On: Fruit Trenches: Cultivating Subtropical Plants in ...

[jungletime]

The most hardy citrus is the ornamental Flying Dragon Citrus, which produces fruit, but the fruit is poor tasting. But can be grown in zone 6 Niagara Falls

http://www.eattheweeds.com/hardy-orange/

https://en.wikipedia.org/wiki/Cold-hardy_citrus

The most tropical fruit that can be grown in cold climates in North America is the Paw Paw. (Survives -30 degrees)

"It belongs to the genus Asimina in the same plant family (the Annonaceae) as the custard-apple, cherimoya, sweetsop, ylang-ylang and soursop."

[pvaldes → jungletime]

Flying dragon or Poncirus is a must if you want to grow Citrus in cold places. Is the only decicuous Citrus.

The fruits are non culinary, but is an excelent rootstock

In any case the "lemon" of cold places is Chaenomeles (and not all are able to bear fruit).