This spring cover crop is an explosion of purple and yellow as mustard and pea vetch enjoy the soft rains and early warmth.
Driving through wine country in the spring, you may see grass and clover growing between the vineyard rows. Weeds? Maybe, but more likely what you are seeing is a cover crop. Many cover crops are annually reseeding grasses or clovers. These are generally mown 1-3 times throughout the spring and the green thatch is left on the ground as a natural mulch and nutrient base for soil organisms. In late spring or early summer, the crop is allowed to set some seed, and then the whole crop is plowed under so that it doesn't compete with the vines during the dry summer months. Some vineyards, particularly those with erosion problems, may use perennial grasses which form a thick sward that holds the soil in place, and also provides traction for farm equipment in wet weather.
Many vineyards utilize a 'natural mix' cover crop of rye, barley, clover, pea vetch,and mustard. This ensures that no matter what the growing conditions are—hot, cold, wet, dry—the cover crop will have a healthy head start. Diversity as insurance.
Annual cover crops are mown several times during the spring. Crews leave the mowings on the ground as mulch and green manure to improve the tilth, biodiversity, and moisture-retaining properties of the soil. The cover crop is then tilled under in the summer after it has gone to seed, and the ground is neatly ring-rolled. Bare summer earth prevents moisture competition while the grapes are developing, and keeps the vines warmer. A lack of summer vegetation also discourages gophers. The cover crop begins growing again during the fall rains. The new growth prevents erosion, and by spring provides ready habitat and nectar for predator insects, when many insect pests are at their most prolific.
Mustard is known for its ability to absorb excess salts and return them to soil in organic mineral form. Clovers and vetch are nitrogen fixers, and add high volumes of nitrogen-rich biomass back into the soil. The cereal grasses—rye and barley—anchor the soil, are drought tolerant, and provide high quantities of lightweight fiber. Blossoming plants like the mustard and vetch attract beneficial insects.
By returning green matter back to the soil each year, vineyard owners maintain the diversity and health of the soil and its water retaining properties.