How Plants Lose Heat

The best way to protect plants against frost is to understand how they can lose heat (note that they lose heat, not gain cold), then take appropriate action. There are three main ways:

Convection
Conduction
Radiation
Evaporation


CONVECTION
Associated with air currents moving around the plant and soil (think of putting on a jacket to keep out the wind). Most plants (and objects) have a layer of still air close to their surface, which acts as insulation. Air currents remove this. Some plants have leaf hairs, which help matters since they help trap this layer of air. Although the main function of this is to cut down water loss, it also helps insulate the plant. (and if the hairs are light in colour, they also aid cooling by reflecting back the sun's rays).

CONDUCTION
Associated with heat loss through making contact with a surface with a different temperature (think of touching cold metal with a warm hand). Since the ground is invariably warmer than the air on a frosty night, this usually warms the plant.

RADIATION
Even bodies in a vacuum can gain and lose heat by radiation, provided the sides of the vessel are clear; no air or contact is necessary. Dark objects are the best (or worst) radiators; light ones vice verse. As recently mentioned, fleece (because it is white) cuts down radiant heat loss (by reflecting back any heat trying to escape this way). We also saw in the summer that black pots could cause compost temperature to rise so much that roots could literally boil. Covering the pot with shiny aluminium foil helped prevent this and reduced temperatures considerably in the tomato pots. Foil will work the other way in winter.

EVAPORATION
Plants cool themselves in summer by 'perspiring' water through the pores (stomata) in their leaves. The act of turning water at 100 deg C to vapour at 100 deg C uses up much energy and takes this as heat from the plant, thus cooling it. The heat needed to do this is called 'latent heat of vaporisation'. The process of plant water loss is called 'transpiration'.

Water acts in the opposite way at freezing point, when it turns from liquid to ice - it GIVES UP HEAT to the plant surface and thus lowers the freezing point slightly, often just enough to prevent damage. Market gardeners spray their crops before nightfall when spring frosts are forecast.