Of Trees and Vines Passive Frost Protection
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Passive Frost Protection
of Trees and Vines
Summary of Recommendations
Several management p r a c t i c e s can reduce t h e p o t e n t i a l for f r o s t damage
i n an orchard or vineyard. The p r a c t i c e s include:
1. Site selection o r choosing a l o c a t i o n for an orchard or vineyard less
prone t o f r o s t damage. Frost- sensitive crops should not be grown i n low
a r e a s where cold a i r is trapped by n a t u r a l topography, vegetation, or by
manufactured o b s t a c l e s t h a t i n h i b i t drainage of cold a i r .
2. Soil and ground cover management t o maximize t h e s t o r a g e and l a t e r
r e l e a s e of h e a t from t h e s o i l within and upwind of a crop. To reduce t h e
c h a n c e s of f r o s t damage, make sure t h a t t h e ground i s firm, moist, and
exposed t o s u n l i g h t i n s i d e and upwind ( u p s l o p e ) from a c r o p by ( a )
e l i m i n a t i n g or c u t t i n g ground cover, ( b ) keeping t h e t o p 1 foot of s o i l
moist, and (c) n o t c u l t i v a t i n g .
3. Delay bloom and crop development through p l a n t i n g on north- facing
s l o p e s and by s e l e c t i n g crops or v a r i e t i e s t h a t develop l a t e r i n t h e s p r i n g
is b e n e f i c i a l for avoiding frost damage.
These passive management p r a c t i c e s a r e less expensive than a c t i v e f r o s t
p r o t e c t i o n with s p r i n k l e r i r r i g a t i o n , and wind machines, and can be more cost
e f f e c t i v e i n some cases. Even when a c t i v e p r o t e c t i o n methods a r e used,
employing t h e s e p a s s i v e p r a c t i c e s can minimize t h e need for a c t i v e p r o t e c t i o n
and reduce expenses.
Cooperative Extension University of California
Division of Agriculture and Natural Resources
Leaflet 21429e (Scan of Leaflet 21429. No content was updated from 1992 printing.)Introduction
Freezing temperatures can have a d e v a s t a t i n g effect on tree and vine crop
production, if they occur during c r i t i c a l developmental periods. During
dormancy, most crops a r e relatively i n s e n s i t i v e t o a l l but extremely low tem-
peratures. The s e n s i t i v i t y of crops t o f r o s t i n c r e a s e s r a p i d l y , however,
from t h e onset of flowering t o small nut o r f r u i t s t a g e s when crops a r e gen-
e r a l l y t h e most s e n s i t i v e . Fortunately, t h e p r o b a b i l i t y of damaging tempera-
tures occurring decreases r a p i d l y during these e a r l y developmental s t a g e s .
F r o s t damage can a l s o occur before a f a l l h a r v e s t i n some crops, and t h e
p r o b a b i l i t y of damage i n c r e a s e s i f h a r v e s t is delayed.
Proper management of your orchard or vineyard before a f r o s t n i g h t can
minimize p o t e n t i a l damage. These management p r a c t i c e s , before a f r o s t n i g h t ,
a r e c a l l e d "passive protection." During a f r o s t n i g h t , such p r o t e c t i o n prac-
t i c e s a s s p r i n k l e r s and wind machines a r e categorized a s " active protection."
This l e a f l e t d i s c u s s e s passive p r o t e c t i o n methods t h a t can improve effective-
ness or e l i m i n a t e t h e need for a c t i v e f r o s t p r o t e c t i o n .
