Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
Zt3'~'4
METHOD OF CONTROLLING ENVIRONMENTAL CONDITIONS IN THE
PROXIMITY OF VEGETATION OR CROPS WITH THE AID OF
PROTECTIVE SHEETING, PROTECTIVE SHFETING FOR USE IN THIS
METHOD, AND A PROTECTIVE DEVICE COMPRISING SHEETING.
BACKGROUND OF THE INVENTION
This invention relates to a method of controlling
environmental conditions in the proximity of vegetation
or crops in the cultivation of plants with the aid of
one or more protective sheets of plastics material, said
sheets being provided with perforations, each perforation
having a protruded rim.
The use of protective sheeting made of plastics
material provided with perforations having a protruded ri.m
as covering for cold-ground cultures, haystacks etc. is
known from DE-B-l 163 5~6.
Described is a foil and its use wherein the foil is
provided with a relatively small number of perforat.ions of
relatively large dimensions whereby each perforat:ion shows
a r.im which projects out of the plane of the sheet.ing.
This foil can be used, with the perforations point.ing
outwards, to cover haystacks to promote drying thereof and
to prevent humidification by rain.
The main purpose of such known foil however seems to
be a uniform supply of sir and water to the soil and to
prevent at the same time excessive evaporation of soil
moisture. The latter is reached by the fact that condensation
of moisture will occur at the inside of a foil covering a
culture area, the foil having its projections pointing
inwards, and that upon saturation the condensate will be
guided by the projections to flow back to the soil.
~Z9Z874
Condensation of water vapour on such a material will result
in drop-for-ma-tion first, if the drop-formation is excessive
the flowing back of water will be observed. The presence of
condensate in dropform is a serious drawback as the light-
transmittance is thereby reduced; such a reduction has a
marked influence on the growth of the plants.
Such known foil is described to be used for open air
applications only; no indication is given of its use in
greenhouse cultivation. If such foil were used in greenhouse
cultivation, the problem oflighttransmittance reduction upon
condensation of water vapour would also exist.
Further in greenhouse acultivation, control of humidity,
temperature and luminance in times of intense sunshine is
of great importance. The use of such known foil having a
relatively small number of relatively large openings for
the purposes as indicated does not offer any advantage.
~ he present invention has as a goal to provide a method
as is indicated in which a plastics protective sheeting is
used whereby for above given drawbacks a solution is provided
and which foil may be used with advantage both in cold
ground applications and in greenhouse applications.
SUMMARY OF THF INVFNTION.
Aforementioned goal is according to the invention
achleved by a method of the indicated type in which use
is made of plastics sheeting which at least partly is
provided with perforations having the form of micro-
perforations, each of said microperforations having a
protruded rim whereby the diameter of the smallest section
of each microperforation has a value of between 50 and
2000 ~m and the number of microperforations per unit area
is between 25 and 40000 per cm2.
In particular use is made of plastics sheeting which at
least partly is provided with microperforations having a
protruded rim whereby the diameter of the smallest section
lZ92874
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is 100 - 1000 ~m and the number of microperforations
is 40 - 80/cm2.
With use of a sheeting as indicated in the method
of the invention a number of advantages is reached such as:
- Moisture retention is much higher than concentional
smooth foil or foil as descirbed in DE-B-l 163 596. Up to
very high humidity no dropform condensation is observed;
moisture is present as a homogeneous layer on the foil
between the microperforations
- Overall light-transmittance is substantially not
influenced by the presence of condensate
- The presence of microperforations provides the
protective sheeting with light-scattering characteristics
as a result of which shadows in the vegetation are reduced
and at the same time burning as a result of excessive light
intensity is prevented. sheeting with
The method of the invention in which use is made ofY
microperforations is particularlyuseful in greenhouse
cultivation as will be describedhereinafter.
In greenhouse cultivation, particularlyin regions
where the climate is temperate, use is made on a wicle scale
of protective structures constructed with one or more sheets
for the purpose of restricting the radiation of heat, f`or
example when the atmosphere cools at night. For this purpose
use is made of closed sheets or sheets provided only
sparingly with perforations in order to achieve the best
possible thermal insulation. The sheets are applied in the
form of webs, which can be rolled up, along the side, and
the top of the greenhouse, so that when the sheets are
completely unrolled protection is provided on all sides.
