As we start the winter season today, many locals and tourist to our area look forward to the end of winter when the Napa Valley comes alive with the beauty of yellow mustard flower that has been celebrated for many years in the ‘Napa Valley Mustard Festival.’ No one can argue the aesthetic beauty of a hillside vineyard covered in the yellow flower of mustard. Working as the Farm Advisor who oversees vineyard floor management in the Napa Valley, I am at times troubled by the sight. Is there an invasive weed that has ‘taken over’ the vineyards? If it is a covercrop, is it good cover crop? And the question I get most often from grower and city folk alike, ‘What kind of mustard is that?’ Consulting the ‘Weeds of California and Other Western States’ it appears that the Napa Valley has at least five “mustards”; Short-pod mustard (Hirschfeldia incana L.) that can become a short-lived perennial, and four species that are all at some point referred to as ‘Wild Mustard’: Wild mustard (Sinapis arvensis L.); Rapeseed mustard (Brassica napus L.) Black mustard (Brassica nigra L. Koch); and Birdsrape mustard (Brassica rapa L.). How do you tell the difference? To really tell the difference you need to look closely at the flowers and the orientation of the mature fruit in relation to the stem.
Are these mustards weeds or covercrop? The answer, as with most weeds, depends on your perspective. Mustard as a group may be one of the best examples of both. In many parts of the country mustards are a serious weed problem in vegetable and cereal production. However, they also have several properties that make them a good covercrop: large tap root that can break up hard soils, usually germinate and grow quickly, providing erosion control and weed suppression, large biomass that can contribute to the organic matter of the soil, and contain chemical constituents that can provide limited nematode and weed suppression. These ‘mustards’ usually germinate in the fall when the rains start, then flower, and set seed in late winter, in time to mow for frost protection.
So, if wild mustards can act as a covercrop, why, according to many long-time Napa Valley locals, do we have much less mustard than before? There are some properties that make mustard a less than ideal covercrop. Deeply buried seeds of some species can survive for up to 50 years. Early flowering reduces growth and weed competition. Wild mustards break down very quickly and add little organic matter and almost no nitrogen to the soil. A wide variety of more suitable plants are available as covercrops, such as domesticated mustards(White mustard or Daikon radish) that have shown promise of more positive properties without as many of the negative. Other covercrops are better suited to the specific needs of the vineyard. Cereal grains, such as oats or barley are often used where vines are too vigorous or in vineyards that tend to hold moisture in the spring. Many growers utilize a ‘no-till’ system comprised of low-growing annual or perennial grasses, and where organic matter and nitrogen are needed a cereal/legume mix of barley or oats with winter pea or fava (bell) bean is very popular.
The amount of mustard in the valley may have diminished, but there will continue to be an abundance of this attractive yellow flower to enjoy for years to come…