Advection and Radiation Frosts
Two types of frost s i t u a t i o n s can occur. One is c a l l e d an “advection
f r o s t , " which occurs when f r e e z i n g a i r blowing i n t o t h e a r e a d i s p l a c e s warmer
a i r t h a t was p r e s e n t before t h e frost occurrence. Moderate-to-strong winds ,
no inversion, cloudy or clear s k i e s , and f r e e z i n g temperatures even during
d a y l i g h t t y p i c a l l y c h a r a c t e r i z e advection frosts. I n some cases, t h e temper-
a t u r e can remain below f r e e z i n g f o r s e v e r a l successive days. Advection
f r o s t s a r e d i f f i c u l t t o p r o t e c t a g a i n s t . The o t h e r t y p e of s i t u a t i o n ,
" radiation f r o s t , " is characterized by l i g h t winds, temperature inversions,
c l e a r s k i e s , and daytime temperatures above 32°F (0°C). Fortunately, radia-
t i o n f r o s t s a r e much more common than advection f r o s t s i n C a l i f o r n i a .
Mechanisms of Heat Transfer
Radiant h e a t is what you feel when you stand near a burning f i r e p l a c e or
when you are exposed t o s u n l i g h t . Everything t h a t has a measurable tempera-
t u r e emits r a d i a n t h e a t . The warmer t h e source of r a d i a t i o n , t h e g r e a t e r t h e
amount of h e a t radiated.
Radiant h e a t is important i n passive f r o s t p r o t e c t i o n because more
nighttime heat is r a d i a t e d from t h e s o i l t o t h e crop i f t h e s o i l s u r f a c e
temperature can be maintained higher. The sky a l s o has an effective tempera-
ture and downward r a d i a t i o n t h a t c o n t r i b u t e s t o p r o t e c t i o n . Clouds, fog, and
high humidity help t o maintain a higher e f f e c t i v e sky temperature, which
provides more p r o t e c t i o n than during c l e a r s k i e s .
I n convection, h e a t is t r a n s f e r r e d by moving a i r . For example, heat is
convectively t r a n s f e r r e d from a furnace t h a t warms t h e a i r and then moves it
by a fan t o d i f f e r e n t rooms i n a house. When wind moves a i r over an o b j e c t
w i t h a d i f f e r e n t temperature, h e a t is t r a n s f e r r e d t o or from t h e object,
depending on which is warmer. This process occurs as cold a i r moves over
t h e ground and through a crop during a f r o s t night.
I n conduction, h e a t is t r a n s f e r r e d without movement of t h e t r a n s f e r
medium. An example would be t o p l a c e the end of a f i r e p l a c e poker i n t o a
fire. Eventually, heat w i l l be conducted up t h e poker from t h e end i n t h e
2f i r e t o y o u r hand and t h e t e m p e r a t u r e of t h e p o k e r i n y o u r hand w i l l
increase. Conduction is very important i n s o i l h e a t t r a n s f e r and it can be
affected by c u l t u r a l management. A primary o b j e c t i v e of p a s s i v e frost pro-
t e c t i o n is t o maximize heat conduction i n t o t h e s o i l during t h e day s o t h a t
more h e a t can be released t o the crop a t night.
Another mechanism of heat t r a n s f e r is t h e movement of water t h a t c a r r i e s
l a t e n t h e a t and r e l e a s e s it t o t h e surroundings when condensing, cooling, or
freezing. S i m i l a r l y , water removes h e a t from t h e surroundings when it melts,
warms, o r evaporates. Freezing, melting, and evaporation a r e t h e main pro-
cesses t h a t t r a n s f e r h e a t t o or from water on a f r o s t n i g h t . Approximately
four times a s much h e a t is released t o t h e surroundings during f r e e z i n g than
i n cooling water from 68°F (20°C) t o 32°F and four times a s much heat
must be t r a n s f e r r e d from t h e surroundings t o melt water as it t a k e s t o r a i s e
t h e t e m p e r a t u r e from 32°F t o 68°F. Approximately 166 BTUs of h e a t a r e
released when a pound of water changes from a l i q u i d a t 32°F t o ice a t 32°F.
Evaporation can a l s o be an important f a c t o r because it removes approximately
7-1/2 times a s much h e a t from t h e surroundings a s f r e e z i n g an equal q u a n t i t y
of water. Fortunately, evaporation r a t e s are low during f r o s t nights.
