COOPERATIVE EXTENSION WORK IN AGRICULTURE AND HOME ECONOMICS
STATE OF FLORIDA
COLLEGE OF AGRICULTURE AG ULTUREAL EXTENG;roN S L;I CS E
UNIVERSITY OF FLORIDA, AND Vegetable Crop Specialists COUNTY AGENT AND
UNITED STATES DEPARTMENT OF HOME DEMOF4STRATION WOI':
AGRICULTURE. COOPERATING V E G E T A R IAGAINESVLL. FLORIDA
TO: COUNTY AGENTS
EFFECTS OF COLD WEATHER AND PROTECTION TECHNIQUES ON VEGETABLE CROPS
Weather of sufficiently low temperature to cause injury or retardation of
growth of some vegetables occurs most every winter in Florida. Vegetable growers
should understand the possible effects from cold and know ways to minimize this
TYPES OF COLD INJURY
A Injury without frost: On a number of crops this may occur at temperatures
anywhere below 70 F, depending on the crop and the stage of development. Symp-
toms vary and may appear immediately or weeks later.
Examples: Tomatoes- Reduces plant growth and plants exhibit phosphorus
deficiency. Fruit set reduced or fruit deformed or "cat-faced", puffy and mis-
shapen. Fruit near maturity may not ripen properly after harvest and will bruise
easily and soften prematurely if harvested and handled at low temperatures. The
lower the temperature and the longer the duration the more severe these condi-
tions will be.
Several other crops express similar symptoms to tomatoes, i.e. eggplant,
peppers, squash and beans.
Bolting Celery is a good example of bolting or premature seeding caused
by exposure to cold temperatures during the growing season. Young plants will
grow normally for a long period then shoot up to seed. The later in the grow-
ing season the cold occurs the quicker bolting will begin. Varieties differ
greatly in their susceptibility to bolting. Lettuce and several other crops
require high temperatures to cause bolting.
Seed Germination On most warm season crops seed planted during cold
periods will germinate slowly and erratically or not at all even with optimum
conditions following cold.
Wind burn Plants and leaves tend to dessicate and appear to be burned
with cold winds. Plants take up water more slowly than it is transpired and
evaporated in cold weather.
Pollination: Pollen may not be as viable or stigma as receptive or both
during cold weather. For crops needing cross pollination, the necessary insects
are not as active particularly bees,
Diseases Cold tends to predispose a number of crops to certain diseases.
B Frost Injury: Injury due to radiation cooling may occur when the air tem-
perature is as high as 40 F above the plant. The plant surface, however, must
drop to 320F for frost injury to occur. The acute symptoms are the killing of
the exposed plant surfaces. The symptoms described for cold injury may also ap-
Ice crystals form outside and inside the plant tissue, which ruptures cells
and evaporation and thawing dessicates cells. This gives the watery appearance
of leaves immediately following frost. Plants vary in their susceptibility to
frost. Some'very tender plants are able to recover and regrow depending on their
stage of development and severity of the cold. Examples are: Potatoes, peppers,
tomatoes and eggplant. Some cold hardy plants may be injured by frost so that
they become unmarketable or require excess trimming depending on their stage of
development. i.e. lettuce, cabbage or other cold hardy leaf crops.
C Freezing Injury: Freezing air below 32oF which moves in is the most des-
tructive. The symptoms described for cold injury and frost will occur. Usually
all but the most hardy crops will be destroyed. Even the most hardy can be des-
troyed with intense cold of long duration.
VEGETABLE GROUPING IN COLD HARDINESS:*
A Cool season crops tolerant of slightly freezing weather.
Cabbage, broccoli, spinach, beets, brussel sprouts, kohlrabi, parsnips, radishes,
rutbaga, turnips, watercress.
B Cool season crops damaged near harvest by freezing weather.
Cabbage (chinese), cauliflower, lettuce, English peas, potatoes, celery, car-
rots, celeriac, chicory, endive, mustard, parsley, strawberries.
