Frost is the ice crystals that form on plants and other objects and may not necessarily occur right at 32°F, but never above that temperature. It can happen lower than, and it's all related to dew point, some sort of relationship between outside temperature and humidity in the air. The ice crystals themselves (the ones we see as frost) do not necessarily cause the damage to the plant, not even in relationship to being quickly warmed by the sun, but they do say for sure that the plant surface temperature has reached at least as low as 32°F. Now, what comes into play is the frozen cell itself. Whether the plant is damaged at this point is entirely related to the plants adaptability to the nasty ice crystals which are actually forming within the cell itself. Plants highly adaptable to freezing move the H2O in varying degrees between the cells, leaving the interior of the cell with a higher concentration of all other components, which means less crystallization within the cell, meaning no swords puncturing the cell membrane. I would imagine the sun rapidly warming the surface of the plant could mean that the cell wall becomes pliable enough for the still crystallized ice to puncture it, when, otherwise, a slower warming might remove the crystallization of the water more evenly with the softening of the carbon-based walls. I have a lawn that has bermudagrass mixed in with various fescues and ryes. When we get our first frost, whether the sun hits it or not, the bermudagrass leaves die and the fescues and ryes look fine. The bermudagrass does not transport water out of the cells quickly enough (or not at all), so the leaf dies because of massive internal puncture wounds. This will also happen to the bermudagrass at 32°F, even if no frost forms, because the cells are not adapted to any self-protection mechanism for freezing temperatures. I have a tendency to believe the actual reason the frosted plants exposed to morning sun seem more tender is likely a reflection of the temperature changes they experienced the evening and night before. They were also likely exposed to a quicker transition of temperature in the first place. A plant in full sun, also got evening sun, and the temperature drop would have been more drastic. A plant on a west, east or south wall experiences residual heat from the wall and may not react as well as a plant that has not received direct light. A more sudden transition from warm to cold does not allow as much reaction time for water transport between the cells. I guess I am suggesting the damage has already been done even before the sunlight shines on the frosted surface. Mr. Kelly M. Irvin 10850 Hodge Ln Gravette, AR 72736 USA 479-787-9958 USDA Cold Hardiness Zone 6a/b mailto:kelly@irvincentral.com http://www.irvincentral.com/