Fog, Clouds, and Wind
Fog, clouds, and wind tend t o p r o t e c t crops from r a d i a t i o n frost. Fog
and clouds slow dropping temperatures because r a d i a t i o n from t h e sky downward
is increased r e l a t i v e t o t h a t emitted upward from t h e ground. Under these
conditions, t h e r e is u s u a l l y l i t t l e or no n e t l o s s of h e a t from t h e crop and
t h e temperature w i l l drop slowly o r can even i n c r e a s e ( f i g . 1 ) . S u r p r i s i n g l y ,
even very high t h i n clouds o r l i g h t fog can g r e a t l y slow temperature drop.
L i g h t wind can markedly a f f e c t slowing temperature drop. Temperatures
w i l l d e c l i n e most r a p i d l y when t h e r e is no wind and w i l l drop less slowly a s
wind speeds i n c r e a s e during a r a d i a t i o n frost. The b e n e f i c i a l e f f e c t s of
l i g h t winds a r e less when cold a i r has s e t t l e d and an inversion has formed.
Therefore, l i g h t wind e a r l y i n t h e evening w i l l slow temperature drop more
than t h e same wind speed l a t e r during a f r o s t night. A hanging p l a s t i c r i b -
bon on t h e edge of an orchard o r i n an open a r e a can be used a s an i n d i c a t o r
of l i g h t winds. The ribbon should be about 3 feet long and should be t i e d s o
t h a t t h e bottom hangs about 3 feet above t h e ground. If t h e bottom of t h e
ribbon is obviously f l u t t e r i n g and displaced from v e r t i c a l by a f o o t or more,
Fig. 1. Air temperature measured over citrus during fog
formation on the nights of January 8-9,1981.
3t h e wind speed is probably g r e a t enough t o slow temperature drop. If it is
hanging s t r a i g h t down, t h e r e is no wind and temperatures w i l l d e c l i n e more
rapidly.
Humidity
Humidity is very important during a r a d i a t i o n frost night because t h e
minimum temperature w i l l usually not drop much a f t e r reaching t h e dew-point
temperature. If you have a low dew-point near 32°F or below, then t h e danger
of f r o s t damage is g r e a t e r . H i g h humidity reduces t h e danger because down-
ward r a d i a n t heat tends t o be g r e a t e r . Also, fog, dew, or white f r o s t ( i c e )
a r e more l i k e l y t o form, r e l e a s i n g l a t e n t h e a t (heat s t o r e d i n t h e water) t o
t h e s o i l , a i r , and crop when t h e humidity is high. I n most cases, formation
of dew or white frost is b e n e f i c i a l , except i n c i t r u s where research has
shown t h a t white f r o s t forming on t h e f r u i t e a r l y i n a f r o s t night can lead
t o more damage than when no f r o s t is present.
Soil, Water Content, Cultivation, and Ground Cover
S o i l s can a f f e c t temperature drop because h e a t capacity (heat s t o r a g e
c a p a b i l i t y ) and conductivity vary, depending p a r t l y on s o i l t e x t u r e . Gener-
a l l y , when they a r e dry, sandy and peat s o i l s do not store or conduct h e a t a s
r e a d i l y a s loam and c l a y soils. The r e s u l t is t h a t t h e r e is a g r e a t e r d a i l y
temperature range a t t h e s u r f a c e for l i g h t s o i l s than for heavier s o i l s , and
t h e minimum s u r f a c e temperature is lower. S o i l s with a darker color o f t e n
absorb more s u n l i g h t than a l i g h t colored s o i l and store more heat. Conse-
quently, a r e a s within an orchard with lighter colored s o i l (and no ground
cover) a r e prone t o f r o s t damage.
Conductivity and h e a t s t o r a g e i n s o i l s a r e g r e a t l y a f f e c t e d by water
content because water stores considerable h e a t , and moist s o i l conducts h e a t
more r e a d i l y than a dry s o i l . Figure 2 shows a sample d a i l y temperature
p r o f i l e change of a moist and a dry s o i l . The c r i t i c a l f a c t o r t o f r o s t pro-
t e c t i o n is t h a t t h e s u r f a c e minimum temperature is higher for a moist s o i l .
