Passive Frost Protection
of Trees and Vines
Summary of Recommendations
Several management p rac t i ces 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 rac t i ces include:
1. Site selection or choosing a loca t ion for an orchard or vineyard less
prone t o frost damage. Frost- sensi t ive crops should not be grown i n low
areas where cold a i r is trapped by n a t u r a l topography, vegetat ion, or by
manufactured obs tac les 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 torage and later
re l ease of h e a t from t h e s o i l within and upwind of a crop. To reduce the
chances of f r o s t damage, make sure t h a t t h e ground is 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)
el iminat ing or c u t t i n g ground cover, (b) keeping t h e top 1 foot of s o i l
moist, and (c) not cu l t iva t ing .
3. Delay bloom and crop development through p lan t ing on north- facing
s lopes 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 the spr ing
is benef ic i a l for avoiding frost damage.
These passive management p rac t i ces a r e less expensive than a c t i v e frost
protec t ion with sp r ink le 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 protec t ion methods a r e used,
employing these pass ive p rac t i ces can minimize the need for a c t i v e protec t ion
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 devas ta t ing 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 increases rapid ly , however,
from the 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 the most sens i t ive . Fortunately, t h e p robab i l i ty of damaging tempera-
tures occurring decreases rapid ly during these ea r ly developmental s tages .
F ros t damage can a l s o occur before a f a l l harves t i n some crops, and t h e
p robab i l i ty of damage increases i f harves t is delayed.
Proper management of your orchard or vineyard before a frost n ight 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 ight ,
a r e ca l l ed "passive protect ion." During a frost n igh t , such protec t ion prac-
tices a s sp r ink le r s and wind machines a r e categorized a s "act ive protect ion."
This l e a f l e t d iscusses passive protec t ion methods t h a t can improve effective-
ness or e l iminate t h e need for a c t i v e frost protec t ion .
Advection and Radiation Frosts
Two types of frost s i t u a t i o n s can occur. One is ca l l ed an “advection
f r o s t , " which occurs when f reez ing a i r blowing i n t o t he area d isplaces warmer
a i r t h a t was present before t h e frost occurrence. Moderate-to-strong winds ,
no inversion, cloudy or clear s k i e s , and f reez ing temperatures even during
dayl ight t y p i c a l l y cha rac te r i ze advection frosts. I n some cases, the temper-
a t u r e can remain below freezing f o r seve ra l successive days. Advection
f ros ts are 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 ther t y p e of s i t u a t i o n ,
" radiat ion f ros t ," is characterized by l igh 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 frosts a r e much more common than advection frosts i n Cal i fornia .
Mechanisms of Heat Transfer
Radiant heat is what you feel when you stand near a burning f i r e p l a c e or
Everything t h a t has a measurable tempera-
The warmer t h e source of rad ia t ion , t h e g rea te r t h e
when you are exposed t o sunl ight .
t u r e emits rad ian t heat .
amount of hea t radiated.
Radiant hea t is important i n passive frost protec t ion because more
nighttime heat is radia ted from t h e so i l t o the crop i f t h e s o i l surface
temperature can be maintained higher. The sky a l s o has an effective tempera-
ture and downward rad ia t ion t h a t cont r ibutes t o protec t ion . 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 protec t ion than during c l e a r sk ie s .
I n convection, heat is t rans fe r red by moving a i r . For example, heat is
convectively t r ans fe r red 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 fe ren t rooms i n a house. When wind moves a i r over an objec t
with a d i f fe ren t temperature, hea t is t rans fe r red t o or from the 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 frost night .
I n conduction, heat is t rans fe r red without movement of the t r a n s f e r
medium. An example would be t o p lace 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
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f i r e t o your hand and the t e m p e r a t u r e of t h e poker i n your hand w i l l
increase. Conduction is very important i n s o i l heat 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 objec t ive of pass ive 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 so t h a t
more heat 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 heat and re leases it t o the surroundings when condensing, cooling, or
freezing. S imi lar ly , water removes heat from t h e surroundings when it melts,
warms, o r evaporates. Freezing, melting, and evaporation a r e the main pro-
cesses t h a t t r a n s f e r hea t t o or from water on a frost n i g h t . Approximately
four times a s much hea t is released t o t h e surroundings during f reez ing 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 ans fe r red from t h e surroundings t o melt water as it takes 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 hea t from t h e surroundings a s f reez ing an equal quant i ty
of water. Fortunately, evaporation r a t e s are low during frost nights .
