# Understanding Growing Degree Days



## Deezil

I'm still trying to wrap my head around this concept completely, hopefully some can help, but lets see what i understand this far. 

Growing Degree Days - a measurement calculated to gauge the progression of plants through the annual life cycle. 

In grapes, im assuming this is how they calculate bud break, flowering, fruit setting, verasion, and harvest dates (all approx.).The measurement itself is calculated by taking the average of the minimum daily temp & maximum daily temp.. And subtracting a base factor. The base factor is the temp at which the plants no longer grow - whats the base for wine grapes?

As a formula:
[(Min. Temp. + Max. Temp.) / 2 ] - Base Factor

The answer provided by the formula, is the Growing Degrees for that specific day. This is then added to the days following throughout the year, accumulating as a number from which to judge the progress of the plants and environment in question.

The things i know, i dont know:

1A. How many Growing Degrees does it take on average to get to the various stages of grape growing? Or are these different for everyone and something you actually dont learn until you're in the field?

1B. Do the averages vary from species to species, variety to variety, rootstock to rootstock, location to location... All of the above? 

2. I dont know the Base Factor - the temp at which grapes stop growing. 

3. How about information on the Growing Degrees needed for the vineyard pests to become real problems? Japanese beetles for instance.

4. Are growing degrees only calculated in Celsius, or does Fahrenheit work too? All the examples ive seen favor Celsius.


Theres probably more to this ball-o-wax, and probably a bunch i've yet to understand but hopefully this is a good start.

Input


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## grapeman

This is a complicated issue and varies by variety. In general, it shows how many heat units are available for plant growth during a year. There is scant research done in the field so far and usually it is a general thing, like frost free days during the year.

It is tricky to track.
1A. This information is not readily available yet, but
1B. yes it does vary for all of the above.
2.The base can be 50, but some use 55. These use F and yes that is commonly used in the US- same formula, just use your base.
3. This is starting to become known and is used in the NEWA pest modeling systems.
4.Repeating, Fahrenheit is common in the US.

Tracking growing degree days along with growth stages is just beginning to be tracked and it can be tricky. To do it, you need to record the phenological stages (growth ratings) and record all the dates. Then the records need to be matched to growing degree days for the site the phenology was recorded on. That gives you gdds to phenology stages and landmarks. Here is an example from the University of Vermont.
http://pss.uvm.edu/grape/UVMvineyard/2009KeyPhenologyDatesWinegrapes.pdf

here is a NEWA page showing the Grape Berry Moth model- obviously this is not a period of concern with them
http://newa.cornell.edu/index.php?p....nrcc.cornell.edu/newaModel/berry_moth/cu_wlb

I have tracked this information for 4 years at the Cornell Willsboro Baker Farm where I have been the vineyard technician. I have all the phenology ratings I scored weekly during the growing season. That information could be tracked along with weather data from the on site weather station. I have not yet done that, but I am getting ready to pull the information out of the 100 page Excel workbook into Access or SQL so we can analize it further. I guess you have found one of the few people out there that has actually done this stuff.


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## Deezil

Thanks for teaching me, appreciate it.

When you say some use 55 for a base, is this people & their preference or do the same people use 50 & 55, depending on the varieties?

So i was poking around online last night, and signed up for a weather station data service through WSU & AgWeatherNet, because i wanted to look at the growing degree information for last year - just to see what the data looks like - and see the beginnings of this year. 

Along with having the Base Heat Units, they also track something called Bloom Units or Bloom Heat Units. Any idea what this is and how it relates?

Oh! Just noticed they list First Fall Frost / Last Spring Frost dates too.. Hadnt seen that before

Besides the actual walking involved, how intensive is it to track the growth rates of plants? I'm only asking because i hate to assume anything, and im interested in giving the process a trial run this year on my blackberry & raspberries, just to see if i can work through the steps properly.


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## grapeman

I use base 50 as do most grape growers I know, but some use 55- not sure why.

There is a ton of information on services like that that you can find a use for.

Rating the vines isn't bad once you get used to the scales. The following link shows the modified Eichorn Lorenz scale I use. Pictures are on page 108 along with descriptions. I rate the vines once a week. I can rank 300 vines in about an hour or 5 per minute. I make up a scoresheet I use ahead of time and enter them in. Since we have a 25 variety trial, it is more involved. You could just go along and rank a representative sample. I use a seperate column in the spreadsheet to show when they hit certain landmark dates, such a budbreak, capfall, berry set and veraison. Then you can match them up with gdds for that date. 