Since a relatively high degree of humidity prevails in
greenhouses, condensation quickly occurs when the air
cools in the greenhouse, which leads to problems,
particularly in the case of webs disposed horizontally
above the plants. The condensate collecting on the top of
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the sheet, together with any water resulting from leaks,
gives rise to local sagging of the sheet. In order to
avoid damage to the sheeting through excessive local
accumulation of water, it is customary to inspect the
sheeting every morning and to pierce it in the places
where water has collected. This is a time-consuming
operation, while in addition the water flowing out may
cause damage to the plants situated beneath it. Moreover,
when cooling occurs, condensation also takes place on the
lower face of the sheeting, together with locally
concentrated dripping.i It is known to counter this dripping
by adding anti-condensation media to the material of which
the sheeting is made, but these additives usually have a
short working life because they diffuse out of the sheeting,
while in addition the transmission of light is adversely
affected by such additives.
In cultivation on cold ground the plants and the
ground are in some areas protected against drying out and
intensive cooling in the first growth phase by covering
them with perforated sheeting~ The presence of perforations
are on the one hand desirable in order to ensure that rain
can reach the plants and the ground under them, while on
the other hand the perforations are necessary in order to
permit the ventjlation required for the photosynthesis
process.
These known perforated sheets in use at present are
smooth and have a perforation density of 500 to lOOû holes
per square metre, with a passage rate of about 5 to 10 for
an average hole size of about 10 millimetres.
The use of these known perforated sheets has the
disadvantage that, because the large holes formed in them,
the plants situated under the sheets are more easily
accessible to harmful insects, while in addition damage
due to extreme cold cannot be entirely prevented because
of the large holes.
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The present invention now seeks to provide a method
whereby the favourable results of the use of the previously
mentioned smooth, closed sheets and the subsequently mentioned
smooth perforated sheets are combined, while however the
disadvantages of the use of these two types of sheets are
avoided.
Another aim of the invention is to provide a method
of the kind in question which uses a protective sheet which
is suitable both for applications where smooth, closed sheets
are generally used and for applications in which smooth
perforated sheets are used.
In addition, the invention seeks to provide a method
which can be used, for example, for the drying of crops.
According to the invention these aims are achieved by
a method of the abovedescribed type which is characterized
in that the protective sheeting used is a plastics sheeting
which is at least partially provided with microperforations,
each microperforation having a protruding rim. When the
plastics sheeting provided at least partially witl- micro-
perforations is used, it is in the first place ensured that
the average light intensity in the greenhouse is higher than
when a smooth unperforated sheet is used. Because of the
presence of microperforations a direct increase of the light
intensity is on the one hand achieved, while in addtition
the presence of irregularities in the form of the projections
around the perforations enables a dispersion of light to be
obtained.
This dispersion of light results in reduced shading
action, so that on the average the leaf surfaces receive
more incident light than is the case when an ordinary
smooth sheet is used.
Because of this increased intensity of light, the
photosynthesis process necessary for growth is assisted.
When use is made of a plastics sheet provided at
least partially with microperforations in the manner
lZ9Z874
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described above, it is in addition ensured in the case of
greenhouse cultivation that condensed moisture will
accumulate in and between the projections of the micro-
perforations. Through the presence of the microperforations,
the effective area of the sheet is in fact greatly
enlarged, so that the chance of water being retained is
very much greater, while in addition it is ensured that
there is less tendency to drip, and the tendency for
water to accumulate at lower lying parts of the sheet
will be considerably reduced. The surface of the sheet
is in a manner of speaking covered with a uniform thin
layer of moisture with the result that during the night-
time the sheeting has an increased insulating action and
the irradiation of heat is considerably reduced. During
the daytime the water uniformly distributed over the
surface of the sheeting will be able to be evaporated
fairly quickly through irradiation with sunlight, so that
the sheeting will regain its permeability.
