Growing Degree Days, why not widely used?

fabaceae_nativeMarch 10, 2010

Hi folks,

I never bothered to think about something called Growing Degree Days (GDD) before, until I saw mention of it for ripening requirements of pawpaw.

After looking into it more, it is THE climate data that I would find most helpful for predicting what the gardening potentials are in a given area. Over the years I've found the Hardiness Zone concept is oversimplified, and useful mainly with ornamentals (where flowering times, ripening needs, and days to harvest are of no importance, but only whether or not the plant will survive the winter). The Heat Index is also useless, if it does not take into account nighttime temps (as GDD does).

I live in a high-altitude semi-arid climate that can experience some significant summer heat, but also has cool nights. The result is that it takes most garden plants (and trees and shrubs) much longer to ripen fruit, and the days to maturity listings on seed packets are usually way off. Many varieties of tomatoes, chiles, melons, and others simply won't ripen fruit even 4 or 5 months after being transplanted out to the garden.

So, GDD would be very useful for gardening (currently it seems to be most used in insect pest management, where temps above 50 degrees or some other threshold are so important). The problem is that GDD is not commonly talked about in relation to plants. If seed catalogs listed ripening times in GDD for different crops, it would make choosing varieties for many of areas of the country much easier.

Anybody out there know of any work done towards this end? Are there websites which list GDD requirements for different plants? I found some sites that generate GDD maps for different regions, although nothing as glossy and complete-looking as the USDA Hardiness Zones.

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Very interesting. Thank you, Fabaceae Native, for introducing the topic. I would like to see gdd information expanded, for as you say, this would be as important as knowing your zone in order to avoid wrong plant selection. I wonder where the amount of sunlight for photosynthesis fits in. After a little research I found the University Extentions in South Dakota, Montana and Spokane provided good gdd information for grain growers, but couldn't find anything for the small gardener. There is an Excel spreadsheet provided, if you want to set up your own data. I would use native Mesquite as a model as it seems to be the most reliable indicator for when to safely leaf out and go dormant in this area.

    Bookmark   March 10, 2010 at 2:38PM
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I looked up a lot of info on gdd's, and it seems pretty complicated for the average home gardener. I also wonder how applicable it is for low elevations in the Southwest, because all of the temp ranges I saw stopped at 86 F, while many of the subtropicals we grow here don't get cranking until the temps are over 95°. Many other plants just poop out at those temps, and gdd's apparently don't take that into account.

Still love the Sunset Zones!
Kevin : )

    Bookmark   March 15, 2010 at 1:44PM
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Thanks for your viewpoints. This is why I love this forum... always something I had not thought about before.

Kevin: It is fascinating that you find the Sunset Zones so useful, they are not so useful here. Just another example of how things differ so much from place to place. It seems that outside of lowland California and Arizona, the Sunset system loses its appeal. For example, much of New Mexico, our fifth largest state, and a very diverse place, is thrown into the same zone, zone 10. I find myself having to fall back on the USDA Hardiness Zones (not my favorite system either) to make sure.

A final word on GDD. I'm not suggesting that it could take the place of a plant hardiness system, its a different thing entirely. But it doesn't have to stop at any particular high temperature either, that's just dependent on the biology (insect or plant) of whatever the calculations are intended for. It also does not have to be so complex, just like any zone system it could be separated into ranges and placed on a nice color-coded map. Just like the Sunset Zones, they take into account winter lows, humidity, air masses, etc... that you don't even have to know about when looking at the map.

The reason that the GDD idea struck me is that it explains all the ripening requirements in a way that no other system does. I would just love it if fruit and vegetable varieties listed their GDD requirements, and we could choose what to grow in a better informed way.

    Bookmark   March 16, 2010 at 10:51AM
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I like the Sunset zones because they dont try to lump Phoenix in with Tacoma, WA, and northeastern Florida. However, you're absolutely right about zone 10! Arizona has the same problem, with "zone 10" actually being a mosaic of only loosely related individual climates. About the only thing they have in common is that you can't grow saguaros there. : ])


    Bookmark   March 17, 2010 at 3:47AM
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Kevin, to add more confusion to the mix, Las Cruces, NM, also listed as Sunset Zone 10, has many large Sonoran Saguaros that are doing great (understood that we could lose them in a record cold year). I have had a Saguaro, which I bought in Tucson 10 years ago, in the ground since then. So far so good. Just to be on the safe side, we also planted the more cold tolerant "Argentinian Saguaros", Trichocereus Terscheckii. Those guys went from 2-3 foot specimens to 8-10 feet in 8 years.

