The Crush - May 2021 - 1

Volume 48 Issue 5 May 2021


Heat Stress Mitigation in Vineyards

By Ted Rieger
Based on climate trends, average temperatures and the number
and duration of heat waves are expected to increase over the next
several decades in California. Overnight low temperatures are
also increasing, affecting diurnal temperature shifts important for
fruit development and quality.
Heat damage to grapevines occurs from elevated air temperature
and from direct sunlight heating exposed berries and is
exacerbated by vine water stress. These stresses can result in
reduced photosynthesis, leaf losses, berry dehydration/losses
and the inability to mature a marketable crop. Heat stress and
high temperatures can reduce yield and negatively impact berry
chemistry and wine quality factors such as sugar levels, acidity,
anthocyanins and tannins.
UC Cooperative Extension (UCCE) viticulture specialist Dr.
Kaan Kurtural has measured berry skin temperatures of 130 F
on black-skinned grapes exposed to direct sunlight at the UC
Davis Oakville Station in Napa County. Kurtural said berry
temperatures are not just an issue during extreme heat waves.
Exposed berries post-veraison can absorb heat and maintain high
temperatures up to three hours per day, and these conditions can
exist up to two months until harvest.
Mark Battany, UCCE viticulture advisor for San Luis Obispo
and Santa Barbara Counties, suggests several strategies to
mitigate heat impacts on vines: vineyard/trellis design and row
orientation, canopy management, irrigation management and
vineyard floor management.
During high heat events with exposed
vineyard floors, temperatures are highest
near the ground and can be up to 7 F cooler
at 5 feet above the ground. " Bare ground

Overhead shade film trial at UCD Oakville Station.
Photo courtesy: S. Kaan Kurtural / UC Davis

when heated by direct sunlight is radiating a lot of heat, " Battany
said. He advised maintaining cover crop or native cover residue
on vineyard floors during the growing season to reduce ground
heat accumulation that radiates into the vines. Mowing, rather
than tilling, can be more favorable for heat mitigation.
Kurtural is conducting vineyard cover crop trials in Oakville
and the Central Valley, comparing till and no-till systems for
effects on carbon sequestration, greenhouse gas reduction and
monitoring temperature and evapotranspiration (ET) for each
trial. " Cover crops reduce heat loads in vineyards and the ET of
the whole vineyard system is less, particularly early in the year, "
Kurtural said. A newer hybrid of Poa bulbosa looks promising as
a low stature cover crop that forms a thick mat, goes dormant in
spring and requires no mowing.
In new vineyard plantings for heat mitigation and mechanization,
Kurtural recommends one of two trellis designs - a single
high-wire or a high-quadrilateral trellis with catch wire with
cordons 60-72 inches above ground level. " Not much vertically
shoot positioned (VSP) trellising is going in currently with


The Crush - May 2021

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