Hi all. Considering the mild Fall we've had, when is a good time to cover inground trees? My two inground large Celeste trees just dropped their remaining leaves. I know it's preferable to allow the trees to off. Thank you for your advice.

I know there has been a lot of discussion about avoiding overheating trees in the spring time, when there are periods of warm weather followed by colder weather, up to and including a late season frost/freeze. You could have the same situation in the Fall, with some early cold weather followed by some late warm weather. I would suspect that if you insulate well enough to keep the cold out, you would also keep the heat out.

You would definitely want to insulate and cover before you get frost or freeze, which can damage the tree.

See http://figs4fun.com/basics_Pruning.html for one person's solution, in Kansas if I remember correctly.

We just had a couple weeks of high 80s and low 90s here in San Diego, so many trees are waking up, not going to sleep. Whole different set of issues.

This is somewhat intuitive (i.e., no systematic data) but I prefer to let the plants experience temps 25-28F so that they have reason to go seriously dormant.  Anytime after that seems OK provided I protect before temps get below 20F.  I figure that if a dormant plant can't survive 20-25F without protection, it won't survive -5 (my typical winter low) with it.  But I wouldn't want to risk <20F without protection.

I suspect that cold wind is more of a risk than cold per se.  So I'd accelerate if you have a forecast of both cold temps and heavy winds.

That generally implies protecting in early Dec, more or less, up here. 

This approach has worked well for me for 3 winters.

I see that "pitangadiego" and I posted almost simultaneously.  If I were you reading these two messages, I'd be confused.  I'm going to try to reconcile the differences.

Let me caution you that "p" has way more experience than me, so you should read what I write with that in mind.

Scenario #1:  Fig plants are experiencing 65 degree F days and 45-50 degree F nights.  The plants are thinking about going dormant but are not yet convinced.  Then suddenly, nighttime temps drop to 30 F.  I agree with "p" that the plants could be damaged.  This scenario would represent a really bad time in San Diego (and lots of other places in late fall / early winter across the South and Southwest).

Scenario #2:  Fig plants are experiencing 55 degree F days and 35-40 degree F nights.  The plants are going dormant.  Growth has stopped cold; leaves are yellowing then dropping.  Then nighttime temps drop to 30 F.  I don't think the plants would be damaged at all.  FWIW, this scenario would represent business-as-usual in my RI (and maybe in your NJ).  This year, for example, we've already experienced light frosts / ~31-32 degree temps here, and the figs I've planted here (the usual cold-hardy varieties) don't seem to care.  

Within limits, the absolute temperature seems less important that the path there.  You don't want growing plants, used to warm temps, to freeze.  But fig plants that are exposed to cold gradually and, as a result, fully dormant should laugh at 30 degrees F.  

I feel very confident that the babies would be fine.  Putting my money where my mouth is, I've got roughly 40 1-yr old cuttings (also without leaves) in SIPs in my driveway, deliberately exposed to cold weather to drive them into deep dormancy.  They've already endured 31F once or twice.  I'm not putting them in the garage until the forecast says 28F or lower.

That said, there are lots of people participating in this forum who have way more experience than me.  They come from PA, OH, NY, NJ, WV, MA, RI, etc.  Let's see what they say.

p.s.  If you haven't done it already, cut back on water.  The pots should be slightly damp, not soggy. 

Fo not forget about the condensation water that can occur under winter protecton. It will make funghi grow, which can harm the buds. And of course there is more humiditiy in "warmer" air. So if you wrap your trees in a mild period, maybe you should consider doing so at nighttime.
In general I would never cover fig trees before the have hab several nights of frost.
Also keep in mind that the trees will never stop to harden their soft green shoots as long as there is even a little bit of sun...even if temps are down most regularly.