I find that you almost need to take your own local weather data and take into account elevation, wind protection, aridity of your site. USDA and Sunset zones, as well as heat zone data are most useful to me in Las Cruces, but that is probably because we have mostly native and arid adapted plants, so growing degree days would be less useful in our setting.

    Bookmark   March 18, 2010 at 9:07PM
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Thanks for that interesting bit desertlvr! That's neat to hear that Saguaros are being planted quite a bit in New Mexico (even heard of some old ones in Albuquerque on this forum!).

I totally agree with your belief in creating your own zone designations from all the different data. I've more or less done the same where I live.

In terms of GDD data, I guess I should stress again that
it does not necessarily inform about what will survive in a given area, but rather what will mature a crop (of fruit, seed, buds, etc...). I see GDD info as being useful primarily for those growing edible crops in cooler (by GDD calculations, not simply by daytime summer highs) areas. I can certainly see why it does not much apply to hotter climates or to purely landscaping. Maybe I should take this to one of the edible forums?

    Bookmark   March 19, 2010 at 11:06AM
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My ignorance is showing again! I had no idea that Sonoran saguaros could be so cold tolerant!

Live and learn.
Kevin : )

    Bookmark   March 20, 2010 at 1:38AM
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Hello Southwesterners. I am new to gardening - had taken it up furiously self taught in Maryland. We moved to Qatar - in THE gulf - and I am trying to continue my new hobby/passion.

Do any of you know of good reference sites/books ANYTHING - that can help me in this DESERT climate. Already we are into the 80s in the days with nights perhaps getting down into the 70s. Sun is FULL and strong.
The good news is that I've discovered the wonderful aroma of Frangiopini (spelled wrong I'm sure) and Gardenia and Jasmine.
Also trying to grow CITRUS.

Garden centers do exist - however the gardeners are mostly from Syria and there is a languag barrier. (Arabic)

Also - the garden centers are fairly basic and there is not much in the way of fertilizers or pesticides.

I was fully organic in Maryland - but that is not an option here. The bugs/pests are over abundant. Also I'm growing most things in pots....

Any ideas on sources/resources????

    Bookmark   March 20, 2010 at 3:07AM
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It sounds like Desert Gardening, Fruits and Vegetables by George Brookbank is a good place to start. Desert Landscaping by the same author is another possibility, though some items of the plant palette may be impossible to get there. You may have to rely on local sources for ID and treatment of local pests and diseases, too. Remember that there are lots of organic gardeners in SE California, S Arizona, and S Texas, which are all just as pestiferous as your new home! At the very least, I recommend looking into Integrated Pest Management.

Best of luck!
Kevin : )

    Bookmark   March 21, 2010 at 8:06PM
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I certainly agree with Kevin about the resources for the US Southwest. I imagine the hottest, driest parts of SE California and SW Arizona (around El Centro and Yuma, respectively) might be most similar to where you are?

Also, the Permaculture movement has tons of useful information for desert areas (this geared more toward edible gardening, farming, sustainability), you might want to check out the Permaculture Design book by Bill Mollison.

I also would be very interested to know if they use pigeon guano as fertilizer. I've read about how important it was, (and probably still is for people wanting to grow organically) especially in that part of the world. I've seen photos of huge ancient adobe dovecotes that house pigeons specifically for fertilizer. In any event, it is apparently one of the best organic fertilizers, has high NPK values, and can be applied directly in its raw form (in other words, it won't burn plants like chicken manure, even when its fresh). Don't know much about the pest control side of it, but I'm sure there's loads of info out there...

Well, good luck to you, I'm sure you'll enjoy being able to choose from a wider variety of plants than in Maryland, at least based on the milder winters. I know desert gardening can be difficult, but with the right planning, it seems a whole lot is possible.

    Bookmark   March 22, 2010 at 10:51AM
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I also amazement how applicative it is for low elevations in the Southwest, because all of the worker ranges I saw stopped at 86° F, while many of the subtropicals we farm here don't get cranking until the temps are over 95°. Numerous added plants honorable simple out at those temps, and gdd's seemingly don't jazz that into record.

