Freezing degree-hours

[mjff]

What do you all think of this?  I have started collecting data for my area regarding not only the low temperature, but the amount of sub-freezing temperatures experienced.  

Obviously, just about any palm could be walked through a deep freeze at -10 and survive with no damage, while I imagine many would be toast if you subjected them to 2 weeks of even mild freezing temperatures continuously.

To capture both the severity and length of the freeze I am taking the hourly temperature observations for my area, then starting when temperatures drop below freezing and continuing until they rise above freezing, subtracting the actual temperatue from 32 degrees each hour, and adding up the total.  For example (actual data from last night and this morning):

Time         Temperature     Freezing Degree-Hours

7:51 PM        34                             0

8:51 PM        30                             2

9:51 PM        28                             4

10:51 PM        28.9                          3.1

11:51 PM        25                             7

12:51 AM        24.1                          7.9

1:51 AM        24.1                          7.9

2:51 AM        21                            11

3:51 AM        19.9                         12.1

4:51 AM        18                            14

5:51 AM        18                            14

6:51 AM        16                            16

7:51 AM        18                            14

8:51 AM        25                              7

9:51 AM        37.9                           0

10:51 AM        50                              0

Total                                               120

At this point I am using the airport data for this, but plan to get a weather station that will report to my computer setup here at the house, since on cold mornings the airport is generally 2-3 degrees colder since it is located at one of the lowest spots in the area.

[bubba]

This sounds great.Where is your location in Texas?I have a weather book on Florida that reports over a large section of coastal South Florida 100 hours or less over a 30 year period.

[richtrav]

I have seen a freeze severity index for Florida citrus described in a book before. It is called the Georg Index and is calculated by the following:

multiply the number of hours at 24 or below by 1

multiply the number of hours at 23 or below by 2

multiply the number of hours at 22 or below by 3

multiply the number of hours at 21 or below by 4

etc, etc

add all the of the numbers above and divide that by the minimum temperature to derive a severity index.

For arguments sake we'll say you had 8 hours at 24 or lower, looks like about 6 at 23 and 22 or lower, about 5 to 5.5 at 21 or lower 4 to 4.5 at 20 or lower, probably 3.5 hours at 19, 3 at 18 or lower, maybe 1 at 17 and .5 at 16 (that's just guessing). Assuming your minimum was 16, the formula would look like this:

24: 1 x 8 = 8

23: 2 x 6 = 12

22: 3 x 6 = 18

21: 4 x 5 = 20

20: 5 x 4.5 = 22.5

19: 6 x 3.5 = 21

18: 7 x 3 = 21

17: 8 x 1 = 8

16: 9 x .5 = 4.5

That sums to 135. Take 135 and divide by 16 and you get an index of 8.44

Note that this index is for citrus in Florida and does not take things like hardening etc into consideration, but it is an interesting way of measuring the severity of a freeze. The starting temp of 24 sounds a little low for palms but a similar method could probably be employed (though it still doesn't take hardening into account)

For anyone who's gotta know, it came from A History of Florida Citrus Freezes by John Attaway, Chapter 21.

[mjff]

(bubba @ Dec. 23 2007,12:33)

QUOTE

This sounds great.Where is your location in Texas?I have a weather book on Florida that reports over a large section of coastal South Florida 100 hours or less over a 30 year period.

I am in San Angelo, TX which is about midway between Dallas and El Paso.

I should note in case it wasn't clear that I'm just looking at our most severe events each winter, not giving a total for the whole season, to establish how much cold the palm species I am growing can take.

BTW, I 'm a Gator too.  I received a degree from UF in 1985.

[bubba]

Martin,Sounds like a great spot.As to our Gators,wait till next year!

[mnorell]

I think it's a great idea to develop something along these lines, as of course length of a freeze is very much a factor...however as an example, we've already had two freezes here at my place, one was seven hours, with a low of 30.1, and last night we had four hours, with a low of 31.0. This would add up to a decent accumulation, though damage has been slight except to very exposed bananas and a few others.

The problem is that one couldn't equate, say, 4 hours at 31 and 3 hours at 30 (=10 using your proposed scale), with, say, 1 hour at 30, 1 hour at 25, 1 hour at 31 (also =10). The plant damage no doubt would be far more significant with a low of 25, even with the shorter duration. I still have several Heliconias that look great (one in bloom), as well as rather exposed Dypsis lutescens unscathed, plus Calatheas, Spathiphyllum, Pinanga coronata, containerized Licuala elegans, etc. that look great, though usually with at least some slight canopy overhead. 25F even for a short time would no doubt paint a very different canvas, even for plants under canopy.

Perhaps a graded multiplication factor could be devised, but ultimate low temperature is certainly a factor, so I think it would be difficult to make a single scale work. I agree, though, that average temps are quite significant; and of course repeated nights even below 35-40F take their toll on many borderline tropicals. I think defining zones of sensitivity and severity, such as 33-36, 28-32, 24-27, 20-23 and then giving hours experienced within each zone could then be used in conjunction with threshhold sensitivities for given plants (e.g. the general direction being attempted with the 24F citrus example given previously) for a pretty useful scale. Food for thought...

[_Keith]

It is not just about the temp.  We get worst damage here during frost, where the temp may not even drop below freezing, than we do in many of our freezes, which are most times fairly mild.

As to football, for some of us, this year is not over yet, then we will see about next year.  

