The zones are rules of thumb. So are the evaluations of hardiness. You can have a rose that is hardy to zone 6. But that 6 can take a dive to 5 during a really bad winter. Or, your individual rose is a wimp that can't take more than 6b.

The real concerns are 1) how much freezing can the roots take, and 2) how much cane die-back can you take.

Water freezes at 32. Frozen is frozen, whether it is 31 or 20 or -40. What matters is, what is the temperature at the roots of the plant vs the plant's ability to generate heat. The colder it is, the more heat it takes to keep the water in the organism over 32. Most of us can survive a very long time at 32, esp if we keep moving/generating heat.

Plants, like people, have differing tolerances for cold. Some metabolisms generate enough heat to ward off cold for longer periods.

On own root, Teas, Hybrid Teas, Chinas and Noisettes have the most tender roots--sort of like skinny people, they get cold easier/faster, esp if they are just standing still.

Well-established roses with deep roots can take a lot freezing weather because it takes a while for the earth to freeze, even tho there might be a lot of above ground die-back.

Plants in pots can take less cold, regardless of class, because the cold penetrates the pot faster and meaner than it does mother earth. The less moisture in the pot/soil, the better chance they have. (You'd get colder faster when surrounded by ice cubes, as opposed to just cold dirt).

Plants in the ground do much better, esp if they are well-mulched and/or have lots of leaves piled on top of them, because the earth stays a lot warmer than the air.

So, rules of thumb: If it will be 32 or less for a few hours in the middle of the night, don't worry about it.

If it will be below 32 for several days, get your pots indoors or put them under a tent with a light bulb to warm the air. If your plants are in the ground, you don't have to begin worrying until the ground is frozen at least 6" deep (unless they are baby bands, in which case, you might lose them).

If it will be below 32 for several weeks, cross your fingers and hope. Or put out smudge pots like they do for the citrus groves in CA & FL.

I never use blankets. I let the leaves pile up in my beds. It's better than mulch because they create pockets of warm air. The igloo effect.

This would probably take a book to explain. So let's start with Chapter 1...

Before there were zone maps, there were Arboretums. So a lot of older books will say of a plant something like 'it is successfully grown at the Arnold Arboretum'. Somebody like me would read that and think I could probably grow it, but maybe not. On the other hand, something that was described as growing at the Montreal Botanical Garden would have to be hardier, and would definitely survive my winters.

When the first zone maps came along, the idea was to connect two sets of data. The first set is how cold a particular place got under remotely normal conditions. The second set was what type of places a plant could reasonably survive. A big reason that Sunset Zones never caught on in the east was that while they could create the first data set, the second took many years, and many, many more data points. The USDA zones essentially took the existing arboretum data and assigned them number codes. Arnold Arboretum mapped to zone 6, Morton Arboretum to zone 5, and Montreal Botanical Garden to zone 4.

This isn't that plants have been tested in an environmental chamber to set temperatures, but that they have survived real world conditions for several years in known places. So a zone 7 plant should be able to handle a colder than average year where the temperatures go below zero because it has handled a colder than average year where the temperatures go below zero at somewhere like Longwood Gardens. Whether or not it could handle record setting cold is a different matter, and often becomes a judgement call based on the life expectancy of the plant. A shade tree that will take several decades to prove its worth should be able to handle more extremes than a subshrub that may die of old age within a decade.

(the shower is still running so I don't have to give up the computer for the pursuit of homework quite yet)

Chapter 2

Plants in pots are generally considered to be a zone or two less hardy than plants in the ground because they don't have the benefits of dirt insulation around the roots. Most plants keep their roots under ground. Soil is often decent insulation*, and there is a small amount of geothermal heat coming up from underground. So the roots don't have to adapt to the large temperature swings that the above ground part of the plant are exposed to. They only have the insulating properties of the soil in the pot. Plants cannot generate heat (I can accept warm blooded dinosaurs. Warm blooded plants are a different matter) so any heat has to come from some external source. Without a heat source, the only thing the insulation can do is slow the transfer of heat from one place to another. So the pot will slowly heat up during the day, then cool down at night as the heat leaks out to the colder air. The same will happen to the ground, but the ground is only in contact with air at the top, not the sides, and is much larger.

*Air is a good insulator, which is why storm windows and fiberglass insulation work as well as they do. Water is a good conductor of heat. So the insulation value of soil is very dependent on how much of the pore space is full of air, and how much is full of soil.

I don't know if there is a Sunset Eastern Garden Book or not, but the Sunset Western Garden Book has interesting short essays on the differences between climates and how they affects plants. It might be educational for you to read those just to get a sense of how differences in humidity, rain, elevation, nearness to bodies of water, etc, influence the success or failure of garden plants. Comparing one similar zone to another gives a better feel for what plants need and how they respond.

Susan, if you're in the city of Nashville you may actually be warmer than a zone 7b. Urban areas tend to be warmer. Looking at a zone map, I should be a 7 but it never gets that cold here -- we're in the outer suburbs of DC and feel the 'heat island' effect.

The zone maps have their purpose, of course, but Cecily is right. There are smaller zones within the main zones right down to your own yard. You have to learn where the warm and cold spots are in your yard and work with those.

Yes, there is a Sunset Northeastern Garden Book. I haven't searched for others, but I would suspect there should be other editions to cover other areas. Worth investigating if you're interested. Kim

Here is a link that might be useful: Sunset Northeastern Garden Book

Thanks everyone! And a big thanks to Mad for all that transcription!!
Now I am wondering the best positions in my yard for thermometers...I want to start charting a few areas....primarily I'd like to know how my garden varies (on average) with the "official" temp of the day. Obviously one outside my bathroom window in direct sun was a bad choice. HA.
Susan

Susan, in zone 7 and colder you must insulate around the pots with something like a pile of leaves. The smaller the pot, the quicker it cools down at night. And rose roots are more winter tender than hardened rose canes, because in nature they are protected by the ground heat. Push the pots together to create thermal mass, leave them in contact with the soil, and put leaves all around the outside.

This post was edited by michaelg on Fri, Dec 13, 13 at 13:21

Damn. I think I've killed some of mine. They feel frozen solid and some of the soil has pulled away from the side of the pot. Have them in black nursery plastic pots.
Oh no. Will there be any change to foliage or stem if the roots are frozen but the plant still had leaves (IOW will I be able to tell if they are dead before spring?).
I went out and pushed them together and insulated them with leaves and pine straw. Some of them were on concrete dang it.
What a dunce I am...
Susan