    Bookmark   April 15, 2010 at 10:44AM
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jilldotchom - Any gardening guides aimed at Phoenix would get you started on techniques. Mulching, composting, and deep watering are your friends.

As for growing things, okra loves heat, as do eggplants and some chili peppers.

Take a local with you to the nursery if you can.

    Bookmark   April 16, 2010 at 11:13AM
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I highly doubt mother nature works off GDDs. Some kooks like it for some reason.

    Bookmark   April 23, 2010 at 1:33AM
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Suzi AKA DesertDance


I live in the Southern California desert, about 4 hours from Phoenix. Same climate! I love to make wine, and decided I needed my own vineyard. In the desert? Are you kidding?

I had to do a lot of research when deciding I wanted to grow wine grapes here. There are miles of vineyards of table grapes here, and I couldn't understand why no wine vineyards.

So, being very determined, I was forced to find out our average temps day and night, and only grow varietals that thrive in the heat! My search took me to the Greek Isle of Santorini (a close twin to our temps), and I ordered the varieties that grow there and also in other searing climates throughout the world. One of my most vigorous and happy vines is from Portugal. It is called Toriga Nacional. They make port from it, and it grows in searing heat, high winds, and rocky soil. Sound familiar?

My little backyard vineyard is 2 years old now, and I keep adding to it. Learning to make cuttings.

It's fun to accept a challenge, and run with it! You can find your average temps on yahoo weather, and then you need to research whatever the growth requirements of the plant you are wishing for, and see if it will fit!

My grandma always said, "Where there's a will, there's a way!

Good luck with all your planting dreams!

    Bookmark   April 29, 2010 at 10:44AM
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Posted by fabaceae_native . . . If seed catalogs listed ripening times in GDD for different crops, it would make choosing varieties for many of areas of the country much easier.

I'm with you on that, Fabaceae_Native! But, seed companies do list GDD on their farm seed. You can see many sweet corn varieties listed by North Carolina State U, have GDD information. Those numbers appear on the tag of every bag of seed, for all sorts of crops.

The horticulturalist calls this "crop phenology" and it amounts to predicting growth stages like ripening based on warmth and time. Here is a crop scientist talking about the crop phenology of Arizona cantaloupes and "heat units" or GDD's. This is a Big Deal since there's $87 million in that AZ crop!

Where I find most information about GDD's is in the north. Canada is fairly interested in maturing crops in farming regions with short growing seasons. That shouldn't come as any surprise.

You can also find a fair amount of information from North Dakota State U and other northern land-grant universities. Here is some from Montana State U. Of course, you have to be interested in growing barley or chick peas to make much use of it.

Posted by fabaceae_native . . . I live in a high-altitude semi-arid climate that can experience some significant summer heat, but also has cool nights. The result is that it takes most garden plants (and trees and shrubs) much longer to ripen fruit, and the days to maturity listings on seed packets are usually way off.

You live in the Southwest by definition just as I live in the Northwest by definition. With less than 20" of precipitation (most falling as snow) and nearly every day of the growing season ending with less than 20% humidity, my area doesn't experience exactly what most folks think of as a "characteristic" Northwest climate.

I spend a lot of time in the Rocky Mountain forum and the gardeners there tolerate the fact that I garden at only 2,000 feet elevation because the growing conditions are very similar.

Anyway, Growing Degree Days have become a very, very common issue in farming commodity crops. You can even buy crop insurance pegged on your crop's requirement and how many GDD's the Weather Service count through your growing season.

It is time for the seed companies to provide this information for all varieties including those grown by gardeners.

Of course, we can lean on our hoes and squint knowingly at the horizon and take a guess based on vast experience. And, I'm pretty good at that after a lifetime in agriculture and scratching around in a home garden. But, ag is probably the most thoroughly researched scientific field and the corporate food industry tolerates little squinting at horizons. If they want to sell seed to us, it's time for the Burpee's, Park's, and Jungs' of the world to move on into the 21st century.


    Bookmark   May 22, 2010 at 4:51PM
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Thanks Digit!

That's just the type of response I was after. I was beginning to think I was crazy for recognizing the importance and lack of accessible info on GDD, but obviously you've reaffirmed my initial intentions with this thread.

Thanks also for the links...

    Bookmark   May 24, 2010 at 3:04PM
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