From Thanksgiving through March, Terra Firma’s fields are “covered” with lush green vegetation that we never harvest. These “cover crops” protect our soil from pounding rain and wind, and give our soils a nice vacation from their job of growing vegetables. But winter cover crops have two major potential downsides.
In a very wet winter, they grow very tall and lush, forming a living mulch that keeps the ground wet. This can delay a farmers’ ability to get in and prepare the soil for planting by weeks. That was the scenario we faced both last year and in 2017.
This year, however, we are on the flip side. The 10 inches of rain we received, mostly in December, was not enough moisture to bring the cover crops to maturity properly. With no rain and frequent dry winds in February, our cover crop used up all the available water in the soil. In the face of this water stress, it did what most plants do when faced with certain death — it began the process of producing seed.
At Terra Firma we use “vetch” — a relative of lentils in the legume family — as our cover crop. Vetch is a vigorous and aggressive grower that wraps itself around weeds and smothers them to death. It is not a plant you want to deal with as a weed in your vegetable crops. About three weeks before setting viable seed, it makes thousands of bright purple flowers that serve as a giant “WARNING” sign to a farmer.
Most years, vetch begins to flower in very late March at the earliest. We normally use sheep to graze most of our cover crop fields. The sheep are extremely efficient at converting the vetch into manure and spreading it throughout the field. This process reduces the amount of fossil fuel (diesel for tractors) that is required in preparing the field for planting, by about 50%. It takes the sheep about 6 weeks to graze all of our cover crop fields, so we usually start in mid-February.
This year, we were just a week into the grazing process when I saw the first flowers while driving past the cover crop. I immediately stopped to get out and see if possibly I was mistaken, but no. By Monday of this week, all of the cover crop fields were in full flower.
With not nearly enough time to graze all the fields before they set seed, we began the daunting task of tractor mowing all of the cover crop fields — about 60 acres. Most years we have to mow a few fields that the sheep don’t get to in time. But I have no memory of having to mow all of our winter cover crop. The sheep have been doing it for over ten years.
Mowing the vetch — long, viney and tough plants — is not a fast job. Most years it takes about 2 hours per acre with our biggest tractor. This year it’s not taking quite as long, because the cover crop is less than two feet tall instead of 6 or 7 feet. Unfortunately, the benefits of the cover crop — nitrogen for fertilizer and carbon for biomass — are measured directly by how tall it is. Most years the cover crop provides all the nutrients our summer crops need to succeed, but this year we will have to supplement with additional fertilizers.
Thanks to the lack of rain, the warm weather and the constant dry wind, the soil is already drier than it was by the end of April last year. We started irrigation a month ago — three months earlier than in 2019 — and all our crops will require more water this year. But the fields growing cover crop are even drier than the rest of the farm.
It’s possible that 2020 is the first year of California’s next drought. But there’s no concern about us running out of water at Terra Firma anytime soon. This time last year, Lake Berryessa (our primary water source) was fill to overflowing, and all the creeks in the watershed ran high and strong for over a month. The wet spring meant that very few farmers started irrigating until May, so the reservoir is still almost full and the groundwater table still high. We’re in good shape to get through this incredibly dry year.