Winter provides numerous benefits to agriculture in areas like the Central Valley of California. First among them is rain, which replenishes the soil and fills the reservoirs. But the cold temperatures also send deciduous fruit and nut trees into the hibernation period — dormancy — that they require to bloom uniformly and produce a good crop each year. Humans who work in farming also benefit, getting their own annual rest period.
But until this year, there was one beneficial phenomenon of winter that I took completely for granted : the destruction of summer weeds every fall by frost, which forms when temperatures are below 36 degrees, or by actual freezing temperatures (32 or below). All of our most aggressive and persistent summer weeds at Terra Firma — both annual and perennial species — have limited tolerance to cold temperatures and zero tolerance to freezing. Every year at some point after October 1st, frost or freezing temperatures arrive and destroy those plants. The same is also true of most of the crops we grow during the summer. The cold air freezes the water inside the leaves, exploding the cells from inside out.
Cold weather does not destroy all weeds. There are dozens or more species of winter weeds that thrive in the rain and cold, but most of them cannot tolerate the heat and dry weather of summer. Their seeds sit in the soil and await the cold temperatures and moisture that indicate to them that their season has arrived. They sprout with the first big storm of the season, then grow through the winter months before flowering and making seed in the spring — essentially sacrificing themselves for their offspring. It’s very rare that we have winter and summer weeds happily growing side-by-side in our fields.
It’s worth noting that freezing temperatures also kill many species of bugs, keeping their populations down from exploding. (In areas where winters are warmer, such as the Salinas Valley, they have to have special restrictions on farmers to avoid providing habitat for certain pests to break that lifecycle instead) In that regard, cold is not just an herbicide, but also an insecticide. And while that benefits all farmers, it is particularly helpful for those of us who farm organically.
Freezing temperatures are not without risks. Our winter crops can tolerate cold temperatures to a certain degree, but they can’t handle temperatures lower than 28 degrees for very long without starting to experience damage. Luckily that doesn’t happen here very often. About once every ten years or so, we experience weather cold enough to farm some or all of our winter crops.
Most years we have some frost in late October or early November here, and our first freeze between Thanksgiving and Christmas. In my 33 years here, we have always had frost by Christmas and usually a full freeze. But this year, it looked as if we might reach New Years Eve without either. It was so warm in November that summer weeds were still sprouting, and by the end of December they were still growing happily — if a bit slowly due to the short days — all over the farm. And not just the weeds. In our tomato fields, the plants were still growing and producing fruit, although it was not warm enough to ripen them. Meanwhile, all the seeds from fruit that had fallen on the ground were sprouting — a literal carpet of tomato seedlings.
By the time our first freeze arrived on December 28th (we skipped right over the frost, which usually precedes a hard freeze by weeks or even a month) I was getting pretty worried. If the weeds survived into mid-January, they would have likely responded to the longer days by putting on a growth spurt. Then we would have been dealing with controlling both winter and summer weeds at the same time, in the middle of winter when weed control is never guaranteed thanks to the frequent wet weather. And the winter weeds got a big Headstart with the early rains in October — they are already twice the size they normally are by now.
With the freeze arriving as it did just before New Years, 2025 set one record — latest freeze ever in TFF history — but avoided setting a scarier one: The Year of No Autumn Frost. But that year will likely come. Maybe not in my lifetime, but eventually. Winters are warming even faster than summers thanks to climate change. The year-round survival of summer weeds — and warm-weather insects — will be just two of many challenges posed to California.
Thanks,
Pablito