Johannes and I found ourselves in the middle of Napa on my birthday last weekend bickering over whether the mustard had always been there (me, the idealist) or rather planted (Johannes, the realist, but also the one with much more experience working in vineyards). Well, after some research, we determined we were both right.

If you’ve ever driven through Napa on a winter day, you’re probably familiar with the vibrant yellow mustard that spreads over the valley like a blanket, contrasting with the green hillsides and old, dark brown vines. It brings cheerful color to the normally gray skies that winter brings.

The belief is that mustard is newly native to the Napa Valley, and quite honestly, an invasive weed. It is thought to have perhaps brought up by the Spanish missionaries in the 1800s. Either way, mustard has taken root and is here to stay. Hard to argue with it’s beautiful and bright color it contributes to the landscape though.

But mustard is so much more complex than just a pretty weed. It is first and foremost a very beneficial cover crop. The seeds are spread easily by wind, plant themselves and Voila!, a beautiful bright yellow mustard plant springs up after only a little rain. We are lucky that it springs up all over the valley though because it has a number of highly beneficial responsibilities that contribute to the great wines that are made in the vineyards

Erosion – Mustard’s root system holds the ground in place during heavy rain or wind, preventing the soil from washing away and disturbing the complex underground system.

Nematode growth – Mustard has a surprisingly high level of biofumigants, a natural gas released from plant tissue, that prevents population growth in nematodes, or roundworms that populate soil and can potentially be parasitic and damaging to vines, especially in a young vineyard.

Nitrogen Replacement – When planted next to other cover crops such as legumes, like fava and bell beans, the naturally occurring Nitrogen-fixing bacteria can break them down and distribute it to the appropriate places.

Water Restriction – Vines thrive off restricted water allocation during their rapid growth months, and as you know, our rainy season can continue well into Spring. Without getting too into it, vines need to be stressed and not given abundant water in order to produce high quality fruit. This way, they will put all of their effort to making sure the offspring, the clusters. Too much water will make a vine lazy and the wine dull. No one wants lazy, dull wines. Getting to the point, mustard and other cover crops will soak up some of the excess water so the struggling effect occurs.

On the other hand, in a year of drought, or several years of drought like now in 2015, the thick mustard sprouts up only in patches. With abundant rain, mustard will thrive, but during droughts, it is actually competing with the vines for what limited supply of water we receive. In this case, the dry soil is unfortunately very likely to experience erosion if large storms do finally head our way.

The good news about any mustard at all is that it attracts beneficial insects, which will in turn take care of the harmful insects. If you’ve got the right combination of these different plants, amounting to a diverse cover crop, pesticides will not be needed, vineyards and their environment will be healthier and visitors to the valley will have something beautiful to look at.

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