This is an interesting subject. There are certain things and situations that absorb heat and also retain heat. Generally darker soil will absorb more heat than light colored soils...everything being equal ...which they are not. Darker soils tend to be wetter. Wetter soils will be a lot colder than dryer soils. It takes a lot of heat to bring those wet soils up to heat and evaporation cools things and with a lot of moisture being evaporated, the soil is cooled.

The sun angle makes a difference. Soils on south slopes are a lot warmer. Masonry picks up heat as does black objects. Another important thing is that sand picks up heat..being small rocks. Soil inside a box might block sun rays all around the edges and in a small box, that adds up to a lot of area. My raised beds are unlined and open to the sun rays....that might make some difference.

Thank you tcstoehr for doing this experiment. There are things that we all assume are true because they seam to make sense and we have all heard them so many times. I always say the only way to really know is if somebody does it. Then there is proof. At this point you have experience and now no one can tell you that raised beds are hotter than the surrounding soil.

A person with experience is not at a disadvantage to someone who read about it in a book or a website.

I grow in raised beds and I notice that the edges of the beds dry out faster than the interiors. Rapid evaporation does not mean that the soil is hotter. It may be exposed to more heat but evaporation releases cold and faster evaporation releases the same amount of cold per volume in a shorter space of time.

To illustrate;

Burlap water bags were and still are often used by logging crews in remote areas. They hang a large tightly nit water bag on the limb of a tree. They put room temperature water in but the water is ice cold when it comes out. This because the water soaks through the burlap so that the entire outside surface of the bag is wet. As it evaporates some of the cold thats produced is stored within the water in the bag. Even when its 110 degrees in the shade the water is still ice cold. The higher the rate of evaporation means more water is evaporated in a shorter amount of time which means more cold being stored in the bag. This is evaporative cooling at its best.

Thus the bed could be could be hit with more heat and the temps not be noticeably affected do to a higher rate of evaporative cooling. Of course I'm sure the mulch and plants growing over the soil also have a positive affect.

Thank you again.

It's well known that dryer soils will warm up more and raised beds tend to be dryer, but not necessarily so. It would be interesting to have data for both temperature and moisture at about a 3-4" depth. Soil surface measurements are very deceptive.
I've heard that in a hot & dry season crops on raised beds suffer from lack of moisture. In a cool & wet season those crops often fair better. I usually lay over a linear mile of black plastic mulch to warm the soil, but never invested in a bed shaper to achieve raised beds and also only put down drip tape on a small fraction of my overall crop. I wonder whether the extra expense would be warranted unless you experience poor drainage.

If only morning and evening temperatures are taken, some mid-day heat raises could be missed.

Last summer on a raised bed with a fair amount of sand in the mix, the soil got so hot in this potato bed after the vines died and the soil was nearly barren, that I was concerned that some of the potatoes would be cooked some. I lay a lot of that to the sand picking up heat so well.

Right, that why I suggested that he measure at least 4 times a day. Raised bed soil temps will fluctuate alot more, and might actually read colder (e.g., after a freezing Feb night) than a level spot would early in the morning. But then the temp would shoot up very quickly and be much warmer by mid-day esp if sunny. I guess the OP could only practically measure temps before he left work and when he got home. 2 data points a day isn;t enough to draw any sort of conclusion. Unless you have one before starting the experiment.

Yes, I chose those times as they were the only times I would be reliably available. So I didn't put any other times on the spreadsheet I used for logging temperatures. However, there were many weekend days that I did casually take readings at random times during the day and those results showed the same similar temperatures between the bed and the ground. I meant to try some at night but never managed the gumption to get out of bed at 2AM to go traipsing around in the garden with a flashlight.
These results are not at all what I had expected. I pretty much bought into the often repeated notion that the raised beds would be warmer during the day, although by how much I had no idea. But I also believed, contrary to some, that the raised beds would also cool off faster as well. I figured the morning temperatures would tell me that, and I believe they did.
Of course this is just my location, my climate, my soil composition, etc. Your mileage may vary and I would encourage anyone to take their own readings. The data you get that way will be worth alot more than heresay and assumptions.

Hello. I am currently reading the Four Season Harvest by Eliot Coleman and he states raised beds can have a temperature of 8-13 degrees higher than ground temperature.

I was just assuming that this applied to winter temperatures and not summer. So it would be interesting to check this out during the winter.

I have raised boxes and this last fall I did not pull things out. Right now I am finding lots of things that overwintered that I did not expect in the boxes. I know we had a mild winter, but I was also assuming the raised beds were protected from some of the colder temperatures.

Soils cool off considerably at night. The temperature down about 4 inches deep could be warmer than surface soil on a nippy night. Compact level to ground soil will conduct heat upward on that nippy night while looser raised bed soils do not have so much upper heat conduction at night on those nippy nights.

I love this sort of backyard science, thanks for doing this. I have a hypothesis that your 25% peat is acting as a temperature regulator in 2 ways: 1) It has insulating thermal properties when dry and holds sufficient moisture in cold weather to transmit temp. changes through the trapped water. 2) The peat inhibits bacterial and fungal associations in your soil. I'd expect greater temp rises from microbial soil action alone in almost any garden box, perhaps additions of compost tea to perk up the soil food web, or adding another organic nitrogen amendment to stimulate subsoil heat generated by the soil decomposers (sapotrophs).

But as to the temp discrepancy with Hume, I think it's another variable - the background wall in Hume's yard vs the open placement of your boxes.

{{gwi:15590}}

can't underestimate that reflected heat & wind protection, especially with a white wall. If so, bad science by Hume to neglect that variable.