Green Manure
April 28, 2008
Spring has sprung and with it comes the endless blossoming of flowers, plants, and hay fever in Napa Valley. One cannot help but notice a difference in all of the vegetation. Being ever so curious, I wondered what’s the relationship between the flowers, weeds, and the grapevines. I sought out Dean, our Winemaker, and as I expected there is a purpose for it all…
I was introduced to the function of cover crops.
Cover crops also called “green manure” are broadly defined as any plant grown to improve any number of conditions associated with sustainable agriculture. They are fundamental, sustainable tools used to manage soil fertility, soil quality, water, weeds, pests, diseases, and wildlife.
The endless yellow blooms painted up and down the valley is mustard, which is common is not only Napa Valley but the California coast as well. Although pretty, mustard plays a very small role in the production and management of vines.
Mustard field as seen from WLW parking lot
From Dean I learned there are two different kinds of cover crops: permanent and seasonal. The seasonal seeds are planted in the fall after harvest. They grow during the winter and then are cultivated in the spring. This is done in order to prevent erosion. When I heard erosion I thought, “Huh, we don’t have hillside vineyards?” Dean went on to explain that even though the vineyards appear to be flat there is still a possibility of losing good topsoil in the winter months.
The purpose for permanent cover crops and/or weeds is to have something living in the soil at all times. Other benefits are to aid in weed control, reduce soil erosion, and to increase soil and water quality. Some soils are extremely rich so there is almost a necessity to grow permanent ground coverage. Our Bommarito vineyard located on Galleron Road in the Rutherford Appellation of the Napa Valley has such soil. So these 25 acres have a cover crop year round.
The traditional cover crop we use are fava beans or broad beans. Broad beans have a long tradition of cultivation in Old World agriculture, being among the most ancient plants in cultivation and also among the easiest to grow. Fava’s are utilized because they can over-winter and because as a legume, they fix nitrogen in the soil.
Fava Beans in our Oak Glen Vineyard
Another important thing I learned is how these crops are planted. Cover crops are planted in either one of two ways: every row or every other row. It all depends on how much vigor we want to control in the vineyard. The vines and cover crops compete for water and nutrients. If there are cover crops planted on in every row there is more vine stress and therefore the capability to control the vivacity of the vines. If the cover crops are planted in every other row than the vine is able to put more energy into its leaves and branches thus giving more to produce better grapes.
Every row- Fawn Park Vineyard
Every other row- Estate Vineyard
No cover crops- Fawn Park Vineyard
