I don't have any answers for you.

But it seems that whether you are talking about tender plants such as tomatoes, or hardy trees, shrubs, deciduous or evergreen, or the biochemistry of seed stratification, -- each would require a dissertation in itself!

You'll need to narrow down to one question and hope someone will answer.

Unless someone tells me how to grow a Live oak in zone 6 I may not be interested. If they invent a chemical that allows that, I'll be the first to buy it, cost prohibiting may affect that though.....

I think some northern trees rely on natural antifreeze. A bucket of salt water will take longer to freeze then fresh water. The body fluids of living things tend to be saltier then their surroundings. Glucose is a natural antifreeze as well. A lot of the chemicals in pine trees that give them their scent are natural antifreezes.

You have to ask HOW freezing kills. One major factor is that ice expands as it freezes, and ice crystals are built on triangle shapes (so, pointy corners). As a result, growing sharp crystals burst cells. In addition to lowering the freezing temperature, some natural anti-freezes make ice crystals more rounded. Some plants produce ice structuring proteins (ISPs) that bind to growing ice crystals and inhibit their growth. Some plants push water out of the cells and cause the ice to form around the cells.

Also, it should be remembered that if you dig underground it tends to be warmer. I suspect some herbaceous perennials rely on the fact that it is warmer underground where there roots are.

Here is a link that might be useful: How do Trees Survive Winter Cold?

It won't be a chemical you add, it will be genetic modification.
Scientists have already tentatively studied this and know the chemicals involved; tracing, cloning, inserting and amplifying the genes would not be impossible these days but still not anywhere near dirt cheap enough to motivate some party to do it for any reason other than agriculture. There was a scientist/horticulturalist doing work related to this in DC but in conversations with him the impression you get is he moved on and sold his lab equipment because it would be hard to successfully commercialize. His investors didn't think he was a big enough party to pull it off so didn't give him anymore money, but the big parties (Syngenta, etc) will only be interested in agricultural applications for the foreseeable future.
So...zone 7 hardy Date Palm? Expect it to be a college student's toy/pet project in 25 years, maybe; not a Monrovia Nursery project in the next 5. Horticulture is a very bottom line business, and is getting more so. Their genetic research will be focused on making plants cheaper to produce...not necessarily interesting for us.

This post was edited by davidrt28 on Tue, Dec 16, 14 at 4:08

And btw I have to add, I cite date palm for a very specific set of reasons.
I have found that adaptations to resist dryness tend to confer accidental resistance to cold.
These are known to have recovered from radiative freezes in NM down to -10F! So, in an absolute sense; there is great hardiness potential there. It's just various metabolic factors mean that while they can once in a while survive that there, they can barely handle the outer banks of NC and will eventually die there in a wetter and/or more prolonged freeze.
So that's more feasible than trying to make a zn 7 coconut; obviously. But good to mention coconuts now; because it's more important for me to go on eating them than have a zn 7 hardy date palms; and plant genetics researchers are going to have their hands full w/plenty of disease problems over the next 20-50 years.

I must say that the "late drop" Live oaks from Mossy oaks native nursery is hardier than plain Live oak, it drops acorns later than the plain LO, that was mentioned on their website. They have it listed as zone 7, just like the regular LO (QV), but, after last winter, the worst crap ever BTW, they came back from the roots, and grew just as tall as before the freeze dieback, and the buds looked like they were gonna break. I would've bet money on it, well, maybe. But after waiting til June I cut the top down. The tree had survived the previous winter with just defoliation. I tried reg Live oaks with all croaking in a moderate winter. I know, it still isn't good enough unless it gets even hardier when older. I guess my Hybrid of Live oak and Overcup oak will be fine. It gets sorta the same shape, the texture isn't as nice, but, we can't always have what we want.

I do think that hardiness just may improve with the age of some trees. I hope so when it comes to some "babies" in my yard. I do hope that the plantnerds do come up with some hardier Crepe/Crape Myrtles and especially Live oaks, other than the hybrids that are currently availible. Poaky1

poaky, my experience is the plants don't get innately more cold tolerant, they just get more and deeper roots below the frost line in the soil, and thus better avoid freeze-burning (especially evergreens). YMMV with the specific species tho.

Yeah Beng, I guess that makes sense. I hope so, I have one that should be quite extensively rooted, it's been in the ground for 2 years or more. It's a Live oak "late drop" BTW. poaky1

My weeping Yaupon died back a bit every winter for three years. Finally, even after last winter, it stayed green. Must have finally dug its roots in deeply enough.

Well, the Live oak "Late drop" has been in the ground since spring 2012, so in about 4 months it will be 3 years in the same spot. Live oaks love my soil. I had one from Florida that was in the ground about 4 months. The top died, but when I tried to dig it up to get rid of the dead top, the roots had really gotten settled in good for just 4 months. I wish I had waited to see if the top came back, but I got rid of it. It was labeled as zone 6 hardy at the Lowes in Florida. I figured it was wrong, that's not why I tried it. I didn't believe that tag for a minute, but it was my first Live oak $20 some bucks, why not see what happens? I am currently hoping my Q. Fusi from Joe in Moe does well this winter, as well as the "Late drop" live oak I keep babbling on to anyone who reads it. :)