Image: undated photo.
“While some thrifty growers still burned stacks of old tires, the majority turned to the coke burning units (‘smudge pots’) as they became available and later invested in the more efficient oil-fueled heaters. All methods of orchard heating are costly, not only in the price of the fuel consumed, but in the wages paid the manpower required for the operation and maintenance of the system. I recall that as a general figure it was estimated that fuel oil costs accumulated at a minimum of $50 per hour (1939 dollars!) for every ten acres heated.” — G. Carrol Rice
“Excerpt from http://elcajonhistory.org/pdf/THOSE_DAMN_SMUDGE_POTS.PDF . Click on the link to read more about recollections of protecting citrus orchards from freezing.
Also see https://www.kcet.org/history-society/the-cold-and-the-dark-a-short-sooty-history—— for more info.