C Cool season crops that are tolerant of FROST.
Onion, chives, garlic, leek shallots, asparagus.
D Warm season crops intolerant of FROST.
Sweet corn, snap beans, lima beans, tomatoes, peppers, summer and winter squash,
cucumbers, cantaloupes, chayotes, pumpkins.
E Warm season crops requiring continuous warm weather.
Uatermelens, sweet potatoes, eggplant, okra, hot peppers.
PROTECTIVE AMD CONTROL MEASURES
1. Site selection. Avoid planting susceptible crops in known cold or frosty
areas during season of the year when cold weather is imminent.
2. Crop selection. Grow crops that will tolerate cold during cold seasons
of the year.
3. Timing. Know the average frost-free dates for your area. Specifically
know the particular air drainage, and frost characteristics of your fields and
locations in the fields. Time the planting and subsequent harvest of crops to
try to avoid coldest periods.
4. Covering. There are a number of ways to cover plants to reduce danger
of cold. They are usually used only on young plants but may be used on older plants
such as strawberries.
(a) Hot Caps or Hot Tents: Individual waxed paper plant covers have been
used quite successfully for many crops such as tomatoes, cantaloupes,
watermelons, and peppers. They also create problems that must be coped
with. If the weather suddenly turns warm when covers are in place, they
may have to be removed or the tops opened to prevent plants from becoming
too soft or being killed by excessive temperatures inside the cover.
Weeds germinate and grow more rapidly under the covers and may be more
difficult to control when covers are removed. Insects and diseases may
*From Vegetable Production, MacGillivray J. H. Blakeston Publishing Company
be more prevalent under covers, particularly cut worms, and are diffi-
Scult to control., Finally, covers may be removed or opened for a warm
period and a killing frost may follow.
(b) Row Covers: Recently continuous row covers using waxed paper or clear
polyethylene plastic have been used to protect plants from cold and wind.
In cold sections of the country, complete coverage of plants using wire
wickets to support the cover along the row are used. The edges are cov-
ered with soil. The covers may be applied and removed with machines
either hand drawn or tractor mounted. The same disadvantages exist as
for Hou-Caps. California growers have used the same types of covers, ex-
cept the cover is applied only two-thirds to three-fourths of the way
over, leaving the south side or the leeward side open. This allows good
air movement and minimizes the problems of weeds, insects and overheating
associated with complete covers.
5. Cultivation: Never cultivate the soil if frosts are anticipated. Bare,
compact soil will give greater protection for plants than freshly cultivated or
straw-mulched soils. High moisture content soils give more protection than dry
soil. Loose, dry soil is a good insulator. It retards the rate at which the
soil will absorb heat when the air temperature is warmer than the soil and it
slows down the release of heat from the soil when the air temperature is lower
than the soil temperature. Water has a greater capacity for both absorption and
release of heat than soil. So, compact, undisturbed, high moisture soil gives
greater cold protection than loose, dry coil or compact, dry soil.
This'is of greater importance on organic soils than on mineral soils. Or-
ganic soils (muck & peats) are better insulators than mineral soils and damage is
usually most severe, because they are usually in low areas where cold air drainage
is into them.
6. Chemical Weed Control: Where chemical weed control eliminates cultiva-
tion, it reduces cold damage.
7. Water: Water may be used very effectively for frost control. With sub-
surface, surface or ditch irrigation, the water table should be kept as high as
possible without causing crop injury during frost danger periods.
Overhead Sprinklers: Overhead sprinkler systems designed or modified for
cold protection are quite effective if water is continuously sprayed on the plants.
When air temperatures approach 32F begin application of water. This water gives
off heat as it falls and freezes. One pound of water turning to ice releases 144
B.T.U. Applying 0.10 acre inches per hour will release 3,257,136 B.T.U. per acre
per hour, if all the water turns to ice. The beginning water temperature is of
little benefit. The latent heat of freezing water released to plants gives the
System requirements are:
(a) Apply the minimum amount of water for maximum protection with good
coverage. This is 0.08 to 0.12 inches per hour.
