Water before the soil freezes. While it's frozen, don't water. Check the soil moisture when it thaws.

I'm not a physicist, but I have a basic grasp of heat stuff. You need to regard the slab as a heat source. Potted roses and of course tree roses are more vulnerable in winter because they are removed from the earth, which is warmer than the air until spring thaw. Ground heat in your area is around 40 degrees, or whatever is the annual average temperature. The earth under the barn is heating the slab all winter. The cold air in the barn is taking that heat away, but not as fast as the colder and windy air outside chills the exposed earth.

So my strategy would be to put the pots close together on the slab, and cover them and the slab area with a pile of insulating material such as straw. The idea is to hold in the heat that is coming through the slab from the earth. Also, in spring, it will help keep the pots at dormant temperature on the days when the air in the barn heats up.

The slab feels cold to your 98-degree hand, but warm to a rose that is at 25 degrees. You just need to keep the pot soil around 25.

Well-- didn't mean to kill your thread.

OK, I'll talk to myself, then.

For example, in zone 4/5 NW Illinois, the average annual temperature is 48 F. This means that, if you have a barn outside of Rockford, the earth at a depth of 4' and downward is always at 48 F. If you covered the slab with 6" of styrofoam, the central area of the slab would stabilize at around 48 F, while the air in the barn would be both hotter and colder than it normally is, because it would be cut off from the tempering effect of the earth. My suggestion is to use the tempering effect of the earth-- treat the group of pots as a unit insulated against the air but exposed to the ground heat.

On the other hand, if you bottle the plants individually and insulate them from the slab, they will assume the average temperature of the barn air over a period of days. If the air averages 15 F for maybe a week, the soil will cool to around 15, which is probably too cold for the roots. The smaller pots will chill down faster.

Where are you getting the 48° at 4 ft data? That sounds more like a reasonable frost line. My old CRC handbook was giving temperatures like that for 10 ft down for calculating basement heating requirements.

In my garage, I've always assumed that ground heat is negligible. The heat source is heat escaping from the house. Since the concrete slab abuts the crawlspace, it conducts heat from there. Barns are usually very leaky structures, so there isn't much difference between the outside ambient temperature and the temperature inside. Outside surface soil temperatures are usually cold enough that there isn't enough heat to melt snow.

I think these are very important discussions because so much winter protection information seems to be based more on wishful thinking than anything factual. I just can't get any sense from the physics involved. Maybe I simply don't understand it well enough.

BTW, some of the older Rodale books have things like tables of thermal coefficients for unconventional building materials. So you can calculate thermal loss through something like a 1 ft mound of baled straw and other potential rose protection.

Mad_Gallica, the 48 is from the average annual temperature. The 4' is an error on my part, sorry. Temperatures at 4' depth are fairly stable in my area, and geothermal heat pumps are typically sited at only 6' depth. However, I see that in areas with larger annual temperature swings, the zone of near-stability starts at more like 4 meters. Being inside a barn would raise that up considerably, though. The barn prevents solar heating in summer and night sky radiation and wind chilling in winter, plus it's R3 or R4 insulation including the still air film on the slab.

Nobody here is up to the physics, but I bet an enclosed slab of reasonable dimensions does contribute some ground heat. Kay could check that this winter by putting a thermometer on the garage slab under some insulating material covering a square yard or more-- leave it 24 hours and then compare it with the morning air temperature in the garage. It should be placed as far as possible from edges exposed to outside temperatures.

I wonder if in March the slab could be used as a source of coolth to cut down on the schlepping required by the plants breaking dormancy. That is, the plants could be set on the coldest part of the slab then and insulated against the warmth of the air.

Michael's ground heat idea seems sensible to me. My only concern about putting the pots directly on the concrete is the drainage issue, when I start watering again in spring....but when it it warm enough to start that, it should be warm enough to shift the pots up onto a pallet...
So now the logistics are - what to wrap the cluster of pots in, as one big unit, and what to insulate them with inside the wrapping? I am thinking of doing the leaf stripping/pot surface cleaning/cane reduction thing, pushing them together with the biggest pot in the middle and the little ones in an inner ring with the larger pots around the outside. Then making an "igloo" of hemlock boughs, covering that with old sheets and finally an outer layer of heavy vinyl tarp. How does that sound?

Leave them outside until they are dormant and the soil is around 32. The small pots could go inside before the large ones because they will freeze through sooner. The canes need to be protected against a certain outside air temperature, which might be around 10 or 12 for tender kinds. Then you don't need to insulate as soon as they are in the barn, because it will be warmer there. I think we can all agree that, at least in early winter, the slab will hold significant ground heat. You want to keep the pot temperature below 40. Also you may want to water a bit before the final covering up. I think I would pick off the leaves and trim the taller plants to a uniform height.

I would insulate around the perimeter with something bulky like straw bales or bags filled with dry leaves. If there is anything to the ground heat theory, insulating the slab outside the rose cluster will help in addition to the insulation for the outside of the pot circle. I would cover the soil with something like styrofoam peanuts or bubble wrap to insulate and reduce water loss. Then I would work a mat of some kind of insulating material over that, evergreens might be suitable, and top with the tarp.

