tell me that citrus can still grow in the south island of New Zealand, and you'd be correct (though citrus really has trouble ripening there). However, we are talking about growing very close to the coast. Those on the south island of New Zealand know citrus does not grow well near the coast with wind exposure. And in this part of Japan, close to the coast is all there is, because otherwise there are mountains.
My point is that these climate zone maps can be a little misleading when they are in very cool to cold oceanic climates.
At 2:33 into the video they show a map of the area where Satsuma mandarins (
unshu mikan) grow in Japan. It's not a large area, compared to the country's total area.
According to the climate zone maps, Yamamoto Town (in Japan) appears like it would be roughly equivalent to the coast of Oregon at the central latitude of the state (in the U.S.).
(probably with slightly warmer longer more humid summers)
I also suspect that despite the climate zone designations, this part of Japan may at rare times be subject to a little bit lower temperatures than would be expected for that same zone on the U.S. Pacific Coast. The lowest temperature recorded for Iwaki, Japan is -10.7°C (12.7°F), which is 150 km south of Sendai, while for Sendai it's -11.7°C (10.9°F), but that was probably further away from the coast near a mountain valley. So I think we can assume the lowest possible temperature for Sendai right near the coast would be somewhere between those two. That is definitely cold enough to be able to kill a Yuzu tree.
(Though to be entirely fair, the lowest temperature recorded for Lincoln City, in Oregon, was 4° F (-16° C) in early December of 1972, but I think low temperatures like that are much rarer on Oregon's coast than in Japan, only about once every 40 years. For comparison, the lowest temperatures on record for Fukushima and Sendai happened very recently, in 2024 and 2025.
sources for that here: Coldest Morning of the Season Occurs in Many Locations Across Japan, Yomiuei Shimbun, Japan Times, Nov 19, 2024 ;
and here:
https://www.visualcrossing.com/weather-history/Sendai%2CJapan/ )
for those interested in more detail about the difficulties trying to grow citrus in the more southerly parts of New Zealand, some information was gleaned from a blog post here:
https://www.craftygardener.co.nz/?post_id=24I emphasize again that this phenomena is very different from the growing conditions in the U.S. Southeast or in most of Europe (maybe unless you're in Scotland).
You can also see this video, which shows someone in Cork, Ireland attempting to grow a Satsuma and a desert lime (Eremocitrus Glauca) against a south-facing concrete block fence wall, between two houses. Due to the very cool oceanic climate, citrus is very difficult to grow, despite being in zone 9b.
https://www.youtube.com/watch?v=phpoodc_PVUAt the latitude of Tokyo, or even Osaka, Japan is a little bit of a "cold marine" environment (compared to China) but it really becomes prominent just a little north of Tokyo.
Japan's northern main island, Hokkaido, in fact, has a climate comparable to Germany or Scandinavia despite being at the latitude of northern Italy. Florence is farther north than Hokkaido's biggest city Sapporo. That would be another discussion though, since the citrus growing zone ends much further south than anywhere near Hokkaido.
