xt7jh98zbd9m https://exploreuk.uky.edu/dips/xt7jh98zbd9m/data/mets.xml   Kentucky Agricultural Experiment Station. 1960 journals 092 English Lexington : Agricultural Experiment Station, University of Kentucky Contact the Special Collections Research Center for information regarding rights and use of this collection. Kentucky Agricultural Experiment Station Progress report (Kentucky Agricultural Experiment Station) n.92 text Progress report (Kentucky Agricultural Experiment Station) n.92 1960 2014 true xt7jh98zbd9m section xt7jh98zbd9m A   ,:,, I
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of P IHSIIC Greervhouses
By E. M. EMMERT
UNIVERSITY GF KENTUCKY
AGRICULTURAL EXPERIMENT STATION
* LEXINGTON
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  ( ' ‘   "    44 aff A

 
 CONSTRUCTION`AND   OF PLASTIC GREENHOUSES*
By E. M. Emmert
Six years' use of plastic for greenhouses on the University of Kentucky Agri-
cultural Experiment Station farm has given conclusive proof that good crops can be
grown in such houses. For l2. years, however, crops have been grown in commer-
cial polyethylene houses at the south edge of Lexington but no definite proof was
established that crops grew wellinthem. Plastic greenhouses may make it possible
to grow out—of-season crops cheaper than in glass greenhouses because overhead
and heating costs are greatly reduced.
RESULTS WlTH CROPS
1. Bibb lettuce produced solid, extra large heads in less time than lettuce
grown in glass greenhouses. lt seems to thrive under the conditions in a plastic
- house and is of very good quality.
. 2. Grand Rapids leaf lettuce grew welliand was of good quality, just as
good if not better than when grown in glass greenhouses.
3. Head lettuce has never headed well in plastic houses.
4. Kentucky Wonder beans gr ew vigorously. However, they tended to
vine and not set pods, especially it the concentration of nitrogen in the soil was
high. lf plenty of air is admitted and the temperature is kept at 6OOF or more,
a good set can be obtained. Plastic; apparently keeps the humidity too high if
air exchange is limited.
5. Tomatoes grown at the same time in both glass and plastic houses were
compared. Here again, there was some tendency towards vegetative g1?0W'Cl'1'i1'1 thi?
plants grown in the plastic house, and good fruiting occured about a week later.
The fruits, however, were somewhat la rger than those grown under the glass
house and were of good quality. Under plastic the yields have been better in most
cases. There was some tendent y towards fruit rot in tomatoes grown in the plastic
house when air exchange was limited. Tomatoes grown in the glass house dry out
much more rapidly and are less likely to rot} however, there was more tendency
for blossom—end rot to occur in tomatoes in the glass house. Blights did not seem
to be more severe, but more Botrytis and sclerotinia rot appeared in the plastic
house tomatoes. These diseases are usually not serious if ventilation is used on
all days possible. lf ventilation is good and maneb fungicides are used liberally,
diseases are not too troublesome in plastic greenhouses.
6. Several crops of cucumbers grown in ZOO-foot houses have shown that
cucumbers grow very well under plaetit They apparently thrive in the high hum~
idity of a plastic greenhouse
7. About lO different annual flowering plants have been tried in a small way
and all did well.
*This publication supersedes Progress Report 2.8 "Low—cost Plastic Green—
houses" published in 19%% and now out of print

