When the winter rains come to Napa Valley, soil erosion is at the top of many viticulturists’ minds. Terracing and other structural changes to the terroir are largely a mitigation method of the past, with many vineyards planting the grassy patches between their rows with mustards, legumes, and grasses as cover crops.

And while the blooming yellows of mustard certainly brighten up Napa’s landscape each year, these widespread plants have important ecological benefits as well.

“It’s all about watershed protection,” said Molly Moran Williams, Industry and Community Relations Director for the Napa Valley Grapegrowers. “Cover crops on hillsides prevent erosion, which, in return, protects our river and watershed.”

“On top of that, in vineyards all over the valley, cover crops add nutrients back into the soil, increase microbiome diversity, and are used widely as a climate-smart farming practice.”

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So for Brittany Pederson, who studied viticulture and enology as well as crop science before kickstarting her vineyard management career, cover crops are the perfect way to further explore her interests.

“Most viticulturists would probably agree with this, but as you go through your career and you have been in it enough, you start to really dive deep into certain topics,” said Pederson, who currently serves as Director of Viticulture at Renteria Vineyard Management. “Cover crop was one of those for me that, about three or four years ago, really started to interest me.”

Pederson says that after working with one single crop all year long, she was intrigued by the different types of cover crop and their impact on the terrain based on the soil’s need.

“I got to think about cover crop almost as a secondary crop because we utilize it to create the best type of balance with the terrain,” she said. “And going from somewhere like Carneros to Calistoga are two totally different things because the ground is different and the terrain is different, so once you get to this level of grape growing, you really have to cater specifically what you are doing for that specific site.”

Here in Napa Valley, viticulturists often use a blend of different plants, but across the board, mustards are the go-to option for most vineyards.

“Aesthetically, people really love it, and it has subconsciously become a sort of staple of Napa Valley, because everyone loves to come during that time because those colors just pop,” said Pederson. “But it is actually also used as a suppressant for nematodes, which are these tiny, tiny insects in the soil.”

The composition of the mustard plant and its root system suppresses the nematodes, pushing them deeper into the soil and thus eliminating the pest problem. Other types of cover crop are also used to account for nutrient deficiencies or overloads in the soil, with nitrogen being the clearest example that is monitored in this way.

“When we think about legumes, that definitely adds a lot of nitrogen content to the ground to the soil and eventually it goes to the vine,” said Pederson. “For places that may need more nitrogen because they aren’t getting it in a different form and we think it could use a boost, we think we can remedy it in a more natural way through cover crops instead of injecting a bunch of fertilizers.”

“On the other hand, if a vineyard is too vigorous and it is out of balance on that other side where it does not need any more nitrogen, then we will use a different type of cover crop, whether it be grasses, or we plant every other row as a permanent cover to try to achieve a more natural balance,” said Pederson.

The location and orientation of the vineyard also matter when planting cover crops, as Pederson says she will often plant the same type of grasses both on the valley floor and on steep hillsides for entirely different reasons.

“On the valley floor you may be using them to reinvigorate them and achieve that natural balance, but in the hillsides, you are really doing it to help with erosion,” she said. “I have played around with a lot of different combinations of things to see what worked in different areas … You also may have some hillside properties where you plant a permanent cover crop for erosion control, and it is great and it will naturally reseed itself.”

Garrett Buckland of Premiere Viticultural Services also uses permanent cover crops to prevent erosion in his vineyards, as he says weather is becoming more and more difficult to predict, thus complicating maintenance and the constant need to re-plant.

“And so as a result, the performance of it is meant to be fairly passive. Setting up a permanent cover cropping system will stabilize the soil and is meant to be a kind of set it and forget it kind of thing,” he said. “More and more all of these designs for vineyards that we are doing are really taking it back to a natural slope, and then focusing mostly on the grass cover crop doing the heavy lifting for us to keep the soil on the ground.”

While many vineyards do utilize cover crops, Pederson says she does encounter situations where she helps develop a property’s plan from scratch. Regardless of the state of the vineyard, she said seeding typically takes place between August and November based on when the grapes ripen that year.

“Earlier sites, whether it be Chardonnay or Sauvignon Blanc or something of that nature, we typically always seed those first because we are done harvesting them first,” she said. “In an ideal world, it would be that right after you harvest, you seed and let it go all winter.”

Come spring, vineyard managers will then tend to the cover crop again, mowing it down and following up with a process called cultivation, usually through disking or spading.

“I do both ways every year,” said Pederson. “And basically what that does is it takes that mulch and it drills it down into the soil at different depths, and that is really when you get that aeration.”

From there?

“You put it on repeat and do it all again after harvest.”