Fog, Clouds, and Wind
Fog, clouds, and wind tend t o p ro tec t crops from rad ia t ion frost . Fog
and clouds slow dropping temperatures because rad ia t ion 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 the ground. Under these
condit ions, the re is usual ly l i t t l e or no n e t loss of heat from t h e crop and
t h e temperature w i l l drop slowly o r can even increase ( f i g . 1 ) . Surpr is ingly ,
even very high t h i n clouds o r l i g h t fog can g rea t ly 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 dec l ine most rapidly when t h e r e is no wind and w i l l drop less slowly a s
wind speeds increase during a r ad ia t ion frost . The benef ic 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 the 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 area can be used a s an indica tor
of l i g h t winds. The ribbon should be about 3 feet long and should be t i e d so
tha t t h e bottom hangs about 3 feet above the ground. If t h e bottom of the
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 foo t or more,
Fig. 1. Air temperature measured over citrus during fog
formation on the nights of January 8-9,1981.
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t h e wind speed is probably g rea 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 dec l ine more
rapidly.
Humidity
Humidity is very important during a r ad ia t ion frost night because t h e
minimum temperature w i l l usual ly 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 the danger
of frost damage is grea te r . H i g h humidity reduces t h e danger because down-
ward rad ian t heat tends t o be g rea te r . Also, fog, dew, or white f r o s t ( ice)
a r e more l i k e l y t o form, re leas ing l a t e n t heat (heat s to red i n t h e water) t o
t h e s o i l , a i r , and crop when the humidity is high. In most cases, formation
of dew or white frost is benef ic i a l , except i n ci trus where research has
shown t h a t white frost forming on the f r u i t ea r ly i n a frost 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 heat capacity (heat s torage
c a p a b i l i t y ) and conductivi ty vary, depending p a r t l y on s o i l t ex tu re . Gener-
a l l y , when they a r e dry, sandy and peat s o i l s do not store or conduct hea t a s
r ead i ly a s loam and c lay soi ls . The r e s u l t is t h a t t h e r e is a g rea te r da i ly
temperature range a t t h e surface for l i g h t so i l s than for heavier s o i l s , and
t h e minimum sur face temperature is lower. S o i l s with a darker color of ten
absorb more sun l igh t than a l i g h t colored s o i l and store more heat. Conse-
quently, a reas within an orchard with lighter colored s o i l (and no ground
cover) a r e prone t o f ros t damage.
Conductivity and hea t s torage i n so i l s a r e g rea t ly af fec ted by water
content because water stores considerable hea t , and moist s o i l conducts heat
more read i ly than a dry so i l . Figure 2 shows a sample da i ly 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 surface minimum temperature is higher for a moist soi 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 he upper 1 foo t of s o i l near f i e l d
capacity t o keep t h e s o i l surface 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.
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growers have 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 occur red and t h e
orchard or vineyard s o i l was dry. They have reduced o r avoided damage when
the s o i l was moist. It is not necessary t o wet t h e s o i l below 1 foo t deep i n
t h e s o i l f o r frost protec t ion .
C u l t i v a t i o n is 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 shou ld n o t be
prac t iced during f ros t season. Turning t h e s o i l reduces hea t capacity and
conductivi ty, and it removes heat from t h e s o i l because water brought t o t h e
surface evaporates and takes hea t away i n the process. Often cu l t iva t ion
before a f r o s t n ight can result i n severe y i e l d loss where none would have
occurred otherwise. Cul t iva t ion 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 ad ia t ion ( sun l igh t ) from reaching t h e ground
surface and reduces t h e amount of heat s to red i n t h e so i l . This r e s u l t s i n a
lower minimum s o i l surface temperature and less rad ia t ion from t h e ground
surface 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 hea t from the s o i l t o the crop a t night . Idea l ly , there would be
no ground cover t o optimize passive frost protec t ion . Because ground covers
a r e benef i c i a l f o r o ther purposes, it is bes t t o keep the ground cover s h o r t ,
e i t h e r by mowing or through use of chemicals, but not by cu l t iva t ion .
