UC Master Gardener of Napa County Published in the Napa Valley Register By Tony Nunes

1. Early in the growing season
By Tony Nunes – UC Master Garden of Napa County
Published in the Napa Valley Register – January 27, 2017
Knowing basic botany and grapevine structure and selecting a variety and rootstock suited to the location can
help vineyard owners succeed. Master Gardeners call it putting the right plant in the right place. We are
fortunate to live in a region with soil types and microclimates that produce world-class wines. Just like
commercial vineyard managers, home vineyard owners need to make decisions based on their unique site,
goals and conditions and make adjustments driven by Mother Nature throughout each growing season.
It starts with a vineyard management plan for the season. Napa Valley has kicked off the year with rainfall
amounts and low temperatures not seen in many years. Determining pruning dates is the first step. In some
places, pruning has already started as vineyard managers allocate their resources throughout the valley.
Home vineyard owners may have the flexibility to delay pruning, which, in turn, delays bud break and
minimizes the potential for damage from a late frost. Some people make two passes, first to remove last year’s
old canes, then to make the final command decision: the pruning of buds created during the 2016 growing
season that will produce the crop for this year’s harvest.
As our growing season progresses, days get longer, soil temperatures rise and grapevine sap begins to flow.
This awakening from winter dormancy is one of many milestones of the growing season.
One element of integrated pest management is a plan for controlling powdery mildew (Uncinula necator) and
botrytis (Botrytis cinerea) from bud break to veraison, the color change in grapes that signals the onset of
ripening. The spores that cause powdery mildew or botrytis are ubiquitous in Napa Valley vineyards.
Powdery mildew infections from the previous year appear as a blue fungus on individual berries or clusters.
Botrytis affects the internal tissues of the berry or the entire cluster. Berries split and fungal spores can grow
out of split skins. The spores from infected clusters are distributed easily by wind. Spores can survive the
winter on the grapevine bark and cause infections the following year.
Early in the growing season, vine shoots grow rapidly. Like a jack-in-the-box, buds produce shoots and a self-
pollinating flower blooms to produce the year’s crop. Bloom and fruit set are critical periods driving yield and
fruit quality. Temperature swings and rain are not welcome during this time.
Grapevines do not require special fertilizers but should be kept in balance with nutrients available in the soil.
Too much nitrogen in the soil can cause vines to produce excess canopy, creating issues with pest
management and crop quality.
When applying fertilizer, first ask yourself why. Testing can help you determine what the vine needs. The first
test of the growing season, petiole testing, should be performed after fruit set. Petioles connect leaf blades to
the vine shoot. Samples of 25 to 50 petioles are selected for each block and tested to determine what nutrients
might be deficient or present in excess. Fertilizers are then applied according to the specific needs of that
block.

2. Grape clusters need leaves and sunshine to ripen the crop. Additionally, new buds require sunshine to
produce energy for next year’s crop. Managing the amount of leaves and their health is referred to as canopy
management.
Removing leaves at the base of the canes surrounding grape bunches allows for better sunlight penetration
and airflow. To guide decisions about leaf removal, growers observe weather conditions and how the
vineyards are oriented to the sun. Leaf removal on the west side facing the afternoon sun is limited during July
and August to minimize sunburn and bunch rot. Home grapegrowers may also need to observe their
microclimates or placement of buildings when determining how to manage the canopy.
Irrigation decisions also affect canopy health. Vineyard managers monitor soil moisture levels and visual clues
from leaf blades and tendrils during the early season’s rapid shoot growth, flowering and fruit set. When
irrigation is needed, it is best to irrigate deeply according to the site’s soil-moisture holding capacity.
Roots don’t grow in dry soil. The goal is to encourage roots to grow as deeply as possible given the site’s soil
conditions, with just enough water stress to produce high-quality fruit.
Home vineyard owners are generally optimists or at least try to be. Each year, they look forward to a
successful harvest, drawing on the knowledge gained from previous growing seasons.
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