More talk about Growing Degree Days, less about USDA zones/minimum temperature

It seems that all nurseries only give the minimum temperature that a certain fruiting plant can take, but nowhere ever (unless you grow corn or wheat) anyone gives andy information on how much warmth fruit needs to mature. . It is vital to us here up in the north (Finland), since while the golf stream makes us approximately equivalent of Ontario and Michican in terms of minimum temperature, that does not take into account of the GDD that we get. We count here the temps above 5 Celsius, somewhere else in the world they might calculate above 10 C , depending on the plant I guess. Last year we almost reached 2000 heat units.
Every cultivar takes different amount of heat to mature. When searching for a persimmon to try, or pawpaw, this information is valuable. But no-one records them, why? So I am planning to get a cheap weather station that can calculate mean temperatures, and try to get some info on the trees and bushes that I have.
If anyone has reliable info on ANY cultivars in this regard, let it be known to all others also!
-Janne

The reason you don't find that type of information is because of the USDA Zone method that has become the global "standard".
The Zone method uses assumption to exclude information that as you point out is vital for people living in "extreme" types of weather or locations.

Pawpaw, for example, is a tree that while it can be grown as far north as Zone 4, it does best in areas that have a minimum of 60 days of above 15C  and a maximum of 30 days above 35C.
It thrives in the south, an area known for high humidity and high temperatures for at least 3 months of every year and usually the high temps last from June 15th all the way to the end of September or even into November.

If every plant or every seed packet contained such information it would indeed be a boon for those living at the "margins of adequate temperature ranges.
The only reason(s) I can come up with for no one doing so is the amount of data must be sparse or the time it would take has been deemed to high to be profitable.

It is possible to do some research in order to derive time/temp data via mathematics and historical recorded weather charts.
I think it just depends upon how deep one wants to delve into such a project. (I would think that if one were to do this work and publish it in book form, there would be a market for the tome)

Redhawk

It's actually not very "marginal" the entirety of western Europe has a climate that doesn't fit into USDA zones unless you use the heat zone as well. I'm 7b I can't grow half the things people in a continental 7b can because we don't get warm, infact we are cooler than most of Finland in the summer! we will go 5 years without ever hitting 30C this year we did for about 4 days. It's August we may make it above 20 again this year but the 10 day forecast shows the highest temp expected to be 19C.

Everytime I see a new (to me) exciting plant on these forums I have to check it's heat requirements, and I have a horrible time explaining to me that no I cannot grow rice or apricots or peaches or or or...

hau Skandi, that's one of the reasons Conservatories were built, even in England there are plants that simply don't live when planted outside, they have to be under glass to make it.

Even where I live which is considered by most to be the "deep south", I can't grow citrus unless I put it indoors for even our mild winters.
My solution is that I'm going to have to build a small conservatory just so I can have some lemon, lime and maybe a mango tree.

Back in the 1970's a friend of mine put up a clear geodesic dome on his land in Colorado so he could have springtime in the dead of winter.
I pointed out to him that he could have grown at least 4 trees in that thing but he didn't seem interested in growing his own citrus trees.

Redhawk

Some years ago, I calculated GDD for my garden, and for some of my crops. The math is straightforward, and the weather data is readily available.  

Here's a couple of  charts comparing my growing degree days to that of a friend in Norway. I based it on 10 C which is a good temperature for warm weather crops like tomatoes, squash, or beans. A lower temperature of 5C is often used for cool weather crops like peas, grains, etc. The charts show the daily GDD, and the cumulative GDD. I get almost twice as much heat as my friend in Norway.

A chart comparing growing degree days in my garden to those of some of my collaborators.

A chart showing GDD for one of my varieties of sweet corn. The colored dots are corresponding to planting/harvest dates. Of note, is that spring planted corn takes much longer to mature than corn planted in hot weather.

Sounds like your summers may be similar to the cool Pacific Northwest. We struggle with gdd's also. You may find helpful info from Washington State ag dept. which partners with mt Vernon research station- they are a tremendous resource for those of us looking for fruits varieties that excel in moderate weather.  Just double check the hardiness and you may be surprised at the diversity you will find.  Also look through the web sites of PNW fruit catalogs although you may not be able to order from them. A few I find helpful are Raintree nursery, one green world, restoring eden, burnt ridge nursery and rolling river nursery. Good Luck

Now we're getting somewhere, I thank you all for the input! I myself live near a place that has a large weather station, and about next to it an arboertum that has some fruiting plants also. I figure there is a place to start to collect info on certain cultivars that they have, and thus the spreadsheet is born . It would indeed be nice if there would emerge a more holistic way to say in what kind of area one lives, not just "zone 5". Before I figured that Köppen charts might be good, but they do not seem to be so accurate either, if one is a gardener .
-Janne

The signature on one of my social media accounts mentions my growing degree days. And specifies that I'm using 10C for the calculations.

Silt/clay, high-altitude, super-arid, sun-drenched, irrigated-desert garden. Cold radiant-cooled nights. ~100 frost free days. Grow most of my own locally adapted landrace seed. GDD10C ~1300. Author of Mother Earth News: Landrace Gardening Blog.

I am working on this very thing. The limit of a tree variety depends, not on how cold the winter gets but, on how warm the summer gets. Every plant has a GDD requirement. To find that requirement, find the optimum growth temperature.
(Optimum grow temp minus base growing temp) multiply by days to maturity and you get GDD requirements.
This is easy with annual crops because they give days to maturity on the package.
Here's a carrot example:
It's a cool season crop, so base temp is 5C
It's optimum growth temp is 16-18C
Touchon carrot has a maturity of 65 days
(18-5)*65=780GDDs
But a carrots maximum growth rate uses a max of 18-5=13GDDs per day
So any day with average temp over 18C has wasted heat.
This year, my area took 71 days to amass those 780GDDs.

I need to work on perennials now. That's this winters work.

I don't know if I'm allowed to do this but here is a link to my YouTube playlist I am building concerning this Growing Degree Day issue
https://www.youtube.com/watch?v=57SamxQ3QRs&list=PLkEF2saHcnSmkZrtPywczBRd-81UaBvDW

FYI
A chart at https://en.wikipedia.org/wiki/Growing_degree-day#Plant_development lists GDDs for a number of trees & other plants.
A GDD calculator for US (input your zip code) at https://www.greencastonline.com/growing-degree-days/home

I'm only beginning to use GDD as a tool for growing plants.  I confess I'm not sure how to use the information.  My calculations need to include humidity I think, since no matter how much hand-watering I do, plants don't grow well till the summer rains begin in July (in a good year).  Microclimates might be the answer for me.  I have so much to learn!