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Copyright 2005, Board of Trustees, University
HEATING FOR LEATHERLEAF FERN COLD PROTECTION
liU- E LU .BPA.
C. A. Conover, R. T. Poole and G. A. Sanders
Agricultural Research Center Apopka
University of Florida ib3
ARC-A Research Report RH-78-6
I.F.AS.- Univ. of Fiori
Sprinkler irrigation of leatherleaf fern for frost and freeze control
has proven to be an effective method of freeze prevention. However, in recent
years hydrological considerations have had an influence on this method, and
some producers are considering other ways to heat.
One method of heating used in the past was use of jumbo cone or return
stack citrus grove heaters. These units have performed well in protected
ferneries, but are inefficient, expensive to operate (fuel and labor) and
present considerable fire danger in some ferneries.
Other systems of heating fern have been used at the Agricultural Research
Center, Apopka during the last 6 years, and include enclosing the fernery with
2 or 4 mil polyethylene and heating beneath the plastic. Several factors that
must be considered prior to utilizing this system include type of heater, type
of fuel, method of installing plastic and type of plastic to use. Enclosing
the fernery not only allows heating during cold periods, but also the increase
in air and soil temperatures increase yield.
Although some ferneries built in recent years do not provide attachment
points for the plastic, there are many constructed in the proper way. These
are generally wood frame units where the plastic can be stapled along the edges
of purlins between 10 to 14 foot sections. Between the sections, plastic is
supported by 14 guage galvanized wire running 900 to the purlins and spaced
every 2 to 4 feet, depending on the amount of sag that is acceptable. The
S2 to 4 mil plastic used on the top should be perforated with 3/8" drill bits
while in the roll, so that 1 to 2, 3/8" holes exist per square foot*. The
plastic used on the side walls should not be perforated. The holes drilled
in the top allow rainfall to enter, which prevents destruction of the flat
top structure and allows for air exchange.
Several types of heaters have been used inside the enclosed structures,
but pollution from unburned hydrocarbons or other compounds can be a problem.
Based on experience at this research center, and on observation in ferneries
and foliage operations, the following information is presented for consideration:
1. Standard "Modine" type unit heaters can be used with or without venting,
provided they are in good operating condition. Units not properly cali-
brated for the proper air-fuel ratio can emit excessive pollutants. Heaters
of this type in our experience can be used to maintain temperatures as high
as 55F without danger from pollutants if vented or if a perforated top is
used. However, we suggest maintaining a temperature of about 34 to 360F,
which reduces heating costs, provides adequate fern protection and reduces
possibility of pollution damage.
Natural gas may also be used in this type of heater, but sulphur content
of natural gas is sometimes high. Thus, prior to purchasing heaters designed
to burn natural gas, determine the amount of sulphur present in the gas
source since it produces sulphur dioxide during the combustion process.
The natural gas supplier can provide the information needed, and levels of
less than 1/10 of 1% sulphur are most acceptable.
Keep the bit cooled by dipping in water because a hot drill bit will melt
the layers together around the holes.
2. Several barrel-type heaters of local and regional manufacture are also
available that burn propane or natural gas. Unlike "Modine" types these
units are not designed for venting, but this is not a factor for this
type of perforated top installation. These units when properly adjusted
will operate satisfactorily in a fernery operation, and data listed in
(1) will be applicable.
3, In recent years a number of cone-type heaters that burn kerosene or diesel
fuel have become available. Generally, these were designed as industrial
spot heaters, and not for agricultural heating. However, if caution is
exercised, they can be used in ferneries. Generally, if low sulphur (less
than 1/10 of 1%) kerosene is used there is not much danger of pollutants
damaging fern if the fernery has a perforated top and a maximum temperature
of 36F is maintained.
A number of these heaters also burn diesel fuel. Generally, a notice-
able increase in pollutants has occurred where we burn diesel fuel, but when
we only attempted to maintain 35F, fern damage was avoided. However, it
is possible that damage can occur under the same conditions if burners are
not efficient or adjusted correctly. Any of the portable heating units
have the advantage of being easily moved during production operations and
to a dry storage area during summer months.
4. Of the several types of grove heaters available, return stack heaters are
the most efficient and produce the least pollutants. If kerosene is used
as fuel, very little pollution occurs provided temperatures of 300 to 35F
are maintained. Where diesel fuel is used in these heaters, excessive
levels of pollutants may occur if they are operated for 8 hours or more.
This build-up of pollutants relates to the rate of air exchange through
the perforated top, and presents the most serious problem on a still cold
night. Soot from these heaters may also make the fern unsalable if heaters
are operated for a lengthy period. These heaters also present fire hazards,
require watching and do not distribute heat well because they do not provide
forced circulation. When these heaters are used, a 3 foot square piece of
transit or asbestos should be placed above these units to deflect the hot
air and prevent melting a hole in the cover.
WARNING Any producer planning to entirely enclose a fernery with non-
perforated top and sides should remember that only vented heaters can be used
in such an installation. Generally, "Modine" type heaters are best for this
purpose and can be used to maintain any temperature desired. If non-vented
heaters are used in a non-perforated top installation, pollution damage is sure
CAUTION Where non-vented heaters are used in structures with perforated
tops, remember that pollution is more likely to occur where an attempt is made
to increase temperatures to 400F or more, heaters are out of adjustment or
pollutant containing fuels are used.
Our experience with heating of fern indicates a winter yield increase of
up to 25% is possible without any increase in disease, insect or nematode prob-
lems. One problem we have observed is cold water damage, which occurs beneath
holes in the perforated top when water of near 32F runs through a hole onto
fern that is 10F or higher in temperature than the water. This damage appears
as a grayish water soaked area in the tissue, and occurs most frequently on
young, immature fronds.