As a result of thinking about the cold weather protection for my in ground citrus and in part by the heated discussion here: link, I’ve been thinking about the general factors that go into affecting the ability of citrus to handle cold weather from a plant physiology viewpoint and thought it would be fun to start a discussion on the subject. Most of this post based on research literature so it would be great to see how these research observations match with peoples personal experiences.

The first factor, that I’m sure every one is aware of, is cold-acclimation or cold-hardening of the citrus. This occurs when citrus is exposed to cool <50F, but above freezing, temperatures for several weeks before the first freezing temperatures are observed and improves the plants ability to tolerate sub-freezing temperatures. The simplest thing do do here I would think would be to try and prevent the exposure of the Citrus to below freezing temps in late fall/early winter, to ensure the tree has had time to properly cold-acclimate.
Here is an interesting paper on the subject: Paper 1

Once cold-acclimated, there are two primary mechanisms that citrus use to survive sub-freezing temperatures:

1. Freeze-Avoidance (i.e. supercooling). This includes physiological changes to the plant brought about by cold-acclimation that result in a lowering of the critical temperature at which ice begins to form in the plant tissues. Apparently (and interestingly) this temperature is relatively similar across most varieties of citrus as mentioned briefly here: Paper 2

2. Freeze-Tolerance. The ability of the plant to tolerate some degree of ice formation within its tissue without dying. From my understanding, this can vary quite widely; up to 7C (11-12F) among common citrus varieties. (This is discussed in the in the preceding link).

The general rule of thumb is to minimize the duration of time that the plant sees temperatures that are cold enough to cause freeze damage. But what about multiple freeze events and sub-freezing temps that do not result in ice formation in the citrus tissues?

Below are some interesting excerpts I found in: Paper 3

“Multiple freezes during one year are of additional concern based on observations that slightly freeze damaged citrus trees are likely to be severely damaged under similar freeze conditions. This increase in damage may be partly due to earlier ice formation in the tissues, which increases ice duration, which increases potential for greater damage. Freeze damaged citrus do not cold acclimate nor supercool as well as non-damaged trees (Yelenosky, unpublished data). Seemingly, the greatest freeze tolerance in citrus is before the first freeze. Once trees are freeze damaged, their ability to cold acclimate decreases regardless of favorable cold acclimating temperatures between freezes. It is not known how long it may take for once-freeze-damaged trees to regain their pre-injury potential to cold acclimate.”

and

“Freeze damage is largely a function of ice that presumably causes dehydration and membrane damage which disrupt the function of cells and tissues to sustain life. Without ice, there is no significant damage within the limits of Florida's freezes, except possibly for chilling injury of grapefruit.”

and

“Expectations of first freeze damage possibly could be lowered by 1C (2F), from -2C (28F) to -3C (26F), without much risk except for flowers and succulent new growth.”

Do the above statements imply that citrus can tolerate prolonged time periods below 32F (but above 26-28F) so long as no freeze damage to the trees occurs (i.e. supercooling of the citrus tissues is in effect, but no ice crystal nucleation occurs)? I have found little to no data on how long and how often cold-acclimated citrus can tolerate temps that are between 32F and the point at which ice nucleation becomes a concern (26-28F). The physical chemistry behind the phenomenon of freezing point depression suggests that the duration of such conditions should not matter, as long as the temp does not drop below 26-28F, the plant tissue should not freeze. I wonder how the theory matches with observation?


It would be great to additional input from those w/ more knowledge than I on this subject.

-Scott


Also, does anyone have a pdf copy of: Yelenosky, G. Cold hardiness in citrus. Hortic. Rev. 7, 201-238 (1985).
It is mentioned frequently in many of the research papers I have read. However, this article is not freely available, and I can’t access it through my work e-journal database.

Quiet in here...


Oh well...

Very interesting subject and also one with much, much more questions than answers. The lower one go (with temperature) the bigger uncertainity is.
The topic where I'm collecting similar information is here:
link

Yes Ivica, there are certainly many unanswered questions, especially since most of the testing/research is focused on traditional citrus growing areas that only see occasional freezes. I have found very little data on how cold-acclimated citrus handle prolonged periods of cold weather.

