I received the following question and thought the response was important enough for an entry:
“… The forecast shows that 28 and but also chance of rain for the same night. Does that mean we will moisture from the ground to hold the temperature up. Would that mean no irrigation is needed?”
Most likely, it will be a clear night in order to drop to 28F. If not, the event would be considered an advection frost event, which are very difficult to provide protection as the wind displaces the heat gained by the various strategies.
It is correct to assume that if the surface of the soil is wet from rain, that irrigating more to wet the soil surface will not provide too much of a benefit – i.e. flood and/or drip systems. This is especially true if the surface water freezes. Once the water freezes, heat will no longer be released. Keep in mind that drip system lines may actually freeze if volumes are not high enough (less than 15 gallons/minute/acre). If volumes over this rate can be applied, drip systems may provide some benefit.
In regards to micro-sprinkler or solid set systems, applying water during the night will provide a heating effect even with a wet soil surface. This effect is due to the heat that is released off the water as it freezes. In essence, with these systems, more water equals more protection. Targeted amounts of water should be 30-40 gallons per minute per acre. Minimally, water should be applied at rates higher than 15 gallons/minute/acre to avoid freezing of the spaghetti tubing/ irrigation lines.
These rates were determined through research conducted by Joe Connell (UCCE Butte County) and Richard Snyder (UCCE Specialist).