[bubba]

Keith,Are those fine looking bananas at your place still OK because I know you guys have skirted with 32F.I hope they defied that battle because they were certainly fine looking.As to another important subject for any man growing up in the city of the"Red Stick",all I can say is Geaux Tigers and I am looking forward  (love your colors)to being a Bengal on Jan.7,2008.As to my Gators,I probably should have said I can't wait till next year. We do,however,have a little ordeal with the Maize and Blue in Olando on Jan.1,2008 that I will admit to more than a passing interest regarding.

No question frost is an ugly thing when you see it on a prized quasi-tropical specimen.Merry Christmas!

[mnorell]

Yes, frost is definitely a huge factor, and one that has to be addressed. Chamaedorea cataractarum has burned where it's exposed to open sky despite the slightness of these frosts, but I know from experience they'll take low twenties without budging if under good canopy. Meanwhile, fairly exposed Dypsis lutescens has not yet shown any burn. But there are a number of plants that seem to be affected whether or not they get the frost or not. In particular, several Costus gingers, even those that are not normally winter-dormant, have turned quite ragged and the tops are dying back, all under good cherry-laurel canopy. This next to Alpinias, Hedychium, Calathea, Heliconia, aroids etc. that still look great.

The awful thing about watching these light freezes come through is that the first one will take out the top layer of foliage, then you lose that layer and the next one goes down, etc. I don't know about yours, Keith, but all my tall bananas look like hell. Still growing very slowly, and I have lots of green foliage down below, but it's shredded-paper-bag syndrome up top. The question is whether I should trim the leaves off as they're still providing a (little) bit of frost-protection for the plants beneath. I'm keeping a Musa coccinea and a Heliconia latispatha 'Distans' looking just about perfect by just loosely constructing a framework of stakes and draping frost-cloth or a blanket overhead. Not even extending to the ground. It sure works, but of course wherever the cloth touches the foliage there's burn...

Of course I don't know if I'll be so positive when the temps fall below the 28-30F range. It's kind of crazy I'm still zone 10a as of right now. I think it must be a record here, as usually it's been down to 27 or maybe a little lower by now. It has been 25F twice at the airport out in the countryside, but we've done well downtown and near the river. Brugmansias and Hibiscus still blooming in slightly protected spots. As I recall you've just barely hit 32 so far, but I assume that down in Cajun Country you've normally usually had several brushes with the 30-degree mark by this time each year, am I right? Let's keep the fingers crossed for that warm winter they've been promising! (knock wood)

[mjff]

(mnorell @ Dec. 24 2007,23:41)

QUOTE

I think it's a great idea to develop something along these lines, as of course length of a freeze is very much a factor...however as an example, we've already had two freezes here at my place, one was seven hours, with a low of 30.1, and last night we had four hours, with a low of 31.0. This would add up to a decent accumulation, though damage has been slight except to very exposed bananas and a few others.

The problem is that one couldn't equate, say, 4 hours at 31 and 3 hours at 30 (=10 using your proposed scale), with, say, 1 hour at 30, 1 hour at 25, 1 hour at 31 (also =10). The plant damage no doubt would be far more significant with a low of 25, even with the shorter duration. I still have several Heliconias that look great (one in bloom), as well as rather exposed Dypsis lutescens unscathed, plus Calatheas, Spathiphyllum, Pinanga coronata, containerized Licuala elegans, etc. that look great, though usually with at least some slight canopy overhead. 25F even for a short time would no doubt paint a very different canvas, even for plants under canopy.

Perhaps a graded multiplication factor could be devised, but ultimate low temperature is certainly a factor, so I think it would be difficult to make a single scale work. I agree, though, that average temps are quite significant; and of course repeated nights even below 35-40F take their toll on many borderline tropicals. I think defining zones of sensitivity and severity, such as 33-36, 28-32, 24-27, 20-23 and then giving hours experienced within each zone could then be used in conjunction with threshhold sensitivities for given plants (e.g. the general direction being attempted with the 24F citrus example given previously) for a pretty useful scale. Food for thought...

I'm not so sure that 3 hours below freezing with a low of 25 is worse than 7 hours with a low of 30.  That's the point of doing this, to answer those questions.

Regarding frost, the air where I am at is usually so dry that frost is pretty rare, and the palms I am growing aren't exactly tropical species.  Those of you that are growing the more tropical type palms probably need to worry about frost a little more.  

The least hardy palm I'm growing is a Queen, and remakably it has taken 16F and 18F with no damage apparent yet.  It has only been in the ground about two months now, and has a rootball the size of a basketball on a 15' (6'CT) tree.

[ruskinPalms]

mjff, I think using your freezing degree hour method is pretty straightforward and does give one an idea of a freeze event's severity and how many "freeze units" a palm experiences. An index can still be created by deviding the freeze degree hours by the ultimate low but I am not sure there is any need to do that. The more simple the method, the more likely it is that people will report some kind of objective data in the freeze forum. I think I am going to report using this straightforward freezing degree hour method. mjff just experienced a 120 FDH freeze. I am forecast for an advective freeze right around 32 (we'll just say 31.99 so the math does not get too fuzzy ) this wednesday night. I will report the FDH that my palms experience if I actually have a freeze. If I were to have a temp around 31.5F for 4 hours during this event then my palms would experience a whopping 2 FDH freeze.  Again, I feel using this simple method captures the duration and severity of a freeze just fine. Thanks for the idea mjff!