<P> Vegetation is not necessarily damaged when leaf temperatures drop below the freezing point of their cell contents . In the absence of a site nucleating the formation of ice crystals, the leaves remain in a supercooled liquid state, safely reaching temperatures of − 4 to − 12 ° C (25 to 10 ° F). However, once frost forms, the leaf cells may be damaged by sharp ice crystals . Hardening is the process by which a plant becomes tolerant to low temperatures . See also cryobiology . </P> <P> Certain bacteria, notably Pseudomonas syringae, are particularly effective at triggering frost formation, raising the nucleation temperature to about − 2 ° C (28 ° F). Bacteria lacking ice nucleation - active proteins (ice - minus bacteria) result in greatly reduced frost damage . </P> <P> Typical measures to prevent frost or reduce its severity include one or more of: </P> <Ul> <Li> deploying powerful blowers to simulate wind, thereby preventing the formation of accumulations of cold air . There are variations on this theme . One of them is the selective inverted sink prevents frost by drawing cold air from the ground and blowing it up through a chimney . It was originally developed to prevent frost damage to citrus fruits in Uruguay . In New Zealand, helicopters are used in a similar function, especially in the vineyard regions like Marlborough . By dragging down warmer air from the inversion layers, and preventing the ponding of colder air on the ground, the low - flying helicopters prevent damage to the fruit buds . As the operations are conducted at night, and have in the past involved up to 130 aircraft per night in one region, safety rules are strict . Although not a dedicated method, wind turbines have similar (small) effect of vertically mixing air layers of different temperature . </Li> <Li> for high - value crops, farmers may wrap trees and cover crops . </Li> <Li> heating to slow the drop in temperature . This is not practical except for high value crops grown over small areas . </Li> <Li> production of smoke to reduce cooling by radiation </Li> <Li> spraying crops with a layer of water that releases latent heat, preventing harmful freezing of the tissues of the plants that it coats . </Li> </Ul>

Hoe cold does it have to be to snow