(b) Pump requirements for 0.10 acres inches per hour is 45 G.P.M. per acre.
(c) Keep ice load to a minimum.
(d) Prevent water-logging of soil.
(e) Keep equipment and operational costs reasonable.
III = =
Type sprinkler required and pressure at nozzle: One single nozzle sprinkler
with approximately 2.50 G.P.M. capacity at 50-60 psi for 5C feet by 50 feet
spacing to give 0.10 inches per hour. For 60 feet by 60 feet sprinkler spacing
use 3.60 G.P.M. sprinklers at 50-60 psi. Sprinklers making one revolution every
two minutes or less are satisfactory.
For irrigation and frost protection a 25 feet by 5C feet, 25 feet by 60 feet
or 30 feet by 60 feet spacing is more desirable. For frost protection every other
sprinkler would be shut off, which will make it a staggered 50 by 50 or 60 by 60.
This system works best on low growing crops; however, it has been used on
staked tomatoes with success.
Flooding: Field flooding may be used on some crops where land is level and
water and pumping facilities are adequate. Water may be run down every 8 to 10th
middle and allowed to stand in the field until frost danger is past. This works
well with potatoes.
A complete covering of plants with water gives excellent protection from
frost and freezing. Water must be removed immediately following frost danger.
This may require pumping water in and out several days in a row. Some crops such
as peppers and beans will tolerate immersion better than others, such as celery.
8. Plowing Under: Plowing young crops under is used extensively and works
quite well with crops that will withstand rough treatment or crops that are field-
seeded thick enough that some destruction does not affect yields. Young beans,
potatoes and even tomatoes can be completely covered with soil without causing
excess injury. Plants must be uncovered as soon as frost danger is over. The
uncovering usually involves considerable labor.(hand)
9. Succession Planting: This is a general practice with watermelons and
cantaloupes. Seed are planted in the same rows at weekly intervals up to four
or five times so that if an early planting emerges and is killed a second plant-
ing is ready to emerge shortly. With these crops the use of peat pots has worked
quite well. The pots are placed adjacent to each other and filled with good pot-
ting soil, and seeded with three or four seed. The area is covered with clear,
plastic over a frame to increase the temperature and hasten germination. The
cover is removed when plants emerge. If frost is expected, the plants are covered
with clear plastic. The plants are held until danger of frost is past before
setting in the field.
10. Black Plastic Mulch: The black plastic absorbs more heat than bare soil
and much more than straw mulch. It releases this stored heat slowly at night
which reduces frost injury to open strawberry flowers which are very susceptible
to frost. Black plastic mulched strawberries recover more quickly and produce
fertile flowers and fruit more quickly following frosts than straw-mulched or un-
S11. eat: Many forms of heat are used extensively. These include old
tiles alone or filled with sawdust and saturated with-oil, wind-rows of pine
needles, straw, sawdust, wood, palmetto roots, etc. are placed on the windward
or north side of fields and sometimes throughout the field. They are then fired
up. This creates much smoke (which is of little value) and some heat. This
heat creates an air turbulence over fields and reduces frost damage. Several
small fires dispersed over an area is much better than one large fire.
Open smudge pots and stack-type, oil-fired heaters are used extensively in
citrus groves and are used some in vegetable fields. The economic value and stage
of maturity of the crops involved determine whether a grower should use heaters.
An open pot burner of nine gallon capacity will consume about one gallon of
fuel per hour. With wind, this is increased to 4-5 gallons per hour. From 40
to 50 heaters of this type may be necessary per acre and cost nearly $20 each.
With the high initial cost for equipment and an average fuel and labor cost of
$50 per acre per night, it is doubtful whether this type of protection is eco-
nomical. With high value crops, such as staked tomatoes near harvest maturity,
and when only two to three degrees Farenheit must be overcome for only a short
time, then it may be economical. When the temperature drops well below freezing
and stays there, the risk of still losing the crop is great.
