Besides, light, sandy soils is usually warmer than heavy, wet clay soils. That has quite much to say about winter hardiness

the warmer the soil general indicates the possibility to growing plants and trees outside your zone.

==>>> hey joe ... see link.. right click and open in a new window ... for background .... then hit the link for all along the watchtower ...

i am glad you are discussing variables.. regarding zone.. other than the usual min winter temp ...

but i dont know if you statement is right ... so i want to start a discussion about it ...

i will mull it over and check in tomorrow .. if i have any insight ...

ken

sounds like this winter ... lol:

All along the watchtower
Princess kept the view
While all the women came
And went bare-foot servants too
Outside in the cold distance
A wild cat did growl
Two riders were approaching
And the wind began to howl, hey.

Here is a link that might be useful: link

While it's true sandy soils get warmer faster, they also get colder faster. In SW Missouri it is clay loam, not much sand. What I see is that while we may hit a low temperture average in the USDA Zone chart, we don't get long stretches of this cold, hence warmer soils. Where other states or areas may not get below the same low temperature as my area, they have longer stretches of cold or dont have the drastic swings to warmer temps that we see in this area. In addition we might also have a higher sunshine index which warms soils faster and easier than a maritime climate of marginally warm cloudy winter days. Warmer soil temps mean roots don't freeze, allowing trees and plants in areas that are in borderline zones to make it through a harsh winter, where as those with colder soil tems but the same zone may not. Just some possible explanations here, not set in stone.

Mulch is a factor..if you have heavy wood chip mulch, it insulates...same with probably any type of mulch...stronger insulation=thicker the mulch, or so i would imagine. They use to insulate huge chunks of ice in wood sawdust in the old days before refrigeration. Probably not much different then todays cellulose insulation that we blow in attics/walls.

RIGHT NOW your soils are warmer than ours...but keep in mind, the East Coast has had a very cool spring relative to normal.

I wonder how this map looks in July, or in January.

Just looking at it day by day, Maryland seems to have somewhat cold soil, it's middle end of April, I'm sure by June July my area will have fairly warm to hot soil.

This is a really interesting map.
The extended cold winter caused cold to penetrate very deeply. In fact until about 2-3 weeks ago...yes this was at the beginning of April...I had a 8' high mound of shredded Juniperus virginiana branches for mulch that was frozen solid under the top 6 inches. My front end loader could not penetrate it! Finally it thawed out. Makes you wonder why we don't build houses out of huge piles of mulch LOL. Stuff must have an R-value of 500. And also shows how icehouses were possible in olden times, even for houses in warm climates like Monticello.