Through the use of sheeting provided with microperforations,
in the case of greenhouse cultivation of plants, the
condensation of water during the night will result in the
formation of sheeting having relatively high thermal
insulation power, while in the daytime it will have just
a reduced thermal insulation power because of the
evaporation of the water condensed on the sheeting during
the night. The number of perforations per unit of area is
in addition of such a nature that in practically all cases
the permeability of the sheeting to air is retained, so
that good renewal of the air lying above the plants is
always ensured.
In the case of cultivation on cold ground, the presence
of microperforations will ensure that the plants are well
protected against harmful insects, while good circulation
of air is always possible. Moreover, the microperforations
1~9~ 7~
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can be so selected that rain falling on the sheeting
will be allowed through in linely divlded form, so that
above the growing plants a uniform spray effect is
obtained and that damage is not in any circumstances
inflicted on the plants lying therebeneath by the
excessively hard impingement of raindrops.
Finally, for example for the drying of plants, the
sheeting provided with microperforations can be used
successfully in such a form that water vapour can diffuse
from the plants through the sheeting to the atmosphere,
but that any incident rainwater will not pass to the
interior through the perforations. This property can be
achieved very simply by judicious choice of the dimensions
of the perforations; with decreasing diameter of the
microperforations a situation may occur in which the
outwardly directed force resulting from the surface
tension of water in the perforations is greater than the
inwardly directed force resulting from the falling of
drops of water on the sheeting. As long as the outwardly
directed force is greater, the entry of rainwater will be
prevented.
In the method according to the invention, all the
projections in the protective sneeting used are in
particular directed in the same direction relative to the
plane of the sheeting, and this direction in relation to
the plants or crops is selected in dependence on the
desired aim. Thus, for example, in the case of greenhouses
an arrangement will be selected in which the projections
around the microperforations will be directed towards the
plants. Water condensing on the sheeting will then be
deposited between the projections and in the perforations,
while the flow of drops along the surface of the sheeting
will be halted by the projections. When the sheeting
provided with microperforations is used for cultivating
plants on cold ground, it will similarly usually be ensured
l~Z87~
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that the projections are directed towards the plants.
Because of the desire in certain cases nevertheless to
allow incident rainfall to pass through in finely divided
form to the plants, the projections will likewise be
inwardly directed, in order in this way to allowacertain
capillary action on the part of the projections to come
into effect.
For the drying of crops, on the other hand, the
projections of the microperforations will generally be
directed outwards in order to ensure the greatest possible
discharge of water vapour to the atmosphere, while at the
same time effectively stopping any tendency for water to
flow into the interior.
In certain cases use is advantageously made of
sheeting in which microperforations project outwards,
in relation to the plants, in certain parts of the sheeting,
while in certain other parts of the sheeting the perforations
project inwards. Sheeting of this kind is used in cases
where increased circulation of air under the sheeting is
desired. Because of the shape of the projections there is
a certain preferred direction in the perforations for the
flow of air; the presence of perfrrations extending in both
directions in the sheeting, enables the circulation of air
from outside to inside and back to be increased.
In another advantageous embodiment of the method
according to the invention, sheeting is used which in
addition to microperforations also has other perforations.
Cases may for example occur in which the need for
rainwater for the cultivation of the plants will be very
great, while on the other hand the plant must definitely
be protected against the action of rainwater or hail
falling with great force. In such a case it may advantageously
be ensured that the sheeting is so constructed that in
positions corresponding to the plants themselves micro-
perforations are provided, while in positions corresponding
~ ~2~'74
for example to paths or hoeing furrows, between the plants,
larger perforations are provided, all of which receive
incident water. The light spray coming from the micro-
perforated areas will thus effect the moistening of leaves,
flowers and fruit, while through the capillary action from
the sides of the plants water will be supplied to their
roots.
The sheeting used in the method according to the
invention can with particularly great advantage consist
of a plastics material to which modifiers are added.
Such modifiers may be one or more pigments, agents
promoting the transmission of light, agents absorbing
infrared radiation, and the like. Throught the selection
of the type and amount of such agents, all kinds of special
ffects can be achieved.
radiation, the sheeting can advantaaeo~s~ly be
Thus, for example, in the case of very lntense solar~
pigmented, for example with the aid of titanium dioxide.
