The process of exposing plants to successively cooler conditions over days or weeks to acclimate them to low temperatures. This particularly applies to plants reared indoors, e.g. in heated glasshouses, so as to avoid chill injury when they are planted out. Cold hardening induces certain biochemical and structural changes, such as increasing the proportion of unsaturated fatty acids in cell membranes, which helps maintain membrane fluidity, and thickening the protective cuticle on leaves.