Grape growers are very concerned with vineyard floor management (cover crops and weed control). Uncontrolled weeds consume limited soil water, compete for mineral nutrients and greatly reduce yield. They also can contaminate harvest, but excessive cultivation increases soil erosion and may prevent grower access to vineyards during critical periods. Increasingly growers have become dependent on long lived, soil applied herbicides to manage their vineyard floors. These have a relatively high potential to enter ground water systems. This research addresses two issues. The first task is to document the relationship between amount of floor cover and vine performance. That will allow growers to make intelligent decisions as to how much weed control they need. The second is to determine the extent to which irrigating the vineyard can supply the needs to both the weeds and the grapevines and so allow greater tolerance of weed competition.