View attachment EichornLorenz.pdf


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## Randoneur

I do think it has a lot of potential. Here in TN, I'm wondering if an upper threshhold may also be needed.


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## SLOweather

Oh boy! Fresh information for our WeatherElement project. Time to do some more research!


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## grapeman

The upper limit is 95 degrees above which vines and most other plants stop growing, but generally is not calculated in. I think some hot areas do figure it in somewhere though. There is some information on gdds and phenology stages in France and other regions, but such information is currently scant.


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## Deezil

Deezil said:


> Along with having the Base Heat Units, they also track something called Bloom Units or Bloom Heat Units. Any idea what this is and how it relates?



Any ideas on this?
According to AgWeatherNet, my area has seen 1.74 Base Heat Units, but 113.10 Bloom Heat Units, accumulated.


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## Deezil

Found an article of interest.. Pages 6-end is some interesting info, even though its a bit outdated now. Some of it is from '97-'01 though.. Japanese beetles are on the very last page, nothing else was really recognizable right off the bat but a better trained eye might see more.

http://www.entomology.umn.edu/cues/Web/049DegreeDays.pdf


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## SLOweather

Back to Growing Degree Days... Is there a threshold/date where it is reset to zero for the next growing season?

If the day's number is < 0, is it subtracted from the previous accumulation or assumed to be equal to zero? (min temp 40 + max temp 50 / 2 = 45 which is less than either 50 or 55)

Never mind... Just found a decent reference in Wikipedia...


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## Deezil

If the days number results in something less than zero, its equal to zero. 

And i was in the process of trying to phrase the other question you asked about the beginning/end of the accumulation & it makes sense to me that you just go by the calendar year. As the results are taken, i would think for most people, a majority of the GDD's would appear in a clump in the middle of the year (march/april - october/november).. 

For those farther south, the beginning and end marks might be a bit fuzzier or blend season to season if you never run out of GDD's at all - in that case i'd just go with the calender year. Someone correct me if i'm wrong, but this is how i understand it. (time to see if i DO understand it  )


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## SLOweather

Thanks for posting this. I'm reading it and playing with January's daily hi/low data from one of my WeatherElement sites imported into Excel to get a feel for how the differing formulas work. 



Deezil said:


> Found an article of interest.. Pages 6-end is some interesting info, even though its a bit outdated now. Some of it is from '97-'01 though.. Japanese beetles are on the very last page, nothing else was really recognizable right off the bat but a better trained eye might see more.
> 
> http://www.entomology.umn.edu/cues/Web/049DegreeDays.pdf


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## Randoneur

I believe most us April 1 as the starting date for accumulation.


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## SLOweather

I back-tested January's data from my main weather station, using 2 of the growing degree days methods, and 2 modifications of my own.

For January 2012

Average Method per the PDF: 193 GDD
Modified Average per the PDF: 298 GDD

The Average Method is really just the mean of 2 samples, so I decided to try using the real mean temperatures for each day as calculated on my server, based on about 2,800 samples per day. That worked out to 123 GDD (A poor match for the 2 accepted methods.)

Then, I used all of each day's readings to calculate a Modified Average style GDD using the temperature of every half minute sample which was > 50 degrees F, subtracting out the 50 F base value, and then converting what amounted to "Growing Degree Half Minutes" back to GDD.

That value came to: 279 GDD which is a good match for the 2 accepted methods, but I believe more accurately represents the actual growing conditions that month (considering the short hours of daylight etc).

I haven't back-tested the Modified Sine Wave method yet.


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## Randoneur

I've used the data tables on this site for several years worth of growing records. http://uspest.org/US/


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## ncwine

On iPhone there is actually an app called growing degree days takes all the calculating out of the equation and pretty spot on accurate. 


Sent from my iPhone using Wine Making


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## montanarick

Randoneur said:


> I do think it has a lot of potential. Here in TN, I'm wondering if an upper threshhold may also be needed.


My literature search and discussions at recent Montana Grape and Winery Assoc. Conference shows a 86°F upper temp limit


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