Weeds compete with the vines for water, nutrients and light. They also reduce the air flow, inviting powdery mildew and other unwanted fungus, e.g. botrytis.
This is easy avoided in a conventional vineyard by spraying herbicides but harming our planet.
In an organic vineyard only mechanical methods are an option and we have invested in an expensive tractor mounted under vine weeder. However, a substantial part of our vineyard has this wonderful stony warm soil, great for all the late ripening grape varieties but causing havoc and wrecking our under-vine weeder. There it comes down to hard manual work. Instead of a hoe we use a string trimmer (weed eater) which is of course still very time consuming, hard labour work and cost intensive.
PS. Powdery mildew is the most persistent fungal problem of grapes in Marlborough and one of the most widespread fungal diseases of grapevines in the world. It is caused by the fungal pathogen Erysiphe necator and seen as ash-grey to white powdery growth on green tissue of the vine.