I've read that grapefruits, kumquats, and some others require hot summers to properly sweeten fruit. No guidelines are given. Does anybody know how much heat is required, and for how long? I think there is a unit of measurement for this but I can't remember the name.

I'm in zone 6b. We get pretty hot summers in southeastern PA but a somewhat short growing season compared so higher zones.

Calculating Heat Unites for citrus. http://www.dpi.nsw.gov.au/__data/assets/pdf_file/0011/218972/Calculating-heat-units-for-citrus.pdf

For most oranges a average number of 2,800 unites . For an example at Univ. Of California at Riverside CA (UCR) = 3,200 unites - Millet

Thank you! And the word I was trying to remember is Degree Days.

According to this map I get ~3400 with 50F base temp. http://www.soilinfo.psu.edu/soil_clim/information/general/PA_clim_atlas/growing_degree_days/map.pdf

It looks like base temp for citrus is 55F but hopefully it is close enough.

[quote="brianPA2"]I've read that grapefruits, kumquats, and some others require hot summers to properly sweeten fruit. No guidelines are given. Does anybody know how much heat is required, and for how long? I think there is a unit of measurement for this but I can't remember the name.

I'm in zone 6b. We get pretty hot summers in southeastern PA but a somewhat short growing season compared so higher zones.[/quote]


Personally,I wasnt aware that heat was so important.....my 2 cents worth....when citrus grow they produce acids.....what i have noticed is generally the bigger the fruit...the more likely it is to be tasty.....nitrogen is the biggest influence on citrus reaching its full potential.....whats more of an issue is winter temps getting down below 10c at night because this when those acids are converted to sugars.....thats why citrus does not go well in places like fiji because their winters dont go go under this temp.....I always knew when a cold front came to auckland...my citrus would be doing their stuff and then i would go for deep colours.
For me californian washington navel taste the best orange and that is due to hot summers and cold temps in winter to develop those sugars.....i grew citrus in Auckland and it never went over 25c in summer....one tree had no sun on it(mandarin) both my lemon and mandarin went big and sweet.......sweetness is decided at ripening time in winter...not summer but i am sure millet knows more than me..But thats what i noticed in auckland NZ......Big is good!
PS
I find feeding fruit organically as opposed to chemically enhances the taste. .

[quote]Calculating Heat Unites for citrus. http://www.dpi.nsw.gov.au/__data/assets/pdf_file/0011/218972/Calculating-heat-units-for-citrus.pdf [/quote]

For most oranges a average number of 2,800 unites . For an example at Univ. Of California at Riverside CA (UCR) = 3,200 unites - Millet

That link is pretty bloody interesting...esp as it relates to where i live.