 PLASTICS FOR GREENHOUSES
While regular polyethylene is cheap and is good for growing plants, it lasts
only one year. lt is entirely dependable for one winter when put on in October, but ,
breaks down in early summer of the next year. Some new, long-lasting types of
plastic are being tested, but the best of these are more expensive than polyethylene.
Weatherable E_qly_ethylene_ - One way to get an inexpensive long—lasting material
would be to make the existing inexpensive one long lasting. A compound has been
devised which gives some promise of prolonging the life of the polyethylene. This is A
a special chemical formulation which is added to the melted polyethylene before being
made into sheets. lt adds very little to the usual cost of polyethylene. The product
looks and seems the same as the regular polyethylene. lt is being tried extensive-ly,
but no one is able to say definitely how long it will last.
Vinyl · Vinyl plastic costs about three times as much per square foot as
polyethyleid-e ofthe same thickness. For use on greenhouses it has to be considerably
thicker than polyethylene: thus, it costs five to six times as much per square foot. _
However, the thicker grades last for two years. lt is questionable whether the thicker
grades will lastthrough the thi rd year, although 8 emil vinyl shows some indication it
might last through the third year. One objection to vinyl is that it collects dirt much
more rapidly than polyethylene or glass.
Weatherable Mylar 3-mil Mylar was used on the house in the cover picture) ~·
Mylar’i_sT;oTt»gTii'AcTea"r  sis-e_s“ofT;N£e»a-the—1Table Mylar are being -
tested at the Kentucky Agricultural Experinient Station. So far, weatherable Mylar
has been commercially available in the 5 lmil thickness only. The current price is
nearly 15 cents a square foot for large amounts and considerably more for small
amounts. This rnakes it cost about the same per square foot as glass. Of course,
installation is rnuch cheaper because a less expensive structure can be used. However,
at the Kentucky Agricultural Experirnent Station Z »mil weatherable Mylar lasted three
yea rs without a break During the fourth year sorne bad breaks appeared on the side
lacing the south.
Weatherable Mylar has one defect in that if it is once broken, it will tear
easily, even in the Seinil thickness. This means that it ·must be fastened securely
on the gz eenhouse so that tears will not start from nail holes or from wind or wind~
carried objects. A jagged stone thrown on polyethylene will make a small hole which
does not enlarge. This same stone rnay make the same size hole on Mylar, but from _
this hole "runs" will start and the Mylar will break down. On a two-foot spacing of
sash bars. Mylar can be run lengthwise and the laps will stay together well. However,
Mylar docs not stay together in laps nearly as well as polyethylene. lf wide spacing
of sash bars is used, the l\/lylar should be placed up and over the frame.
'Festar - Teslar is a vinyl fluoride. lt is related to vinyl chloride but is
t ltiiico  t. aglie. and more resistant to weather than Mylar. lt does not get dirty 4
like regular vinyl does. However, it is rnore costly than Mylar. lf it proves satis-
factory. it rnay be cheaper when niade in larger quantities. One older type has lasted
lour years on a University of Kentut ky greenhouse, and tests in Florida show that it
may last 7 years. The new type has been on the University of Kentucky house only abo
une ytnr. but it appeared to be in good condition after the first surrimer.
4

 , Weatherable Tenite — Still another plastic being tested is weatherable Tenite.
Like the others, it was not satisfactory as first developed. However, chemicals
have been added to make it resistant to oxidation. It has remarkable clarity and
can be readily joined together by the use of a solvent. With this material the
plastic "welds" together and makes a permanent joint. The first year of test-
ing showed that a 3-mil thickness did not last. The 5-mil is doing better, and
the 8—mil has lasted well in the second year. A few tears appeared the third
year. It is not quite so costly as Mylar, but because a thick grade has to be
used makes it rather expensive.
SOURCES OF PLASTIC
A The various types of plastic needed in greenhouses can be obtained from
one or more of the following (Polyethylene can be obtained from all companies
V mentioned):
E. I. DuPont DeNemours & Company, Film Department, 1007 Market Street,
Wilmington, Delaware (Mylar, Teslar).
The Continental Can Company, Mt. Vernon, Ohio.
Gering Plastics, North 7th St. and Monroe Ave. , Kenilworth, N. J.
The Visking Corporation, Chicago, Ill.
The Eastman Chemical Company, Kingsport, Tennessee. (Tenite)
Peter & Co. , 3618 Lexington Road, Louisville, Kentucky.
(Mention of commercial products in this report is made only for purposes
_ of information. The Kentucky Agricultural Experiment Station does not guarantee
or warrant the standard of the products, and the use of the names in this report
does not imply the approval of these products to the exclusion of others which may
also be available.)
HEAT AND MOISTURE RETENTION
Sunlight does not cause as great a temperature build—up in plastic as in
I glass, and the heat loss at night is less. With a double layer of plastic, the
heat loss is about half that with glass. This fact was determined by measuring
the fuel needed to heat plastic houses.
Plants grown in plastic greenhouses retain much more moisture than those
grown in glass greenhouses because plastic has fewer laps and is tighter, This
lessens the amount of watering needed. If the houses are not ventilated, there is
more danger of disease. Ventilation and the application of fungicides are required
to prevent disease.
LIGHT TRANSMISSION
Ordinary types of plastic such as polyethylene will transmit about 90 percent
of the light that glass will transmit. However, the more ultra-violet light and heat,
the quicker most plastics deteriorate. If put on in late September, polyethylene will
hold until the next June in locations from Kentucky on 1’10I‘th.
. ·‘5·