S o i l and ground cover management h a s v a r y i n g effects on minimum
temperatures within orchards and vineyards. The effects are most important
under c l e a r , calm condit ions 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 benef i c i a l effects of these o ther
f ac to rs on t e m p e r a t u r e w i l l o f t e n dominate over 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 forms,
temperatures w i l l general ly be lowest near t h e s o i l surface and w i l l increase
wi th he ight from there ( f i g . 3). With extens ive f o l i a g e on the trees, t h e
effects of floor management a r e less important because the rad ia t ing surface
has moved t o t h e tops of t h e trees or vines and the temperature varies l i t t l e
with he ight within t h e orchard ( f ig . 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
frost nights (1929, 1933) in a walnut orchard near West
Covina, California. Referenced by H. B. Hanson (1956) February 4-5,1985. A similar temperature profile is com-
mon for leafless orchards with minimal or no ground cover. after C. Cole.
(15 m) height over bare ground at Davis, California on
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increase with height . Thus, good floor management is most important a t the
beginning of t h e f ros 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 ferences
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 ros t protec t ion of
c i t r u s and young deciduous trees and vines where wide p lan t spacing always
leaves t h e ground exposed during daylight .
Ice- nucleating bac te r i a a r e present on most tree and v i n e crops, and some
evidence ind ica tes t h a t t h e p o t e n t i a l for f ros t damage is grea te r when t h e
concentrat ion of ice- nucleating bac te r i a is high. Ground covers a r e probably
cont r ibutors t o t h e spread of these bacter ia . 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 area exposed t o sunl ight a s poss ib le . Energy is t rans fe r red
t o or from t h e soi l on a per- uni t a rea bas i s , and hence a l a rge r area of
moist soi l w i l l conduct and store more heat . Thus, using sp r ink le r s or
f looding t h a t wets t h e e n t i r e soi l su r face is best . Furrows should be a s
wide as poss ib le t o maximize t h e surface area 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 capaci ty) . Generally, it is unusual t o need g rea te r than 1 inch
t o 1 1/2 inches appl ica t ion 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 so i l s requi re less water with an appl ica t ion of 1/2-inch t o 1-
inch depth for a dry, sandy soi l . Note t h a t i f water is applied uniformly, a
1-inch depth is approximately 27,000 gal lons per acre (0.62 gal lons per
square foot) .
Another considerat ion is t h e condition of t h e area 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 those downwind or downslope
from other tree or vine crops where passive or a c t i v e protec t ion is prac-
t i ced . Cul t iva t ion upslope can a l s o lower t h e minimum temperature within an
orchard or vineyard. Minimum s o i l su r face temperatures measured over t u r f -
grass or over a cu l t iva ted 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 ferences measured a t
shelter height (5 feet) and above a r e less than 7°F, but colder a i r w i l l
develop over a colder surface and w i l l increase t h e l ikel ihood of frost
damage downslope. This s i t u a t i o n can be avoided by p rac t i c ing t h e same
management used i n t h e orchard or vineyard.
Site Selection
Se lec t ing a good locat ion t o grow your crop is o f t en t h e best method of
passive frost protec t ion . Because cold a i r is more dense than warm a i r , it
flows downhill much l i k e water. Thus, low spo t s where cold a i r w i l l collect
should be avoided. The tops of h i l l s are a l s o of ten cold and general ly it is
best t o p lan t on a slope. One procedure t h a t might he lp i n loca t ing poten-
t i a l l y hazardous sites is t o i d e n t i f y locat ions where fog develops on n ights
when fog is spot ty . The fog w i l l tend t o develop i n t h e cold spo t s first
because t h e cold a i r w i l l sett le t h e r e and the low spot reaches t h e dew-point
temperature before less hazardous areas. On f ros t n ights when the dew-point
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is low, these spo t s w i l l be coldes t . This method does not 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 he r i s k of damage is grea te r i f
bloom comes ea r ly . Warm temperatures tend t o bring on bloom, so plant ing on
a north- facing slope can delay bloom and reduce t h e probabi l i ty of frost
damage. It a l s o is benef ic i a l t o be downslope or downwind from a lake.
Avoid growing crops i n cold spo t s caused by obs tac les such as ra ised road
beds, buildings, o r vegetat ion t h a t i n h i b i t na tu ra 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 robab i l i ty of damaging low temperatures decreases
rapid ly i n t h e spring. Thus,