In particular I am interested in how/if they tolerate long periods of 24 h or more at or slightly below freezing; i.e. 32 to 26F (0 to -3C). From what I have read, freeze damage (via ice formation) should only be a major concern for cold-acclimated citrus when temps drop below 26 or 28F. Apparently it is the ice formation inside the plant tissues that severely reduces the citrus plants ability to survive additional freezes without injury.

With the winter protection I have for my citrus (see here: link ) I believe that it will be difficult for the internal temperature to drop much below 26F even without any external heat sources in my zone 6B/7A location (my tree is surrounded by ~1200 L of water). However, without additional heat, it could result in long periods where the internal temperature is at or just slightly below freezing. Will the citrus be able to handle this? It would be nice to have some idea what to expect before I let my tree experience a long duration just slightly below freezing. My goal is to devise a setup where I can overwinter my citrus without the need for supplemental heating.

Ivica, I think the data from your testing comes the closest I've seen to answering some of my questions. What was the total number of hours your citrus were below 0C in your testing in the cold wave in Dec. 2009? Do you know the total number of hours your citrus were at or below 0C for the entire 2009/2012 winter?

Cheers
Scott

In my experience nothing happened to any of my citrus (even lemons and citrons) for several days at -5°C. Only small spots on external fruits.
It was late in winter so the citrus were already hardened.
It could have been very bad if it happened in autumn.

Sylvain,
Thanks, that's helpful information. Were your citrus below freezing the entire time, or did they get above freezing in the daytime?

-Scott

Only the nights at -5°C, but still freezing in the day.

I am new to in-ground citrus, but not gardening in general. I have so many questions about this topic that I need years of experience for -- even after reading so many informative old posts here!

In the US southeast, we recently had something like four days of rain. It stayed around 50-60s F (day) to 50-40s F (night). Yesterday, the clouds cleared out and the night dropped to freezing or just below (but not all night). I put up cold protection and still I've had no damage.

Today I started wondering if there's a particular concern for citrus after so much rain in the winter when temperatures can suddenly dip. Our patten this year includes many warm days that drop to 29F nights, but many mid-30F nights, too. I learned with other plants that having good hydration in their system can help with cold, but I don't know about citrus. Is a lot of winter rain a help or not or neutral? I will keep observing to give more accounts, but I wonder what others think.

-- Brigid

i think i read somewhere, that moisture in the leafs and temperatures falling below 0 are not good combination for citrus.

Brigid, Adriano,
Here's the most relevant info regarding freezing temps and wet leaves that I am aware of.

https://docs.google.com/viewer?a=v&q=cache:Pfy9685PutAJ:www.fshs.org/Proceedings/Password%2520Protected/1980%2520Vol.%252093/3-5%2520%28YELENOSKY%29.pdf+&hl=en&gl=us&pid=bl&srcid=ADGEESjrJrtA1VQb7okkAwFaTTe--obwHprt16ozMcjMCgXU_tIJan1VswuBE33WowxNrPd4z9ptV7DtEAiHm5UzpiDJ1fyB16MiIDiqlXUbIBQawq7agBvH9iPIf10cIigveW9HcoHy&sig=AHIEtbSGC_bta0XTsIjjsRfFbj8rO5xKzA

Synopsis: If the citrus are cold-hardened, there's not too much to worry about. If the citrus is actively growing, be cautious.

-Scott

Thank you, Radoslav! I have heard of that also, but had no idea it is only good for the short term. So good to know!!

Scott, thank you so much for such information and link! Haha, and the synopsis. I plan to read all the links you've posted on this thread in detail when I have a break.

All the Best,

Brigid

Another useful technique, for protection of citrus trees in open area, against sudden short-lived cold snap, is by creating smoke screen using "smudge pots".
http://en.wikipedia.org/wiki/Smudge_pot
My father used to use this technique to protect apricot and peach trees in the spring, when sudden morning cold snap can easy kill new growth or flower buds.

Unfortunately for the plants, but luckily for public health, this method is no longer environmentally acceptable.
My neighbours would certainly complain if I created a blanket of smoke.
In fact, in the UK it is illegal under the Clean Air Acts of the 1950's which were introduced after several disastrous London 'smogs' which covered the city for many days in winter.