Vegetables and Fruit forum: Degree Days, Growing Degree Days, Heat Units

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Name: Rick Corey
Everett WA 98204 (Zone 8a)
Sunset Zone 5. Koppen Csb. Eco 2f
I helped beta test the first seed swap Plant and/or Seed Trader Seed Starter Region: Pacific Northwest Photo Contest Winner: 2014 Vegetable Grower
Avid Green Pages Reviewer Garden Ideas: Master Level Garden Sages I was one of the first 300 contributors to the plant database! I helped plan and beta test the plant database. Charter ATP Member
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RickCorey
Apr 11, 2012 6:28 PM CST
I just found a calulator at Weather.com for "degree days".

http://www.weather.com/outdoors/agriculture/forecast/98201

You can give it your ZIP code, and a base temperature, and a range of dates, and it will calculate your "degree days" or "heat units" for you

You can even enter future dates, and it will predict how many heat units you should expect by that date.

Does anyone know where to look up how many degree days different crops "prefer"? Or even better, how many degree days different tomato varieties require to ripen?

I think this is how degree days are calculated:

For each day, find the average temperature by averaging the daily high with the daily low. It doesn't matter how many hours were high or how many hours were low.

Subtract the "Base Temperature" from that average. If you are comparing your results to some published result for other regions or a particular crop, "50 degrees F" seems to be the usual choice.

I think that negative numbers are "rounded up" to zero, as if cold days just prevent growth and maturation, they don't put the plant into reverse.

Then the result for each day is added together into a grand total.

For example, 3 days each averging 60F would count as 30 heat units or 30 degree days. One more day averaging 70F would bring the total to 50 heat units.

Several weeks of days eeach averging 50F or less would leave the degree days unchanged at 50 heat units.

U Washington says that the Seattle area averages 2156 degree days between last and first frost, using "50F" as the base temp.
Name: Carol
Santa Ana,Ca. (Zone 10b)
Sunset zone 22
Charter ATP Member Hummingbirder Lover of wildlife (Black bear badge) Orchids Region: California Plant Identifier
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ctcarol
Apr 11, 2012 7:00 PM CST
Thanks, Rick! I'll have to try it just for laughs. I wonder if it can factor in "El nino" and "la nina" years too. LOL! Here in So. Cal. that makes a big difference. This year, I think it is useless for the whole country! Nobody has been average. Always fun to play with these things though.
Name: Rick Corey
Everett WA 98204 (Zone 8a)
Sunset Zone 5. Koppen Csb. Eco 2f
I helped beta test the first seed swap Plant and/or Seed Trader Seed Starter Region: Pacific Northwest Photo Contest Winner: 2014 Vegetable Grower
Avid Green Pages Reviewer Garden Ideas: Master Level Garden Sages I was one of the first 300 contributors to the plant database! I helped plan and beta test the plant database. Charter ATP Member
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RickCorey
Apr 11, 2012 9:22 PM CST
>> I wonder if it can factor in "El nino" and "la nina" years too. LOL!

That would be cool. All they would have to do would be to sort their historical database into around 4 categories, and then do their projections based on which kind of year this is.

I'm still trying to decide what to do with the numbers! I know enough to look for tomato varieties that are "early" and "for cold climates", but there ought to be something more elaborate I could do, involving numbers!

It would be depressing if I had to fall back on common sense and learning from experience. LOL, I think!

I see there is no emoticon for "thoughtfull expression". What does it say about me that I went this long before I felt any need to add a "thoughtfull expression" to a post?!?

Confused
Baltimore County, MD (Zone 7a)
A bit of this and a bit of that
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bitbit
Apr 12, 2012 6:58 AM CST
Great tool, Rick! I admit that I usually skim right over things that talk about growing degree days, because calculating it has always seemed like too much effort (and I'm a scientist!) *Blush*

Looks like Norfolk has an average of 4477 GDD. Shows how useful the USDA zones are on their own - we're in the same zone, but by this measure, I have more than twice as much heat as you.

