Hi everyone,
After completing so many hardy citrus taste tests this season, I compiled my ratings and compared it to the approximate hardiness of these varieties. The numbers for hardiness I used are not exact, but estimates based on my personal experience, as well as the experience of others that I have read. It only got down to 10F so far this Winter, so I can only confirm up to that point, and how good our plants look after experiencing these temperatures with full wind and sun exposure.
The results were pretty similar to what you might imagine, here's some averages based on hardiness:
-15F = average score of 28.8/100
-10F = average score of 27.7/100
0F = average score of 45.6/100
5F = average score of 59.3/100
10F = average score of 76.6/100
15F = average score of 84.7/100
It's important to note the ranges too - at 0 and 5F, the difference between the lowest and highest scores was 34.5 and 28.5, while it was <10 for all other hardiness groups. I didn't go super in-depth with statistical tests, but you can tell pretty easily what data is significant to us. I did remove Prague from the average and range calculations because it skews the results; this information is intended to be more valuable from a breeding perspective, since chimeras may possess unusually good hardiness despite having high-quality fruit.
From my interpretation, it seems there is a reasonable amount of potential for breeding fairly good fruit in the 0 to 5F range. I don't see dessert-quality fruit as a reasonable expectation, but fruit can still be fairly useful and good-tasting to some. The highest scores in these ranges were 60.5 for 0F and 68 for 5F. Considering an Owari Satsuma scores a 90.5, scoring in the 60s is not bad, especially for acid citrus intended to be used for processing/cooking, not fresh consumption.
Our options are more limited in the sub-zero ranges, with Poncirus plus ranking the highest (32.5), followed by the Conestoga selections, and likely other extremely hardy F2 hybrids. Some Citrandarins/Citradia might be hardy a little below zero but it's still uncertain. 75% trifoliate hybrids might also stand a chance.
Keep in mind that this is only based on my own taste tests which are done as objectively as possible, but are still subject to my personal biases. A lot of guesswork is involved in all of this, but we can still establish an inverse correlation between taste and hardiness. This correlation is not unbreakable, but there is certainly an upper limit for fruit quality depending on hardiness. With our collective efforts, we can see many of these categories improve by 5-10 points. This can be very meaningful for the sub-zero range especially - it would be like having Rusk or Swingle fruit quality on a plant that can survive in some zone 5-6s.
In my opinion, once a fruit scores 60+, it is worth growing for some sort of culinary purposes. We are warm enough to get away with many different plants hardy down to the 5-10F range but too cold for anything more sensitive than that. Even then, I prefer to have some guaranteed survivors in case we are ever hit with extreme cold that wipes out even those plants.
