How much heat is required for lemons to ripen?

I have a Eureka lemon tree that’s been in the ground about three years, and have been able to protected enough in the snowy winters that it’s getting to a decent size. I’ve only ever been able to harvest a couple of lemons off of it. Right now, it’s bursting with a lot of flowers and small fruit, but it’s getting very cold now. Last year none of them made it far enough along to be usable. I have a similar problem with my Kinkan kumquat tree, and it looks like my new Myers lemon still in a pot is following the same pattern.
I can’t think of any way to boost the temperature other than what I’m doing which is passive heat through water tanks and frost cloth. I got a feeling this won’t work.
Are there any other tricks that can be used, or are these trees and simular just a waste of my time in this climate?
For reference, I have several other citrus varieties which all naturaly ripen around October, and are doing fairly well.

Lemons are continuous croppers and should bloom 3-4 times per year. The fruit require less than 60 72°F degree-days to ripen.


I always find your explanations of citrus very useful, thank you Richard.