There will be condensation on the underside of the tarp, so the canes will stay damp. When temperatures inside the cocoon are above 40, I would be concerned about canker disease. I might spray (before covering) with Daconil or copper and remove the tarp during warm spells.

This is a fancy version of how I winter my small pots, under leaves on the pavement, with the canes exposed except for coldest weather. I don't know how well it will work for you, but it's the best I can figure.

Thanks, Michael - duh, of course the pots will be "ready" at different times, based on size...
I will put a thermometer under the tarp, check on warm sunny days when I go in to take care of the hens, give them a 'breather'.
Antonia

This is of course a very simplified model, but it is good for some giggles. It's a link to a greenhouse heat loss model. You put in the size of the greenhouse, the temperatures it's supposed to be good for, the amount of insulation, and it tells you how much heat you should throw in. Here are some thermal conductivity numbers to throw around. Unfortunately, I don't seem to have numbers for straw, or leaves.

units are Btu/(hr*ft*F°) so multiply by the depth of material

Earth, dry 0.85
Paper 0.07
Sawdust 0.05
Vermiculite 0.035

or you can use R-values since thermal conductivity = 1/R

Here is a link that might be useful: greenhouse heat calculator

One point is that water vapor moving through insulation carries lots of heat away with it, then condenses when it hits a cooler zone, and wetness kills a lot of the insulating value of materials like straw or sawdust, or even fiberglass. You will have water vapor migrating from the pot surfaces in this case, except when frozen. Bagged leaves are great because they include a vapor barrier and will stay dry. Styrofoam and such things as well. From an insulating standpoint, it would be best to seal the roses in bags to keep the moisture in and away from the insulation, which could then be placed over the bags. You would then get condensation inside the bags instead of under the tarp, but maybe not as much. This might be a better way than the other, especially if you used a fungicide before closing the bags.

An analogy to the issue of the slab feeling cold-- I have a solar-heated mass wall in my house which feels cool to the touch on winter evenings after sun. But if I put a thermometer up there and tape insulation over it, I learn that the wall is radiating heat at maybe 74 degrees and heating the 68-degree room. Also sitting near it, you feel warmer than sitting elsewhere, although you can't notice any heat from it.

Your analogy to the solar mass wall clicked with me, Michael. I have a sunroom on the south/west side of my house, that went in in '86. Tiled floor on top of deep sand base, and I don't know what is below that, but it is not over the basement foundation. But in the coldest time of year, that floor is never as cold as the same tile, on a plywood subfloor over the cellar, which is a walkout to my driveunder garage.
Antonia

sunny, why not put them in the sunroom? i have the same dilemma.i also have a sunroom that sits on a concrete slab.
i have searched for pointers and come up with very little besides the usual methods.i guess i will know for sure after this winter.i will be getting bands that i will pot into 5gal. crack windows 4 ventilation,sun for warmth,glass to protect from wind. maybe cover in burlap.

I'm not sure why, but our unheaded garage seems to be the perfect place for my container roses. It gets almost as cold as outside, as it is not insulated, which means there will be temps as low as zero on the coldest nights (usually more in the 20s and low 30s however). Sometimes the soil in the pot seems to be frozen solid (at the surface at least). Yet they sail through the winter beautifully. I do give them a little water every 6 weeks or so, and I do set them on styrofoam sheets, with styrofoam sheets on the garage wall behind them. But other than that, they are on their own. If I kept them outside they would be deader than a doornail, and when I buried some a few years ago they didn't fare nearly as well as the 'garage guys'. All I can think of is that the wind damage is much worse than the cold?

i'm glad to hear that karen.boy am i ever. that gives me a little more hope. i think the water definitely helps and i agree about the winds. i've read posts about garages that didn't make it but they said they didnt water enough so i wondered if that was the reason.

Sunroom is really really WARM on sunny winter days - it doesn't have a fourth "wall" - it is a step down from my dining room. It was designed to be source of passive solar heat. It is almost sauna like on an early Feb afternoon when there is snow cover and bright sun. So no indoor roses! Midwinter tropical houseplants from the supermarket enjoy it, as did a hibiscus I had for years before unintentionally killing it by moving it outside too soon. And my garage doesn't have room, once the car gets inside, although I may rethink what spends the winter in there on the sides....but right now I am leaning to the barn plan described above.

I thought about this some more. It might be good to focus on keeping the moisture in the pots and away from the canes and insulation. This would reduce heat loss and the risk of disease. You might put each pot in a bag, put a little insulation over the soil, and fasten the bag around the very base of the plant with soft wire. Plastic grocery bags wadded up or styrofoam peanuts would keep their insulating qualities in the moist conditions.

And I do think bagged leaves would be great for the perimeter. If you don't have leaves, you can snatch bagged leaves from the curb on any suburban street. Open the bags for a couple of weeks in the barn to be sure they are good and dry.