12. Wind Machines: These machines have gained wide usage among citrus grower,
however, only a few are used with vegetables. The reasons for this are that the
machines must be permanently installed and are difficult to move and are expensive
Vegetable growers move from area to area and usually operate on leased land. Wind
machines work best with only light frost and where there is a warm air inversion
above an area. With cold, clear, still weather their value is a .ted. ,/ -
- 13. Wind plus Heat Machines: Recently, portable and stationary rotating
boter type machines with an oil heat source have been perfected. At this time,
none have been used on vegetable crops in Florida. They appear to have distinct
advantages over wind alone or heat alone.
14. Foam: In 1959, a United States Patent was awarded for a foam treatment
for shielding plants from frost. Bean-and tomato plants were sprayed with a Sa-
ponin, 2.5 parts, Methocel, 7.5 parts with 1,000 parts of water. This was spumed
(foamed) over the plants just prior to anticipated frost. In one test, the tem-
perature went down to 24F and foam-covered plants survived while uncovered plants
were killed. This method has considerable promise, but requires more research on
practical application equipment and reduction in costs of the ingredients to be
Mason E. Marvel
Assistant Vegetable Crops
.. -" .... 1"
F.S. JaJison James Montelaro
Vegetable-Crops Specialist Associate Vegetable Crops
1. Beahm, R. B., and Kidder, E. H., Frost Protection by Water Application Under
Simulated Radiation Frost Conditions. Quarter Bul., Mich. Agric. Exp. Sta.
Vol 43, No. 2, p.338-344, Nor. 1960.
2. Crawford, T. V., Leonard, A. S., Frost Protection With Multiple Wind Machine-
Heater Units, Calif. Agric., Univ. of Calif., August 1962
3. Dean, Rollo H., Further Studies on Effectiveness of Water Sprinklers Used For
Protection of Citrus Nursery Stock Against Freeze Damage, 1960 W eaher Fore-
casting Mimeo 60-15, U. S. Weather Bureau, Lakeland, Florida.
4. Dowling, S. E., How to Buy On Irrigation System, Fla. Agric., Ext. Serv., Cir-
cular 173A, 1962
5. Georg, J. G., The Effects of Soil Moisture and Tillage on the Nocturnal Mini-
mum Air Temperature Over Sandy Soils, 1960, Weather Forecasting Mimeo 61-1,
U. S. Weather Bureau, Lakeland, Florida.
6. Gray, A. S., Frost Control by Sprinklers, National Rainbird Sales & Engineering
Ccrp., Azusa, Calif.
7. Griffiths, J. T., and C. H. Hendershott, Wind Machine Experience in Florida
Citrus, 1961, Proc. Fla. State Hort., Sec. 1961, Vol. 74, P.112-118
8. Harrison, D. S., Suggestions for Overhead Sprinkler Irrigation Systems Design
Used for Cold Protection of Citrus. 1962, the Citrus Industry, September
9. Harrison, D. S., and S. E. Dowling, Frost Protection of Citrus with Sprinkler
Irrigation, 1962, Mimeo Report, Fla. Agruc. Ext. Serv., Univ. of Fla., Gaines-
ville, Fla., April 27, 1962
10. Harrison, D. S. and Marvel, M. E., Cold Protection of Crops Through Sprinkler
Irrigation, Soil and Crop Soc. of Fla. Proc., Vol. 22, 1962 (In press)
11. Hendershott, C. H., Controlled Freezing of Orange Trees and Fruit, Citrus
Exp. Sta. M1imo Series 62-6, Sept. 19, 1961.
12. Horne, 0. W., Control of Summer Frost Peat Lands, Agr. Engr. Newsletter, No.
40, Univ. of Minnesota, St. Paul, 1935.