 VENTILATION
Ventilation has been shown to be very important. The usual method of `
ventilating greenhouses, is side vents and vents in the ridges and gable ends.
The side vents often let cold air strike the plants. This, of course, is un— ,
desirable Ridge ventilation largely eliminates the cold air problem and should
be used in winter. U
Side Ventilation. Ventilation in the spring or fall can be more effective .
if the plastic on the side is left loose at the bottom so it can be raised. It is
fastened only at the top to the eave plate (usually a Z" x 4"). It hangs down and T
meets a piece of black plastic which extends about 2 or 3 inches above the soil
line.
The dead- air space on the side wall is made by putting a Zemil layer of
plastic on the outside of the side framing or supports and a thin 3/4 mil layer
on the inside of the framing. This dead—·air space reduces heat loss. If these
sheets of plastic are not too firmly attached to the sides or bottom, they may
be rolled or pulled up to provide ventilation during very warm periods. A l"
x l" or Z" piece is placed every 4 feet on the outside of the plastic to keep it
from flopping. The piece is driven into the soil at the bottom and nailed at
the top
In cold weather the clear plastic is pinned to the black plastic, or soil
can be placed along the edges to hold it. The plastic on the side must be long
enough to extend 4 to 6 inches in the soil to insure good sealing. This is readily -
removed in late March after winter is over, and the sides can be raised as
needed.
HEATING
A hot water heating system is best because it gives more uniform temper--—
atures. The cost of the equipment is high and may require somewhat more care,
although the operation can be made automatic. A steam system is also effective.
lt has about the same general layout as a hot water system, but the temperature
is not as uniform as with hot water.
Most steam and hot water heat pipes in a non—bench house are run along
the edges of the greenhouse. This is not the best arrangement to save heat, but
it is the most convenient placement of pipes. Steam and hot water systems heat _
the house uniformly. Convection of the air gets heat to the ground level. In this
process the warm air rises to the top and moves down to plants by convection or
- air movejment. As a rule, the top of the house is the warmest part, sometimes
being 2.0 warmer than the air next to the soil.
Gas heaters are relatively simple to operate; they can be entirely automatic,
and the equipment cost is comparatively low. Gas heaters may burn propane gas °
which usually costs somewhat more than natural gas. Propane gas, if combustion
is complete, gives off only carbon dioxide and water. If the concentration of car-
bon dioxide is not too great, it favors plant growth. Few propane burners, how—-
ever, give coinplete combustion. With incomplete combustion, harmful gases are
given off. If natural gas is used, the burners should be carefully vented because
some types of natural gas and their cornbustion products harm plants. Be sure to
adjust burners so you have complete combustion.
. (3..

