I've read that in order for a citrus plant to achieve it's maximum cold hardiness, as allowed by the rootstock, it is necessary for the tree to have gradually seen reduced temperatures somewhat like the a peach needs a certain amount of chilling hours. And like stone fruits, there are penalties if the temperature rises too high for too long resulting in the tree having to again gradually back into the colder temps. If this is true, what is more important the soil temp or the air temp. Not much you can do about the air temp but a lot can be done to influence the ground temp such as covering it in a dark weed block to warm it up when unusually cold temps are coming too quick or watering the ground near the peak daytime temps to cool it or to try and prevent premature bud break in the spring. I realize that we are only talking about small changes but sometimes a fraction of a degree could be the make it or break it point. Any thoughts.

Stem/leaf/bud temp is the important thing, which is indirectly affected by air temp (in still, clear conditions, the tree will get colder than the surrounding air; with wind or cloud, it may be the same or warmer). Soil temp has little or no effect. In our commercial citrus areas, soil temp never goes below 70 F more than an inch or two deep, and will likely stay above 75 F 6 inches deep. Yet trees on a hardy rootstock will become quite nicely hardened, with cold exposure to the top.

Thanks Malcolm,

So the tree shuts down at the tips and the roots sense the reduced flow and react by decreasing their supplies which in turn causes the tips to stop growing and the cycle gradually shuts the tree down for winter.

Is that how it works?

OK if you can get up off the floor and stop laughing long enough I'd appreciate your comments.

Thanks,

I would not say roots "decrease their supplies." A citrus tree still needs water and mineral nutrients through the winter. I presume (but don't have proof) that the roots likely do reduce their output of cytokinins (which would stimulate top growth).

So is the average air temperature that begins inducing cold hardiness in a citrus tree <64F. - Millet (864-)

Malcolm,

So I'll exchange the word "supplies" for "cytokinins" and I'll have somewhat of a picture of what happens. After reading a bit about cytokinins I found that it is still a very active area of research and it is by no means an established exact science. Thanks for helping me through that.

Millet,

Have they come up with how many hours <64 is required to set the tree and how much or how long at higher temps will undo the set. I use the word set as I'm not sure if dormant would be a correct term. I guess that different root stocks react differently too or else we wouldn't have "Cold Hardy Rootstock" right.

Not sure on how many hours for cold hardiness. Probably depends on the variety. To unset, it would only take a couple days. However, to insure that a citrus tree sets a good crop of fruit, chill hours between 600 - 800 are normally required. If the tree set a heavy crop in the preceding year the tree will do best with 800 chill hours. - Millet (864-)

I'm assuming the temperature required to count as chill hours is the less than 64 and not the lower number required for stone fruits as we only get about 450 chill hours using the stone fruit number. Central FL only gets about 200 - 250 hours. Good thing as I doubt we every get 800 hours at the stone fruit temp. I'm not sure what that number is. Are there charts showing Citrus chill hours like there are with the stone fruits?

Our winters here are crazy. We definately do not have a defined season like you would. We can be in shorts sweating one day and the next be wearing coats and gloves expecting a freeze. It wrecks havoc on plants.

Yes, chill hours are for temperatures less than 64F. The chill hours are accumulative through out the fall and winter, and do not need to be in a row. During the fall and winter I keep my greenhouse night time temperatures between 48 to 55F. This gives me chill hours above 1000. Therefore, the spring bloom is very heavy. Good luck with your endevor. - Millet (863-)

Found this in my bookmarks.
http://aggie-horticulture.tamu.edu/stonefruit/chillacc.htm
Found this on Florida chill hours also
http://www.imok.ufl.edu/misc_news/peach_field_day06/growing_citrus_and_peaches.ppt

I think we may be defining things differently. Florida research has shown that 55 F is the "magic temperature" here, which causes dormancy and begins hardening -- higher temperatures don't seem to do much.

Also, we presume chilling is unnecessary for citrus; certainly our main commercial citrus belt gets fewer than 200 chill units per year, and more than half of it gets fewer than 100 chill units (granted, we define a chill unit as an hour below 45 F for deciduous fruits).

Just from observation over a lifetime in the business, I've never noticed that our winter temperatures have any significant effect on fruit set or the next year's crop size, except, obviously, for the hard freeze years, and that's not a "chilling" effect.

Dr. Manners I find you post very interesting, especially as compared to UF's Dr. Albrigo.. The information that I posted above comes from the Flower bud Advisory for Florida. This is the University of Florida's Dr. Gene Albrigo. According to Dr. Albrigo, Flower Bud Induction begins at temperatures below 68F. Above I posted 64F, but the correct temperature is 68F. For the last 10 years in Florida, Dr. Albrigo's advisories have shown Florida chill hours in the range of 500 to 850 hrs. He puts out monthly chill hour up dates to Florida citrus growers, keeping them advised of how many chill hours have already been accumulated. Below is a short example taken from one Dr. Albrigo's up dates.

Here is an example of one of Dr. Albrigo's advisories (this particular one was for the crop of 2004-2005. I have not transcribed the entire advisory, as it would be too long, but have shown excerpts from the beginning to the end of the report period.

.......So far this Fall, cool weather has reached 150 to 200 hr <68 degrees F., from southern to northern citrus districts. The National Weather Service (NOAA) predicts that there will be about 80-90 addition chill hrs below <68 degrees F. during the next 8 days..............accumulated cool weather has now reached 270 to 400 hr <68 degrees F. from sothern to northers citrus disteicts, respectively...............Up to this week, cool temperature accumulation has been about average compared to the last 4 years.........Cool weather has now reached 320 to 450 hr <68F degrees F. from southern to northern citrus distaicts. NOAA predicts that there will be about 80 additinal hours below 68F during the next 7 days. This means that all areas will still be in the low to moderate range of flowering potential through this week, but the accumulated hours will be past the midway point in that category for all citrus areas and some northern areas should be near the break pont of 600 hours.........

Dr. Manners, eventually by the end of his monthly reports, the cool hours reached 733 to 1015 for southers and northern citrus areas that crop year.

I have his monthly advisories all the way back to the Florida crop year 2001. Anyway, thank you for your interest in this subject. Your comments/remarks are very welcome.

Millet (863-)

Interesting! I have never heard of this. Certainly the concept of a chill unit is fundamentally different from that of deciduous fruits, and I wish he had used a different term. Now I wonder how one deals with citrus in the truly tropical world, where temps less than 70F never, ever happen, yet large crops can be produced?

You know much more about citrus then I, but in tropical areas I have always read accumulated drought hrs instead of chill hours. - Millet (863-)