Edited for incorrect info...

This post was edited by j0nd03 on Wed, May 1, 13 at 11:24

FWIW,
The National Weather Service uses 36F as their guideline for possible frost. I have seen frost at 40 before, but it depends on more than the absolute temp.Dry dead plants frost easier, than vibrant green plant from my experience, and that actually makes sense in the physics of it all.

Arktrees

arktrees is right. Radiational cooling can cause exposed surfaces to dip below freezing, while the air itself can be as high as ~40F. But typically, at least here, the temp needs to get down to 30F or lower to cause any significant damage, except to the most sensitive plants.

There is a lot of variation is how certain plants react to freezing temps, too. Some can take temps down to the mid 20's with little damage while some hit just below freezing and get hammered. Malus foliage should be fine in the upper 20's. The other fruit tree foliage I have no experience with but a little frost shouldn't bother them much.

Now your tender veggies are a whole other story. Generally speaking, they don't deal with hard frosts well at all. A light frost for a short period usually does not usually pose a major setback for most things. The key word there is usually.

Microclimate really dictates what happens. When our forecast low is 34-35 I can count on a freeze in the 29-31* range unless it is overcast overnight. It always happens without fail. The local weather station will hit the 34-35 while my house gets considerably cooler. The opposite occurrence can also happen in the right microclimate.

If you are using the nearest reporting weather station (not one at your garden location) and 28* was their reported low, that should not be low enough to kill all the green on mature trees. It sound like you may have a colder microclimate at your house which is very common in the country especially in valleys. If that made a 4* difference and you actually 3 nights of 23-25*, yeah you should have had some significant damage to the mature trees.

I would say if you are forecast for the mid 30's or lower, cover up the tender veggies and leave the trees alone. I can't see you hitting anywhere in the 20's since it is not supposed to get the cold up here.

Of course if you wanna cover up for upper 30's, nothing wrong with that either.

John

This post was edited by j0nd03 on Wed, May 1, 13 at 10:44

Thanks all. Good info yall given out and interesting too. The last time they predicted lows like we are suppose to be getting, the bird bath water had a layer of ice on top. The temp predicted was 39-40 degrees.
All of post oaks new green got hammered during that 29 degree cold snap. They are having a hard time coming back and they are very mature trees. The trees that that got hit by the 29 degree temps were water oak, post oak, sycamore, most elms, nothern catalpa, crape myrtle, white oak, swamp chestnutt oak, ? dogwood, river birch, thats all I can think of, but theres more out there. When I say took a hit, I am refering to the new green leafs and possibly some of the new growth. Looks like Ill just cover the vegies. Once again thanks. Treenutt

Most of those same trees don't show damage here with temps in the upper 20's. I bet you are in a colder microclimate as I postulated above and got much colder than 28*. It could be the local ones (to me) are more adapted to late frosts but it shouldn't be much different from your local population in that regard. Especially mature oaks, elms, and the sycamore should be resistant to that temperature. A little localized damage sure, but not the black death of entire growing shoots.

My crapes are black as night from a late freeze of 29* the other day with not much damage to most everything else. It doesn't take much to zap their foliage but they usually rebound pretty quickly. Some other sensitive things like gingko got a little singe on the leaf edges but that is about it.

John

In my personal experience 28 seems to be about the cutoff for most plants from temperate climates. Our Ginkgo was fine at 29, but 25 a week or so ago did lots of damage. Same with Acontifolium maple, and many other things. However, serviceberry, freeman maples, sugar maples, and allot of others were not bothered in the least by 25. In 2007, the cold black death descended upon much of the eastern US after everything was well in to leafing out. Got to 12 degree at my house, and cold black death is the best way to describe the after effects.

Arktrees

It really has been a trying spring for many folks, that's for sure!

36F is about the point where there's a 50-50 chance of seeing frost damage here.

My place is about 3 degrees colder at night than the "urban heat core", so I'll see frost damage where my workplace plantings are unaffected.

Post oaks took the brunt of the last freeze, everything else seems to be coming back. The post oaks, as of last weekend, look as bare as in winter time, except for the black leaves. Even my white oaks have full leaves, which you know take a while longer to produce leaves.

Your right cearbhaill, its been one heck of a spring.

My fruit trees green leaves were not effected by the freeze, just the blooms. Everyone of them. Dang man, I really wanted those peaches.

Two other factors to consider are the height of where temps are measured and wind.

Most official readings are taken at the 5 foot level and temps near the ground can be several degrees colder and higher up could be several degrees warmer depending on your terrain.

On a windless night frost could occur up near the 40F mark but a windy night might not allow it until 26F.

tj

The article below seems pretty pertinent to the OP. Read Lee's comments in the comment section for even more specific frost info.

Hope you made out OK and it looks like the fear of frost (for us anyways) IS FINALLY OVER!!!!!!!!!!

John

Here is a link that might be useful: Frost Gets a Bad Rap

i have heard 40 degrees