I live on the Cumberland River in Middle Tennessee. (zone 6-7) This is an area of limestone karst topography with rock outcroppings and sinkholes. (To paint the picture, they sometimes have trouble running water lines in my area, or plowing, due to giant rocks under the sometimes thin soil.)
Anyway, In the middle of my back garden is an area of solid rock about 10 x 12 ft (and probably infinitely deep) There is about 4 inches of soil over the top.
I am thinking of shoveling off the soil and building a 4 x 8 ft cold frame in the middle of this rock, refilling it with good soil, for cold weather seedlings. It would not collect water on top, because it is actually slightly domed shaped, and slightly above grade. I was thinking that the thermal mass would collect the sun's heat and release it at night. However, after this recent deep freeze, and we haven't really had a lot of sun, and now I am wondering if the rock would just be colder than the surrounding soil? The soil froze about 6 inches deep, which is really unusual for here. We had no snow, so I could not tell which would have thawed quickest. This may be a dumb question, but my mind is fuzzy, does the mass of the rock cool off quicker than the mass of the soil? And would it hold the cold longer?
When the ground froze over the rock, did it freeze equally deep in other places in your yard with similar positions of solar exposure?
Another way to check what I'm getting at is to measure the temp of the soil over the rock today and also in other places in your yard to see if that rock is adding or removing heat from the area.
I'm speculating that the solar gain in a cold frame won't heat the rock much as compared to the enormous surface area of the rock that is not under the cold frame. So I "think" it would act as a huge heat sink to maintain the temperature of your cold frame at the temperature of the rock. If the rock is heated by the deep soil temps, that could be a very good thing (say 45-50 degrees). And it would help prevent overheating on sunny days.
Knowing the temp of the soil down against the top of the rock would be very good info to collect. My hunch is that a cold frame would come in pretty close to that temperature in your situation.
Thank you Mike!
Well, I can't really get to that rock right now as it is under a pile of scrap lumber left over from a garden project that needs to be relocated soon. : )
But today after I posted, we had our first snow, and no sun, so I took a walk around the yard and the neighboring farm to look at other rocks that are either on the surface or just under, and the snow was melted above them (like one would see over a septic tank) even one that I know of down the lane that is 4 inches under the soil. Of course, one's sees this all the time in the city with asphalt and concrete, but it doesn't necessarily intuitively transfer to your concept of isolated rock outcroppings out in the country. And all my previous farms did not have rocks in such close proximity to growing areas.
We don't have snow often, and I haven't been at this site long, so had never noticed this before. Pretty neat that nature gave me this demonstration just when I asked the question! I did notice that the smaller rocks, say 3 ft in diameter had negligible melting, but the very large ones (like the one I am considering) where completely melted.
For TN, two weeks of night time lows in the single digits has not happened in my lifetime before, so I guess it got me a little concerned about my plans to utilize the rock. Since the freak cold, we have had two warm days before this next cold snap, enough to warm the rocks again, I would presume. But I don't know if they got "really cold" during that spell or not.
I am thinking that if the coldframe is situated in the middle of this "rock island" I might gain more heat than I previously thought, and maybe in the summer I could somehow harness the solar heat build up for some sort dark enclosed herb drying cabinet.
Anyway, thanks again, and I will take your advice of sticking the soil thermometer in both on the rock and beside the rock and then again for the next week as we are supposed to have some crazy cold again this coming week, good time for experimentation, bad time for plants used to zone 7.
I'm glad the rocks show underground, that's a pretty cool way to know where underground obstacles are. Like a metal detector for boulders. I have one spot on my driveway that always melts first and I'm at a loss as to why. It's near a huge white pine so maybe its roots are doing something. We don't have many big rocks in my plot.
Cady Goodwind wrote: Since the freak cold, we have had two warm days before this next cold snap, enough to warm the rocks again, I would presume.
If the rocks are under some soil, I would not assume that the sun is heating them much at all over a short period like a few days. I would assume that most of that heat is coming from the core of the rock and that the rock is being heated by the warmth of the earth several feet down. It's just better at carrying that heat to the surface than the surrounding soil.
It sounds to me like it's a good place to harvest deep soil temperatures for a cold frame without digging a 10' hole
So, this next cold spell brought us 3 days below freezing with night time temperatures reaching 0, -1 and 3 degrees F. On the third morning, I am happy to report that even though the surrounding garden had soil frozen to a depth of 2 inches, the soil over the rock was not frozen, and was easy to insert the thermometer in.
When the thermometer hit the rock (under 6 inches of soil) I was surprised to find the thermometer reading 38 F . The soil at the surface above the rock measured 34! I located the edge of the rock by poking and prodding with a rod. The temperature at the edge of the rock was 34 F tapering off to frozen at 2 inches away from the edge of the rock.
So, I am guessing that is a very deep outcropping indeed. While I was poking, I think that the rock is bigger and differently shaped than I originally thought, so I may have to rethink the placement of the cold frames, and the surrounding garden (this might explain some poor summer crop results in that part of the garden.) Sunlit areas are at a premium in the limited landscape, so this does change things.
Thanks again Mike for the suggestion of using a thermometer : )