 Burners may be bought from the Bluegrass Butane Company, Lexington, Ken-
tucky; the L. B. White Company, Onalaska, Wis. or the Modine Manufacturing Com-
pany, Racine, Wis.
A heat distribution system made of black polyethylene tubing is efficient and
is much cheaper than pipe. With this system, tomatoes planted along the wall grew
well even in sub—zero weather. Plain, clear plastic tubes can be used but do not
work quite as well as black tubes and deteriorate more rapidly.
Heat Source - Any heater that warms air that can be drawn off by a fan can
be used. The heater should not give off fumes. Steam or hot water coils can be
1 used to warm air to be drawn off by a fan, but as a rule a direct hot air heater is
more economical in both overhead and fuel costs.
_ Tube Installation - The polyethylene tube system is very simple to install.
Connect a 6—inch or larger pipe in large houses to the heat source. Connect this
pipe to a header (Fig. 1) of metal pipe with as many outlets as desired. Place the
outlets so it will be convenient to run the polyethylene tubes between rows of plants
(Fig. 2) or under benches. Black plastic tubes 4 inches in diameter and 3-mils
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Fig. 1--Heating tube header distributes heat to rows. Tubes last longer when
they are black polyethylene. The cover picture (bottom) shows plants
that have just been set alongside the tubes.
-7-

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Fig. 2-- Black plastic tubes running through house from heating header.
Partitions are for experimental tests.
thick may be used if the tubes are run between the rows. Six-inch tubes of the
same material can be used if the tubes are placed under benches. In some cases,
a small amount of metal pipe may be needed just beyond the header to prevent melt-
ing of the black plastic, for it melts at fairly high temperatures. Dampers may be
needed in the headers to insure equal warm air flow through each tube. You can tell
how evenly the heat is being distributed by watching the tubes as they inflate. If one V
tube inflates a lot faster than the others, it needs to be dampered down.
End of Tubes - It usually is best to run the tubes out of the house and have the
heater inside the house draw air in. This keeps fresh air coming into the house at all t
times.
Be sure the division of air flow is uniform and that the heat tubes are long
enough to utilize all the heat so that little goes to the outside. The end of the
polyethylene tubes can be attached to a 4-inch metal pipe which extends outside to
exhaust all possible fumes. A
With this system, the heat is delivered along the row and warms the soil and
air immediately around the plants. In this way, the temperature of the soil is higher
than with other systems.
Handling Plastic Tubes - The plastic tubes need to be stapled in place about
every 10 to 15 feet. These holders can be readily removed and the tubes rolled up
out ofthe way when they are not needed or the house is to be reworked. The black .
plastic will last several years.
Trouble with Plastic Tubes — With long lengths of tubes expansion and contraction
may cause the tubes to buckle when heat is on and the plastic expands. This is usually
not noticeable except with lengths over 100 feet. Stapling the tubes helps correct this T
problern.
..8..

 Sometimes moisture accumulates in the tube when gas heat is used and the tube
is cold at the end. This condition is easily remedied by punching holes on the underside
of the tube at the low points where water accumulates.
RESULTS OF TUBE HEAT ON TOMATOES
The tube heating system produced the best crop of tomatoes ever grown in the
Kentucky Agricultural Experiment Station‘s greenhouses. The yield was 18 pounds per
plant. The same variety (Michigan-Ohio Hybrid) grown in a glass greenhouse with steam
heat yielded 15 pounds per plant. The plastic house tomatoes were also of somewhat
better quality.
CIRCULATING FANS FOR VENTILATING PLASTIC HOUSES
AND SAVING HEAT
Since plastic greenhouses are much tighter than glass greenhouses, it is import-
_ ant to have good ventilation and air movement. In cold weather ventilation is difficult
without chilling the plants and losing too much heat. Any fan systems that keep the air
circulating and mix the warm air from the top of the house with the rest of the air are
_ helpful. It has been found that circulating fans set up as shown in Fig. 3 can save heat
and provide good air movement with a minimum heat loss. The fans were installed
according to the diagram. _
Q;. cm air .1...,,.
` from open vent
  in daytime.
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(regular vent, / I
Open On Warm Warm air drawn
days) from under closed
/ vent at night and
6u intake on cloudy days.
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