At least one million smudge pots stood in Southern California's citrus groves by 1915. A smudge pot was an oil-burning orchard heater, lit between the trees on freezing nights to keep frost off the fruit. The heat protected the fruit; the black smoke was wasted fuel, and it eventually brought state regulation.
That count of a million comes from D. J. Waldie's history of Southern California smudging for PBS SoCal, which also gives the stakes. Five hours below 26°F could ruin a year's crop, and a week of freezing nights could kill the trees. A replanted grove took about seven years to pay again.
How did a smudge pot work?
A smudge pot was a squat round tank of oil with a chimney standing out of its middle. Burning oil sent heat and smoke out the top, along with water vapor and carbon dioxide, as the Goleta Valley Historical Society describes it. The design dates to around the turn of the twentieth century.
There were several kinds. The U.S. Environmental Protection Agency's emissions handbook, AP-42 section 9.2.3, sorts orchard heaters into five types: pipeline, lazy flame, return stack, cone and solid fuel. Most of these carried their own fuel reservoir in the base. A pipeline system fed every heater from one central tank, and solid-fuel heaters were just briquettes set on the ground and lit. The return stack is the one with a second pipe looping back into its chimney.
Placement mattered as much as the heater. Growers put one in the open space between every four trees, staggered from row to row, with extra heaters along the edges of the grove where cold air rolled in.
Did the smoke protect the fruit?
The smoke wasn't what protected the fruit. According to the EPA handbook, heaters protect fruit with radiant and convective heat, and that for a given fuel rate the temperature response is about the same for every type, as long as the heater is clean and doesn't leak. Pipeline heaters had the lowest particulate emissions of any design. A smokier pot burned the same oil and simply sent more of it into the sky as soot. Growers needed the heat, and the old pots delivered it inefficiently.
What frost does to citrus
Fruit can freeze before the air does. UC's citrus frost guide notes that plant tissue can drop to 32°F while the air is still warmer. Damaged fruit shows watery brown pits in the rind, called ice marks, which later let decay in. Eureka lemons and grapefruit are among the most cold-sensitive citrus. Mandarins and sweet oranges are hardier, and so is the Meyer lemon.
Frost nights still come to Redlands. Our family still grows Minneola tangelos here, and the fruit that comes through winter goes into jars such as Minneola Tangelo Marmalade and Navel Orange Marmalade.
A night of smudging
Smudging was night labor, done mostly by teenage boys and college students. Former smudgers interviewed by La Verne Magazine described the routine in the groves around La Verne. Crews went out when the thermometers dropped to 28°F and tried to hold the air at 30°F or above. They lit each pot with a torch filled with smudge oil and gasoline. A full pot burned eight or nine hours, and in a long freeze the crews refilled them in the middle of the night. Pay was about 50 cents an hour.
On a hard night the work started around 9 p.m. and ran until 7:30 or 8 the next morning. In the magazine's account of the 1937 freeze, temperatures fell below freezing 17 nights in a row.
When the soot reached the cities
The smoke didn't stay in the groves. Waldie lists six big freezes between 1913 and 1949. Smudging during the 1922 freeze shut down the Port of Los Angeles. By 1937, drivers were using their headlights at midday.
People in grove towns remembered the aftermath as much as the cold. One La Verne resident told the magazine the soot was so heavy that people left footprints on the sidewalks.
Fuel was costly too. Waldie puts the oil bill at no less than $50 an hour for every ten acres, in 1939 dollars.
How California regulated smudge pots out
According to Waldie, 1947 state legislation outlawed many of the older, smokier heaters. Later, California Health and Safety Code section 41860 set a hard limit: after January 1, 1975, no one could use an orchard or citrus grove heater unless the state board had approved it or it put out no more than one gram per minute of unconsumed solid carbonaceous material. On top of the usual air-pollution penalties, the statute lets a violator be charged "the cost of putting out the fire caused by a violation."
Law wasn't the only pressure. The Goleta Valley Historical Society points to rising oil prices in the 1970s alongside the complaints about toxic smoke. Together they ended the smudge pot as a routine tool.
The approved, cleaner heaters stayed legal, but by 2004 hardly anyone used them: the San Joaquin Valley air district's orchard heater survey counted about 100 heaters in Merced County and 100 in Fresno County, each lit about one night a year. Most of the valley's other counties reported none.
What replaced them
Wind machines took over. By 1949, Waldie writes, growers had begun experimenting with them: 125-horsepower motors turning aircraft propellers on top of towers.
They depend on a temperature inversion, a layer of warmer air sitting above the cold air in the grove on a still night. The propeller mixes that warmer air down to the trees. The EPA handbook makes the related point for heaters: when there's little or no inversion, radiant heat is the only thing that helps much.
Growers also use water. The San Joaquin Valley air district describes running well water at 50–60°F onto the ground at the base of the trees. In that valley, heaters now only back up wind machines and water on the coldest nights.
Photo by Carrie Borden on Unsplash.
