Thanks Joe, Good info and interesting blog too. It seems to make sense that the temp in the center of a 25 gal pot would be warmer than the outside. This however is not my experience. Last winter I placed a probe deep in the center of a group of pots covered with a mass of leaves and compost and it indicated only 2 degrees warmer than exposed reading. Recently i placed a probe inside a towel inside a large sealed insulated cooler. There was no difference at all with exposed temperature readings. Both of these tests were done at around 25F. There are a few nuggets worth pondering in this chat last year at Garden forum.
Poetry , Good you linked to the other forum where a good discussion took place, I encourage forum members to browse through it. I would like to call attention to the link, given in that discussion, concerning 'Preparing Nursery Plants for Winter'. It clearly says that Thus the claims regarding potted plants being killed after exposure to a given low temperature may in fact be due to desiccation rather than to all roots being killed by ice. That's why it is important to protect frozen pots from desiccating winds, covering them with anything that will keep the plants from losing the water in their leaves (if evergreen) and in their buds and tender twigs for the others. The best cover is, of course, to have the pots buried in snow!! It is true that, as I said in the other forum, tender roots are likely not genetically programmed to cold harden (although I have not seen any scientific information on this) and cells will be killed by the ice crystals that will form when the temperature at the root surface will reach about -3/-4°C (by analogy to above ground spring growth which is not cold hardened). From here it is a simple thermal calculation: 'If I have a cylindrical pot of diameter D, with thermal conductivity z and the temperature at the edge of the pot is T (say 10°F), then what is the temp at a distance D from the edge after a lapse of time t?'. (the hard part is to know the value of z) It is also worth considering that maple roots are very superficial and that, if you do not have snow cover, then the distance from the superficial tender roots to the air temperature is comparable to what would happen in a large pot, .....just some geometrical considerations, no claims ;o)). Personally I believe that tender roots are actually killed for plants in small pots, but I also believe that woody roots do cold harden, just like branches, and that in Spring the plant grows from them new tender roots quickly . In addition those tender roots are regenerated all the time during the growing season (this is something I read in a scientific publication from a forestry research group from Finland. They found a threshold for root activity at about 7°C, it is possible that the plant has enough time, before bud break, to regenerate most the tender roots lost to freezing temperatures). This is my theory that may explain the contradictory experiences reported. Gomero
