Frost
Protection March is a beautiful time of year in Napa Valley.
Trees and flowers begin to bloom and grapevines experience bud break
(when the buds push and new growth begins). Bud break usually occurs
in early March in the southern part of Napa Valley, and slightly
laterin the north. Thus begins a new vintage, a joyous, yet worrisome
and often sleepless time for the grape grower. Once the buds appear,
they are susceptible to the frigid nights and early mornings common
in Napa Valley. Something must be done to protect this young, succulent
tissue from freezing temperatures, or the vintage could be off to
a rough start. This is where the sleepless nights come in –
frost protection. Temperatures in the 20-30 degree Fahrenheit range
will likely kill new bud tissue, so frost protection is critical.
There are three major methods of frost protection for grapevines:
overhead sprinkler systems. Wine machines (fans), and smudge pots.
St. Supéry uses only overhead sprinklers for frost protection.
Sprinklers are mounted on top of vine posts throughout the vineyard.
There is a weather station at Dollarhide that triggers an
alarm when temperatures hit 35-36 degrees. The sprinklers turn on
when the temperature hits 33 degrees. When the water comes in contact
with the vine, it begins to freeze, which gives off energy, known
as latent heat of fusion. As ice forms around the bud, it acts as
an insulator while this energy keeps the cells of the young vine
tissue from freezing, slightly above 32 degrees.
In uniform, flat vineyards, wind machines are often used for frost
protection. These machines are turned on at about the same temperature
as the sprinklers, yet provide a different method of protection.
On flat, even ground, such as the valley floor, there can be little
air movement. The warm air settles higher and the cold air hangs
near the ground. Fans or wind machines circulate these layers of
air, bringing warmer air near the ground, which can increase the
temperature surrounding the vines by a few degrees.
A third method of frost protection, the smudge pot, is rarely used
anymore. It requires lighting canisters throughout the vineyard
that produce smoke. The smoke acts like a blanket, not allowing
the radiation from the daytime sun to escape from the soil, hence
keeping the ground temperature warm. You can imagine why this smoky
blanket has fallen from popularity over the years, but you can still
see old smudge pots lining the rows at some Napa Valley vineyards. |
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