Cold hardening is the physiological and biochemical process by which an organism prepares for cold weather.
Plants
thumb|Plant covered in snow after an ice storm in 2013, Ontario, Canada
thumb|upright|[[Rosa canina covered in frost, Swabian Jura]]
Plants in temperate and polar regions adapt to winter and sub zero temperatures by relocating nutrients from leaves and shoots to storage organs. Freezing temperatures induce dehydrative stress on plants, as water absorption in the root and water transport in the plant decreases. Water in and between cells in the plant freezes and expands, causing tissue damage. Cold hardening is a process in which a plant undergoes physiological changes to avoid, or mitigate cellular injuries caused by sub-zero temperatures. The plant starts the adaptation by exposure to cold yet still not freezing temperatures. The process can be divided into three steps. First the plant perceives low temperature, then converts the signal to activate or repress expression of appropriate genes. Finally, it uses these genes to combat the stress, caused by sub-zero temperatures, affecting its living cells. Many of the genes and responses to low temperature stress are shared with other abiotic stresses, like drought or salinity. and makes the cell shrink, as water is drawn out when ice is formed in the extracellular matrix between cells. Overwintering insects remain active through the winter while non-overwintering insects migrate or die. Rapid cold hardening can occur during short periods of undesirable temperatures. The buildup of cryoprotective compounds such as glycerol is one mechanism of cold hardening in insects. This reaction between glycerol and water has been used as an antifreeze in the past. Proteins also play a large role in cold hardening. Glycogen phosphorylase (GlyP) is a key enzyme that increases in comparison to a control group not experiencing cold hardening. Once warmer temperatures are observed, the process of acclimation begins, and the increase in the concentrations of glycerol and other cryoprotective compounds is reversed. There is a rapid cold hardening capacity found within certain insects that suggests not all insects can survive a long period of overwintering. Non-diapausing insects can sustain brief temperature shocks but often have a limit to what they can handle before the body can no longer produce enough cryoprotective components.
<nowiki/>thumb|The common fruit fly
In addition to improving insects' survival during cold temperatures, cold hardening also improves the organism's performance. Rapid cold hardening (RCH), one of the fastest cold temperature responses recorded, Polyols simply act as a barrier within the insect body by preventing intracellular freezing by restricting the extracellular freezing likely to happen in overwintering periods.