And it also confirms my suspicion that 2012 is off to a very weird start. We should've had 94.5 GDD by now, and we've had 537! All those days at 80+ in February didn't do anything for my crops, though... they were still cozy in their seed packs at that point Hilarious!
Name: Rick Corey
Everett WA 98204 (Zone 8a)
Sunset Zone 5. Koppen Csb. Eco 2f
I helped beta test the first seed swap Plant and/or Seed Trader Seed Starter Region: Pacific Northwest Photo Contest Winner: 2014 Vegetable Grower
Avid Green Pages Reviewer Garden Ideas: Master Level Garden Sages I was one of the first 300 contributors to the plant database! I helped plan and beta test the plant database. Charter ATP Member
Image
RickCorey
Apr 12, 2012 1:49 PM CST
>> Looks like Norfolk has an average of 4477 GDD. Shows how useful the USDA zones are on their own - we're in the same zone, but by this measure, I have more than twice as much heat as you.

I agree the USDA hardiness zones aren't good for much other than predicting best-case winter kills. (Even for that, I think the worst-case for killed-by-cold is very unpredictable, sudden, early Fall freezes and spring freezes after a long warm spell.)

I like to compare "Texas Zone 8" with "Seattle Zone 8". Texans are COMPLETING their spring tomato crop and complaining about the heat around the same time my nights are finally getting warm enough to PLANT OUT tomato seedlings in hope of ripening a few by late summer (isn't that when Texans start Fall tomato crops?)

So far this year Dallas, TX has had 967 GDDs, wheras an average year for them would only have had 496 by now.

My ZIP code has had 18 GDD by now. Average: 0.

Maybe, along with "av erage last frost date", we should be looking at "average date to accumulate 500 GDD".

I like "Sunset Climate Zones" enough to keep the link in my signature block (yours looks like 31 or 32), but no seed packet I've ever seen tries to give info for each Sunset Zone.

http://www.sunset.com/garden/climate-zones/

http://www.sunset.com/garden/climate-zones/sunset-climate-zo...
Name: Carol
Santa Ana,Ca. (Zone 10b)
Sunset zone 22
Charter ATP Member Hummingbirder Lover of wildlife (Black bear badge) Orchids Region: California Plant Identifier
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ctcarol
Apr 12, 2012 5:02 PM CST
Rick, If it makes you feel any better, tomatos don't do well in the So. Cal coastal zones either...for the most part. They do tend to like drier climates. I'm putting off planting mine because our weather is still going from 60s to 80s and back, all in the same week. All my plants are confused!
Name: Rick Corey
Everett WA 98204 (Zone 8a)
Sunset Zone 5. Koppen Csb. Eco 2f
I helped beta test the first seed swap Plant and/or Seed Trader Seed Starter Region: Pacific Northwest Photo Contest Winner: 2014 Vegetable Grower
Avid Green Pages Reviewer Garden Ideas: Master Level Garden Sages I was one of the first 300 contributors to the plant database! I helped plan and beta test the plant database. Charter ATP Member
Image
RickCorey
Apr 12, 2012 5:16 PM CST
>> tomatos don't do well in the So. Cal coastal zones either...for the most part. They do tend to like drier climates.

Hmm, I thought all of CA was dry! That shows what I know.

Fortunately my part of the PNW coast is dry and sunny during the few months warm enough for tomatoes. A cool Mediteranian climate pattern.

But now that I know they don't like constant drizzle, my grandiose plans for keeping them under plastic in late spring and fall will be intended to keep them drier, as well as keep them warmer.

My nights are still dipping down to 42-44F, so I wouldn't even be tempted to harden off if I had started seeds. Since the daily HIGHS are as often below 50 as above 50, I'm not really missing any great opportunities.
Name: Carol
Santa Ana,Ca. (Zone 10b)
Sunset zone 22
Charter ATP Member Hummingbirder Lover of wildlife (Black bear badge) Orchids Region: California Plant Identifier
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ctcarol
Apr 12, 2012 9:04 PM CST
Haha! We do have a coastline! It's just a bit narrower than yours, and a lot of it is facing the wrong direction.
Baltimore County, MD (Zone 7a)
A bit of this and a bit of that
Charter ATP Member I was one of the first 300 contributors to the plant database! I helped beta test the Garden Planting Calendar Garden Sages The WITWIT Badge Herbs
Composter Container Gardener Seed Starter Vegetable Grower Dog Lover Plant and/or Seed Trader
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bitbit
Apr 13, 2012 8:42 AM CST
I've always heard the sunset zones are more useful out west, but you're right, I never see them mentioned on seed packs or sales websites in regard to planting dates, or even what plants are appropriate to grow in an area. I'm in 31 by that map... and since "oppressive humidity" is what seems to distinguish it from 32, I think it's a fair assessment Hilarious!

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