13. Kidder, E. H. and J. R. Davis, Frost Protection With Sprinkler Irrigation, Ext.
Bul. 327 (rev.) Mich. State Univ., East Lansing, 1956
14. Marvel, M. E., Ways to Beat Jack Frost In the South, American Vegetable Grower,
April 1962, Vol. 6, No. 4.
15. Mac Gillivray, J. H., Vegetable Production, The Blakiston Co., New York 1953.
16. Rogers, W. S., Protection From Frost by Water Sprinkling, American Fruit
Grower. April 1962
17. Rose, D. H., Wright, R. C., Bratley, C. O., Freezing Injury of Fruits and
Vegetables, U.S.D.A., Circular 713, 1944
18. Rowe, H. B., Moisture Reauirements in Agriculture, McGraw-Hill Book Co., New
19. Shadbolt, C. A., McCoy, 0. D. and Whiting, F. L., The Microclimate of Plastic
Shelters Used for Vegetable Production, Hilgardia, Vol. 32, No. 4, larch 1962
20. Shadbolt, C. A. and McCoy, 0. D., Temperature and Plant Responses to Paper
and Plastic Protectors on Cantaloupes, Hilgardia, Vol. 30, p.247-266, 1960.
21. Stout, G. J., Successful Truck Farming, The Macmillan CoA, New York, 1st edi-
tion 1950, p.111-138.
22. Von Pogrell, Hubertus, Experience in Frost Protection Work by Sprinkling in
Germany, Mimeo Report, Mich. State Univ., East Lansing.
23. Whitemen, T. M. Freezing Points of Fruits, Vegetables, and Florist Stocks,
USDA, AMS, Marheting Res. Report 196, 1957
24. Woodard, G. O., Sprinkler Irrigation, 2nd Edition, Sprinkler Irrigation Ass'n,
Washington, D. C. 1959.
25. Wright, C. S., Report on Frost Protection by Sprinkler Irrigation Equipment,
1956, Sprinkler Irrigation Manual, Chapt. 5, Sec. 3, Wright-Rain Ltd., Ring-
wood, Hamshire, England.
26. Yates, H. E., Adding of Heat to the Air with Sprinkler System as a Means of
Frost Protection in a Nursery. 1959, Weather Forecasting Mimeo 59-25, U. S.
Weather Bureau, Lakeland, Fla.
27. Yates, H. E., Report on Fine-Spray Water Protection, 1961, Weather Forecasting
Mimeo 61-13, U. S. Weather Bureau, Lakeland, Fla.
COOPERATIVE EXTENSION WORK
AGRICULTURE AND HOME ECONOMICS
COLLEGE OF AGRICULTURE, UNIVERSITY OF FLORIDA, STATE OF FLORIDA
UNITED STATES DEPARTMENT OF AGRICULTURE, AGRICULTURAL EXTENSION SERVICe,
CNDBARDSOFA T Y FS COUNTY AGRICULTURAL,. HOME
AND BOARDS OF COUNTY COMMISSIONER, Gainesville, Florida EMOHNTRATON, AND 4-H CLUB WORK
January 9, 1963
TO: County Agents
RE: Vegetarian Newsletter No. 58
Effects of Cold Weather and Protection Techniques on Vegetable Crops
Wind Machines: The last sentence in this paragraph should read: With
cold, clear, still weather, machines are most likely to be of value.
On exceptionally cold nights, the inversion of warm air above the earth
is usually small and is considered a weak inversion. The rate of cooling
of the aarth's surface becomes much slower once it reaches the freezing
point, especially if frost forms. Very cold air is usually very dry,
and the air density is greater than warmer, more moist air and should
be more resistant to displacement, thus rendering the machines less ef-
Machines are least effective when winds exceed 6 mph and should not be
operated both to prevent possible injury to the crop and to the machine.
More detailed information on this subject may be found in Florida State
Horticultural Society Proceedings, Volume 71, page 92, 1958. Wind Ma-
chines for Frost Protection in Florida by James G. Georg.
Mason E, Marvel
Assistant Vegetable Crops