Most of t h e d a i l y change i n temperature occurs i n t h e upper 1 foot, so it is
important t o maintain water content i n t h e upper 1 f o o t of s o i l near f i e l d
capacity t o keep t h e s o i l s u r f a c e temperature a s high a s possible. Many
Fig. 2. Daily range of soil temperature at various depths
for soils with high and low conductivity.
4growers h a v e had n e a r l y t o t a l y i e l d l o s s e s when f r o s t o c c u r r e d and t h e
orchard or vineyard s o i l was dry. They have reduced o r avoided damage when
t h e s o i l was moist. It is not necessary t o wet t h e s o i l below 1 f o o t deep i n
t h e s o i l f o r frost p r o t e c t i o n .
C u l t i v a t i o n i s d e t r i m e n t a l t o f r o s t p r o t e c t i o n and s h o u l d n o t be
p r a c t i c e d during f r o s t season. Turning t h e s o i l reduces h e a t capacity and
conductivity, and it removes h e a t from t h e s o i l because water brought t o t h e
s u r f a c e evaporates and t a k e s h e a t away i n t h e process. Often c u l t i v a t i o n
before a f r o s t n i g h t can result i n severe y i e l d loss where none would have
occurred otherwise. C u l t i v a t i o n becomes less of a problem a s leaves develop
and t h e floor of t h e orchard or vineyard becomes more shaded during daylight.
Ground cover i n h i b i t s s o l a r r a d i a t i o n ( s u n l i g h t ) from reaching t h e ground
s u r f a c e and reduces t h e amount of h e a t s t o r e d i n t h e s o i l . This r e s u l t s i n a
lower minimum s o i l s u r f a c e temperature and less r a d i a t i o n from t h e ground
s u r f a c e t o warm t h e a i r and crop during a frost. Ground cover a l s o slows
t r a n s f e r of h e a t from t h e s o i l t o t h e crop a t night. I d e a l l y , there would be
no ground cover t o optimize passive frost p r o t e c t i o n . Because ground covers
a r e b e n e f i c i a l f o r o t h e r purposes, it is b e s t t o keep the ground cover s h o r t ,
e i t h e r by mowing or through u s e of chemicals, but not by c u l t i v a t i o n .
S o i l and ground c o v e r management h a s v a r y i n g e f f e c t s on minimum
temperatures within orchards and vineyards. The effects are most important
under c l e a r , calm conditions when there are no leaves on t h e trees. If there
are clouds, fog, or s i g n i f i c a n t wind, t h e b e n e f i c i a l effects of t h e s e o t h e r
f a c t o r s on t e m p e r a t u r e w i l l o f t e n dominate o v e r b e n e f i t s or d e t r i m e n t s
r e s u l t i n g from s o i l and ground cover management.
When t h e r e a r e no leaves on t h e trees and an i n v e r s i o n f o r m s ,
temperatures w i l l generally be lowest near t h e s o i l s u r f a c e and w i l l i n c r e a s e
w i t h h e i g h t from there ( f i g . 3). With e x t e n s i v e f o l i a g e on t h e trees, t h e
effects of floor management a r e less important because t h e r a d i a t i n g s u r f a c e
has moved t o t h e t o p s of t h e trees or vines and the temperature varies l i t t l e
with h e i g h t within t h e orchard ( f i g . 4). Above t h e orchard temperatures w i l l
Fig.3. An example of temperature changes up to 50 feet Fig. 4. Mean minimum temperatures averaged over all
(15m) height over bare ground at Davis, California on frost nights (1929, 1933) in a walnut orchard near West
February 4-5,1985. A similar temperature profile is com- Covina, California. Referenced by H. B. Hanson (1956)
mon forleafless orchards with minimal or no ground cover. after C. Cole.
