Mild, radiation frosts occur on still, clear nights, usually with the development of a strong inversion. Cold air accumulates in low spots or in areas where air drainage is blocked. Under these conditions, a few degrees of frost protection may be all that’s needed and it can be provided by running water, by flying helicopters, or by running wind machines in narrow valleys.

Advection freezes are more severe and often result in damage. They occur when cold air blows into a field from outside the orchard. Usually they are associated with wind greater than 4 mph. Formation of an inversion under these conditions is unlikely making successful frost protection extremely difficult.

Frost Sensitivity

If water is used for frost protection, critical temperatures for frost damage help us know when to turn irrigation systems on or off. Buds showing pink are more resistant to cold compared to flowers at the full bloom stage, which are in turn more resistant than small nuts. Estimated frost damage is shown in the following table.

Our work with artificial freezing in 1990 indicated that the early varieties Peerless, NePlus, and Sonora are all similarly susceptible in the small nut stage. Peerless is the most sensitive at full bloom and Sonora is hardier. This was especially so at pink bud when Peerless is still very sensitive while NePlus is intermediate. Of the mid-blooming varieties Nonpareil, Carmel, and Price, Carmel is the most sensitive, with Price intermediate and Nonpareil the most tolerant. Among the late blooming varieties Mission, Padre, and Butte, the Mission is most sensitive while Padre and Butte are similar with Butte possibly being slightly more sensitive than Padre. This summarizes our current knowledge of variety hardiness.

Soil and Groundcover Condition

Groundcover condition affects orchard minimums with any cover taller than 4 inches in height generally being colder. Soil heat storage is reduced because sunlight is reflected and water is evaporated. Keeping groundcovers cut short to 2 inches or less during frost season allows sunlight to reach the soil surface, and increases soil heat storage resulting in a warmer orchard through the night.

Bare, firm, moist soil is warmest, but this is true only when the surface is moist. If pre-frost conditions are dry and windy and a dry crust forms on the surface, then, bare soil can be colder than a surface with a short (less than 2 inches) groundcover that tends to keep the surface moist with dew from the grasses and weeds. The ground surface must be moist for bare ground to be warmest.

Dry or recently cultivated soil has many air spaces, lower heat storage capacity, and low heat conductivity resulting in colder minimum temperatures. Moist soil stores more heat due to water content, has higher conductivity, and will have higher minimum temperatures. Irrigation should ideally wet the top foot over the entire orchard surface, soil moisture should be near field capacity, and these conditions should be achieved in advance to gain the most advantage. A light irrigation to moisten the soil the morning before a frost will help obtain the greatest heat storage.