Plant hardiness is based on USDA Plant Hardiness Zones. These are based on the lowest annual low temperature in the location. In general hardiness zones progress in 10 degree Fahrenheit increments between zones, with a 5-degree increment within a zone. The coldest zone is zone 1, with winter low temperatures of -50F or colder. Around Interior Alaska zone 1 translates to all the low-lying areas. Higher areas are warmer, some are zone 2, the highest perhaps zone 3. Hardiness zones pertinent to Fairbanks include:
Zone 1 - winter low temperature of -50F or colder
Zone 2a - winter low temperature of -45 to -50F
Zone 2b - winter low temperature of -40 to -45F
Zone 3a - winter low temperature of -35 to -40F
Zone 3b - winter low temperature of -30 to -35F
In general apples will grow in warmer zones than they are rated to, so if you are fortunate to live in a zone 3 location you can grow apples hardy to colder zones. If on the other hand you live in zone 1 the other less hardy apples will probably not make it in your location. Based on other sources and also our own experiences, the apple varieties listed on this site break down as follows
Chinese Golden Early, Dawn, Dolgo, Heyer 12, Heyer 20, Noret, Norhey, Prairie Sun, Rescue, Shafer, Trailman. (Alma Sweet, September Ruby, Clair 4, Patterson and Rosthern 18 are possibly zone 1 apples too)
Zone 3 or Colder: (These are apples about which little is known. Some were bred right here in Fairbanks. They are doing well in our orchard, so they are hardy to at least zone 3 but probably a bit hardier than that, as we are borderline zone 2/3)
Adanac, Carroll, Norland, Norkent, Norson, Yellow Transparent, Zestar (Norkent and Norson have thrived for several years in pots, but then died after being planted. Norkent also keeps growing late in the summer and doesn't get ready for winter. We now have it grafted on a hardier rootstock [baccata] and it seems to be surviving, but not thriving. Yellow Transparent and Zestar also perished when grafted onto ranetka, but seem to be surviving on baccata. Fingers crossed for these trees!
What happens if there is a frost when there is fruit on the tree?
The varieties growing in our orchard have taken a frost, down to 28 degrees F with no damage. Kerr was on the tree at 25 degrees and remained undamaged.