This is a picture of a tree that is about 30 miles southwest from where my trees are growing.  The leaves on my trees are gone and many have damage from cold/wind in the form of splitting bark. Meanwhile, the tree in the pic...

~james

Is that fig out in the open, or on high ground? Any surrounding protection like a house or windbreak of some kind?  Even down here by Houston, my figs are now bare. 

James down here is south La. all my trees are bare except one, Latterrula. It's as green as a spring plant. Who knows?

"gene"

To be sure, the climate this fig and my figs are growing in are very different.  This fig is growing in a populated area with a protected by a strip mall to the northwest.  I'm sure that not only provides some windbreak and a small increase in temperature.  I'm guessing if it were in the field where mine are, it would not look the same.  I was just so surprised to see it still green.

~james

I think many folks (speaking in general, not to you directly, James) understimate the crazy microclimates that are caused by asphalt, concrete and masonry. 

Tapla could probably elaborate more on this, but it's my understanding that trees go dormant based on the temperature of the roots.  When your roots are protected by a thick layer of asphalt and concrete - which absorbs heat all day and emits that heat all night - you can create wild temp increases of 15º-20º or more both under the ground and several feet above it. 

Meanwhile, many of us guys at home have our trees planted in open land, with nothing but grass and soil atop the root plane, no nearby masonry or stone walls... or planted in pots with plastic walls less than a millimeter thick, easily allowing cold to penetrate to the roots.

In Atlanta, 98% of my trees have gone dormant.  However, the potted trees I have sitting right up against my house and below the dryer vent (up on the 2nd story) are still as green as can be.  They get heat from the dryer vent and heat emission from being right up against the concrete & granite foundation of my home on the south- and west-facing walls of my home (they're in the corner).

Just for S&G, I took my two-zone wireless thermometer and put sensors on the sides of the pots closest to the house and the pots farthest away (in the same bed) and on a 30º night, the pots next to the house (less than 18" away) never saw temps below 43ºF, while the ones 7' away from the south-facing wall saw the true temp of 30ºF hours before sunlight broke.

Looks like the fig in the picture is benefiting from the solar gain collected by the asphalt surface and the surrounding brick bed border. I also assume that although not pictured it is against some sore of structure and if that is also brick the warmth collected by the exposure to the sun can be enough to create a micro-climate for this plant

Jason,

I know that root growth can occur even when the tree is dormant... or at least they are still ready to grow when the soil conditions are right even if the top of the tree remains dormant (this is one of the reasons why we are encouraged to plant in Oct/Nov instead of during spring.)  I don't remember reading dormancy is based on root temps.  My remembrance is that dormancy begins as the days grow shorter.  In other words trees go dormant in preparation for the cold weather, not necessarily because of it.

I agree with you the difference is that my trees are in a field, without any real protection from the weather other than the T-post that hold up the wire fence.  But  my trees did not go dormant.  They were still green and with leaf when the first freeze came in early Nov and knocked them back.  They started budding out again before the second freeze came a few weeks ago.  When I went out lat week to pull some of the wet mulch away from the trunks of the tree, I found 2 of them were pushing out new suckers. 

It is possible that my thoughts on dormancy are not entirely correct, and there is enough discrepancies on the web to leave some doubt.  It is interesting though that the length of the day in Austin is 1 hour longer (today) than in the Philly/Trenton area.  The length of the day on the first day of summer in Austin is 1 hour shorter than in the Philly area (using 2011 data).  There is only a 4 hour difference in the length of the day in Austin vs. 6 hours in Philly.  I don't know if this is enough (without factoring temperatures) to cause the patterns of dormancy to be so different between the two areas.

~james

James,

Unfortunately, I can only find information suggesting that trees will BREAK dormancy based on soil temps, and that's what searches are turning up.

I am finding a bunch of info on "temperatures cooling, changes in day light/day length", but nothing definitive.

I'm pretty sure that ground temps climbing above 40°F initiate wakeup, so I'm inclined to believe that temps dropping below 40°F would also set in dormancy.

I also found info that suggested the leaves on trees have a built-in "temp gauge" that, given certain air temp, the leaves would send the signal to the plant to enter dormancy.

Not a botanist here...

I know seed break in many varieties is triggered by ground temps also ...

I'm inclined to believe length of day is less of a factor than ground temps.... but I also think that the presence of microclimates and concrete has the largest influence.