My first lesson about the risks that cold weather poses to fruit trees came from reading John Nichol’s “New Mexico Trilogy”, which included the Milagro Beanfield War. In the fictional valley where the novels are set, the farmers never got to harvest any fruit from their peach, apricot or plum trees. Every year, without fail, a warm spell in late winter would trick the trees into blooming, only to be followed by a hard freeze that would knock the blossoms or tiny fruit off the tree.
Different varieties of fruit (and nuts) have varying degrees of tolerance to cold weather. Tightly closed blossoms can tolerate temperatures down to 25 degrees, but the more open they are, the more susceptible. And tiny, just-pollinated fruit — connected to the tree by the most fragile stem you can imagine — will be knocked off starting at 28 degrees.
There are various ways for farmers to fight the cold in their orchards. Historically, they would make piles of prunings and leaves throughout the orchard and light them. Later, the “smudge pot” was invented: a primitive type of heater that created a large amount of warm smoke that would burn more slowly over a longer period of time. They were banned in California shortly after the Clean Air Act was passed into law.
Wind machines are a cleaner option. The opposite of windmills, they use a diesel engine to run a large propeller that keeps the air above the trees moving and thus prevents the coldest air from settling. But wind machines only work in an inversion, when the air up high is warmer than it is on the ground.
California is obviously a better place to grow fruit than New Mexio. Here, citrus is probably the crop most recognized as being cold-sensitive. But that’s because it is harvested during the coldest part of the year. Almonds, apricots, grapes, nectarines, peaches and plums are dormant during the winter. Instead, they are susceptible during and after bloom in the spring.
At Terra Firma, we have lost citrus to hard freezes in the winter, but we’ve never lost a crop of peaches or nectarines. Then again, we’ve only been growing them for 12 years. And over the hundred-plus years that stone fruit has been grown in this area, there have been several bad “spring freezes”. The most memorable occurred in the 1930s, when a very late frost in May wiped out almost every crop in Yolo County.
This week, after a ten-day period of very warm weather, a Canadian airmass descended on California. Most of the almonds orchards in the state are in full bloom, as are a large percentage of peaches and nectarines. If it gets cold enough, over $5 billion dollars worth of tree crops would be at risk of partial or complete destruction.
The only option to protect blooming trees during a large-scale cold weather event like this is water. Keeping the soil wet allows more of the heat stored in the soil to be released at night. And running sprinklers during the cold weather creates heat as the cold air reacts with the continuously flowing warmer water. You can potentially raise the temperature by 4 degrees. Whether this is enough to save the crop depends on how cold it gets, and how far along the trees are in their bloom.
On Tuesday morning, the temperature on our farm got down to 28 degrees for an hour or so, but we kept the peach and nectarine orchard warmer by running the water all night. While the ended up looking like an ice-skating rink, the air around the trees probably didn’t get much colder than 32 degrees. With below normal temperatures in the forecast for at least another seven days, we will be keeping our eye on the thermometer every night, turning the water on again if need be, and keeping our fingers crossed it doesn’t get too cold.