5i n c r e a s e with height. Thus, good floor management is most important a t t h e
beginning of t h e f r o s t season for deciduous trees and vines; it becomes less
important a s t h e ground is shaded by crop leaves. Air temperature d i f f e r e n c e s
of only 0.5°F between orchards with and without ground covers a t 5-foot
height have been reported when orchard f o l i a g e is f u l l y developed (H. B.
Hanson 1956). Floor management is always important f o r f r o s t p r o t e c t i o n of
c i t r u s and young deciduous trees and vines where wide p l a n t spacing always
leaves t h e ground exposed during daylight.
Ice- nucleating b a c t e r i a a r e present on most tree and v i n e crops, and some
evidence i n d i c a t e s t h a t t h e p o t e n t i a l for f r o s t damage is g r e a t e r when t h e
concentration of ice- nucleating b a c t e r i a is high. Ground covers a r e probably
c o n t r i b u t o r s t o t h e spread of t h e s e b a c t e r i a . Therefore, it is prudent t o
minimize ground covers even a f t e r appreciable crop f o l i a g e has developed i n
t h e spring.
When applying water to a soil for passive frost protection, cover as much
of t h e surface a r e a exposed t o s u n l i g h t a s p o s s i b l e . Energy is t r a n s f e r r e d
t o or from t h e s o i l on a p e r - u n i t a r e a b a s i s , and hence a l a r g e r a r e a of
moist s o i l w i l l conduct and store more h e a t . Thus, using s p r i n k l e r s or
flooding t h a t wets t h e e n t i r e s o i l s u r f a c e is best. Furrows should be a s
wide a s p o s s i b l e t o maximize t h e s u r f a c e a r e a t o be wetted. The amount of
water t o apply depends on dryness of t h e s o i l r e l a t i v e t o t h e maximum it can
hold ( f i e l d c a p a c i t y ) . Generally, it is unusual t o need g r e a t e r than 1 inch
t o 1 1/2 inches a p p l i c a t i o n depth t o r e f i l l a r e l a t i v e l y dry, clay loam
s o i l . Lighter s o i l s r e q u i r e less water with an a p p l i c a t i o n of 1/2-inch t o 1-
inch depth for a dry, sandy s o i l . Note t h a t i f water is applied uniformly, a
1-inch depth is approximately 27,000 g a l l o n s p e r acre (0.62 g a l l o n s per
square f o o t ) .
Another consideration is t h e condition of t h e a r e a surrounding your
orchard or vineyard. Crops t h a t a r e downwind from pasture or low-growing
crops a r e i n more danger of frost damage than t h o s e downwind or downslope
from other tree or vine crops where passive or a c t i v e p r o t e c t i o n is prac-
t i c e d . C u l t i v a t i o n upslope can a l s o lower t h e minimum temperature within an
orchard or vineyard. Minimum s o i l s u r f a c e temperatures measured over t u r f -
g r a s s or over a c u l t i v a t e d f i e l d a r e a s much a s 7°F lower than over a firm
bare ground (H. B. Hanson 1956). The a i r temperature d i f f e r e n c e s measured a t
shelter h e i g h t (5 feet) and above a r e less than 7°F, but colder a i r w i l l
develop over a colder s u r f a c e and w i l l i n c r e a s e t h e likelihood of f r o s t
damage downslope. This s i t u a t i o n can be avoided by p r a c t i c i n g t h e same
management used i n t h e orchard or vineyard.
Site Selection
S e l e c t i n g a good l o c a t i o n t o grow your crop is o f t e n t h e best method of
passive f r o s t p r o t e c t i o n . Because cold a i r is more dense than warm a i r , it
flows downhill much l i k e water. Thus, low s p o t s where cold a i r w i l l collect
should be avoided. The t o p s of h i l l s are a l s o o f t e n cold and generally it is
best t o p l a n t on a slope. One procedure t h a t might h e l p i n l o c a t i n g poten-
t i a l l y hazardous sites is t o i d e n t i f y l o c a t i o n s where fog develops on n i g h t s
when f o g is s p o t t y . The f o g w i l l tend t o develop i n t h e cold s p o t s first
because t h e cold a i r w i l l settle t h e r e and t h e low s p o t reaches t h e dew-point
temperature before less hazardous areas. On f r o s t n i g h t s when the dew-point
6is low, t h e s e s p o t s w i l l be c o l d e s t . This method does n o t work for c o a s t a l
fogs t h a t move i n from t h e ocean.
