Gotta know when to mow ’em #covercrops

“The cover crop we grow with winter rain becomes the water-holding soil organic matter that gets us through our long, dry summers.”

This phrase is a staple of every farm tour and school visit that comes to Petty Ranch. But how does the winter cover become summer’s mulch? By mowing.

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Our cover crop usually includes annual grains, which die back during the summer. Winter’s green blanket of barley turns into a golden yellow mat of straw over the summer. This shades and cools the soil, retains moisture,  and inhibits dust.

How do we know the optimal time to mow? Confession time: We don’t.

It isn’t really possible to know the perfect time without knowing exactly what still lies ahead in terms of rain and weather. Without a time machine, we are out of luck.

But perfection is overrated. Farmers are often ruled by practical considerations. Do we need to allow a harvest crew to pick? Are gophers getting out of control while hidden by the greenery? Are the winter’s fig prunings brittle enough to chop nicely?

Most importantly: If it is time to water the trees, it is time for the cover crop to stop using water.

Cover crops do great things for the soil, but they do use water. That is why we like to grow them during the rainy season. When we stop getting water for free it is time to cut the cover crops off. Literally. Sadly, it looks like most of the rain for the season is now behind us. (After only 7.5 inches, making my prediction from December 4 more accurate than I would have liked.)

We started an irrigation cycle this morning that will run through Friday. Once it is completed, it will be time to start mowing.

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Why rain euphoria doesn’t last

Last Thursday we were fortunate to have a very nice rain, 1.3″ that fell steadily over night. Now it’s Monday, and I’m already considering my next irrigation. Why? This graph holds the answer.

moisture

This is a data log of our soil moisture after the rain that fell on the night of December 15/16. The light blue line represents soil moisture at 12″, the dark blue line represents moisture at 36″. The range represented by the shading and dashed lines is the preferred range; a fully saturated soil would be at the top.

What we’re seeing here is that prior to last week’s rain we were quite dry… very close to the lower acceptable limit. We were ready to irrigate had the storm failed to deliver. Thankfully it delivered in line with expectations. Where are we now?

The rain had the clearest impact at 12″ (light blue), which is as you would expect, but it was insufficient to fully saturate the soil. At 36″ (dark blue), the rain barely registered at all. Shallow soil moisture has already dropped considerably as the rainwater wicks through the soil.

Hoping the rain forecast for next Monday turns up!

(Hat tip to our technology providers at Acuity Agriculture for a great tool!)