Frost Risk, Tuesday morning
May 11
The following text is adapted
from a April 24, 2007 article in the Penn State Fruit Times by Dr. Rob
Crassweller. Some of the information is irrelevant presented is of no use
this close to a frost event, but included for
completeness.
In the spring flower damage
is based upon the Critical Temperature for flowers. The critical temperature
is that temperature measured on a properly exposed and calibrated
thermometer at which flowers or fruits will endure 30 minutes or less
without injury. “
Extent of Damage
The extent of the damage to
flower buds can be variable even though critical temperature thresholds are
reached or exceeded. The reasons are varied:
* Not all buds are equally
sensitive
* Not all trees are equally
sensitive
* Not all cultivars are
equally sensitive (Ed. Note – but after Petal Fall, cultivars are more
similar in frost
sensitivity)
Factors impacting upon
Critical Temperature Damage
* Flower development stage -
for apples, King Bloom and Pink appear to be more sensitive than those at
Full Bloom. In general it is believed the decline in sensitivity is due to
increased water content in the floral parts with a concurrent decrease in
the ability to supercool.
* Wind
* Humidity
* Cultivar
* Tree vigor
*
* Weather prior to freeze
event - the exposure to low temperatures and dry conditions
prior to a frost event can
induce greater hardiness and resistance to frost damage
* Rate of thawing – fast
thawing may increase injury
TYPES OF FROST
Advective
Advective freezes occur when
a large mass of cold air moves into the region. It may or may not be
accompanied by clouds. The air mass then migrates into the region and is
usually accompanied by winds that exceed 5 mph. Temperatures at the surface
are below freezing and become colder with elevation. Little can be done to
protect the trees other than possibly covering a crop. Heating is not
economically feasible.
Radiational
Radiational frosts occur when
heat that has been stored in the upper soil surfaces is radiated back into
the atmosphere at night. Wind speeds are generally low and an inversion
usually develops with temperatures near the ground surface falling to or
below freezing. The rapidity and depth of the temperature drop can be
influenced by the dewpoint.
Dewpoint is the temperature
at which moisture begins to condense from the air mass. The general rate of
temperature decrease due to radiative losses can be fairly rapid until the
air approaches the dew point temperature when atmospheric water begins to
condense on the colder plant tissues (which reach atmospheric dew point
temperature first because they are colder).
The more water vapor in the
air the higher the dewpoint. Knowing the dewpoint can help determine how
quickly a grower will need to respond to frosty conditions. When the
dewpoint is above freezing then the temperature drop will be much slower. If
the dewpoint is below the critical temperature then the temperature drop is
faster. Low dewpoints also indicate dry air and potentially more difficulty
in increasing temperatures in the orchard.
Passive methods of frost
prevention
Good site selection is the
most economical and effective method of frost protection. No frost
protection mechanism can overcome poor site selection for the
orchard.
Soil moisture
On a daily basis heat is
transferred into and out of approximately the top foot of soil. When the
soil is wet, heat transfer and storage in the upper soil layer is better, so
more heat is stored during daylight for release during the night.
Considerable differences between thermal conductivity and heat capacity are
observed between dry and moist soils. However, if the soil water content is
near field capacity, wetting the soil is unnecessary. Wetting the soil to a
depth below 1 foot is unnecessary because diurnal temperature is
insignificant below that level.
Soil texture
Heavier soils with more clay
retain heat better than sandy soils. Sandy soils are also often lighter in
color and hence tend to reflect more sunlight rather than absorb it in the
form of heat.
Ground Cover Management
When grass or weeds are
present in an orchard or vineyard, sunlight is reflected from the surface
and less energy is stored in the soil. A tall dense cover crop has the
potential to reduce night time temperatures by up to 10F (5C) whereas, a
closely mowed cover crop is usually only about 2F (1C) colder than bare
soil. Therefore, during the frost season it may prove beneficial to keep the
row middles in the orchard closely mowed to increase potential daytime heat
absorption and reduce nighttime heat loss. Similarly, vegetative mulches
usually reduce the transfer of heat into the soil and hence make crops more
freeze prone. A bare undisturbed moist soil with no ground cover vegetation
can release enough heat to raise the temperature 2 to 3 degrees in the plant
canopy as compared to a sod mulched covered soil.
Maine comments
On May 10, 2010, at 7:57 AM, Maria Schumann wrote:
We survived getting 14 inches of snow dumped on us a week ago; now
they're predicting a cold night in the low twenties or even upper
teens. We have about 100 apple trees in early pink to late
pink.
Too many to cover or hang Xmas lights on!
We'll spray them with
water, make some bonfires and
cross our fingers. Any biodynamic
people out there that use
valerian
spray?
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