One important f a c t o r t o consider is t h a t t h e r i s k of damage is g r e a t e r i f
bloom comes e a r l y . Warm temperatures tend t o bring on bloom, so p l a n t i n g on
a north- facing slope can delay bloom and reduce t h e p r o b a b i l i t y of f r o s t
damage. It a l s o is b e n e f i c i a l t o be downslope or downwind from a lake.
Avoid growing crops i n cold s p o t s caused by o b s t a c l e s such as r a i s e d road
beds, buildings, o r vegetation t h a t i n h i b i t n a t u r a l cold a i r drainage.
Delaying Bloom and Leafout
Any management p r a c t i c e s t h a t delay bud break w i l l reduce the chances of
f r o s t damage because t h e p r o b a b i l i t y of damaging low temperatures decreases
r a p i d l y i n t h e spring. Thus, l a t e or double pruning t o delay l e a f o u t or
bloom is b e n e f i c i a l f o r crops grown i n f r o s t - p r o n e areas. Also s p r i n k l e r
i r r i g a t i n g or fogging during warm winter or s p r i n g days can delay bud break
and reduce chances of f r o s t damage. Research has shown t h a t f r o s t damage
normally occurs when t h e ice f i r s t forms on leaves and then propagates i n t o
t h e buds, blossoms, f r u i t , or nuts. Consequently, delaying t h e development
of leaves w i l l reduce the r i s k of frost damage. S e l e c t i n g crops or v a r i e t i e s
t h a t bloom l a t e r i n s p r i n g can reduce t h e l i k e l i h o o d of f r o s t damage.
References
B a l l a r d , J . K., and E . L . P r o e b s t i n g . 1981. F r o s t and f r o s t c o n t r o l i n
Washington orchards. Washington S t a t e University Extension B u l l e t i n
0634.
B a r t h o l i c , J . F. 1979. S i t e s e l e c t i o n . In: Modification of t h e Aerial
Environment. Gerber and B a r f i e l d ( e d s . ) . American S o c i e t y of
A g r i c u l t u r a l Engineers. S t . Joseph, Michigan.
Hanson, H. B. 1956. Influence of s o i l temperature and ground cover on
orchard minimum temperatures. U.S. Weather Bureau (publisher unknown).
K a s i n a t i s , A. N . , B. E. Beardon, R . L. Sisson, R . A . Parsons, A. D. Reed, and
K. Rowers. 1975. F r o s t p r o t e c t i o n for n o r t h c o a s t vineyards. UC
L e a f l e t 2743.
Opitz, Karl W., Robert F. Brewer, and Robert G. P l a t t . 1979. Protecting
c i t r u s from cold l o s s e s . UC L e a f l e t 2372.
S c h u l t z , H . B . , and R . J . Weaver. 1977. P r e v e n t i n g f r o s t damage i n
vineyards. UC L e a f l e t 2139 (out of p r i n t ) .
7The Authors
The authors are Richard L. Snyder, Biometeorologist, Cooperative Extension, Davis;
Kyaw Tau Paw U, Biometeorologist, Department of Land, Air &Water Resources, Davis;
and James F. Thompson, Agricultural Engineer, Cooperative Extension, Davis.
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Publication 21429e
Most recent printing 1992. Digitally re-issued 2002. No content was updated from the 1992 printing.
©1992, 2002 by the Regents of the University of California
Division of Agriculture and Natural Resources
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Director, University of California, Agriculture and Natural Resources, 300 Lakeside Drive, 6th floor, Oakland, CA 94612-3550;
(510) 987-0096. For information about ordering this publication, telephone 1-800-994-8849.You can also read