In that case, of course, other types of pigment may also
be used, depending on the desired effect.
The invention likewise relates to protective sheeting
for use in the method described above for the control of
environmental conditions in the proximity of vegetation
or crops in the cultivation of` plants.
Protective sheeting of this kind consists of an at
least partially microperforated sheet of plastics material,
in which the edges of the microperforations project out of
the plane of the sheet.
It may be observed that sheeting of this kind is known
per se and is used as a covering layer for a moisture
absorbing material, such as for example diapers for babies
or certain medical compresses. The use of sheeting of this
kind for agricultural purposes has not hitherto been
described.
The protective sheeting according to the invention
has in particular a diameter of the smallest section of
1~9Z~37~
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the microperforations between 50 and 2000 ym; the
diameter of the perforations preferably amounts to
between 100 and 1000 ~m. The number of microperforations
per unit area can be selected as desired, and will
generally be between 25 and 4 x 104 perforations per
square centimetre, but particularly between 100 and
104 perforations per square centimetre, preferably
between 100 and 103 perforations per square centimetre.
- Finally, the invention also relates to a protective
de~ice for use in greenhouses, which comprises protective
sheets, holder means for said sheets, and means for
extending the sheets either under tension or otherwise,
which device is characterized in that at least one of
the protective sheets is provided with microperforations,
while each of the microperforations has a projecting edge.
When they make use of sheets provided with micro-
perforations, protective devices of this kind provide
a noteworthy improvement in respect of heat, moisture
and light economy in the greenhouses in which they are
used.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be explained with the aid of
the drawings, in which:
Figure 1 is a section of a sheet which is to be
useo in the method according to the invention and which
is provided with microperforations, each of the latter
having an outwardly projecting edge.
Figure 2 is a section of a glass greenhouse
provided along its walls with sheeting having micro-
perforations.
Figure 3 shows an arrangement for cultivation on
cold ground with the aid of plastics sheeting provided
with microperforations.
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Figure 4 shows a single microperforation on an
enlarged scale, as present in the sheet of figure 1.
rigure 5 shows a graph illustrating the yields of
product in an experiment in greenhouse cultivation of
paprika with use of two types of protective sheeting.
In figure 1 the reference numeral 1 designates a
sheet which is provided with microperforations, each of
which has an outwardly projecting edge 2 surrounding the
microperforation 3. A sheet of this kind is produced in
kwown manner by bringing thermoplastic sheeting into
contact with a metal surface provided with micro-
perforations, heating said surface, and producing on the
inner side of the metal surface a vacuum by means of which
the plastics sheet is drawn into the microperforations and
deformed, the deformation being allowed to continue until
holes are formed in the sheet at the sites of the
perforations. After the sheet has been removed from the
metal surface provided with microperforations and has
cooled, the sheet designated 1 results.
~ igure 2 shows schematically a greenhouse having
glass walls 11, along which plastics sheeting 12 provided
with microperforations is stretched. The plants 13 are
protected at night through the presence of the plastics
sheeting provided with microperforations against intense
cold caused by irradiation of heat, which is greatly
retarded by plastics sheetiny, while during the daytime,
after the evaporation of the water condensed during the
night, the sheeting resumes its original less intensive
insulating action. In this figure the sheeting is shown
close to the glass walls. It is obvious that at the
discretion of the user such sheeting may also be disposed
in other places or in other positions. In addition, the
sheeting may be arranged to be movable, so that at certain
times of the day the sheeting can if desired be entirely
--"` 129Z87~
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removed from the surface receiving the radiation of the
sun, or can be appropriately disposed.
Figure 3 shows a situation in which the sheeting
is used for cultivating plants on cold ground.
3' designates a space bounded by hoops and sheeting.
The sheeting 31 is stretched over hoops 32, while 34
indicates schematically the plants being cultivated.
As pre~iously stated, it can be ensured through selection
of the dimensions of the perforations that on the one
hand good circulation of air is achieved and that on the
other hand direct damping by incident rain is avoided,
or the rain is distributed in the form of a harmless mist.
In addition, the projections on the sheeting may
all be directed inwards, or else some of the perforations
may be directed inwards and others be directed outwards
in order to ensure increased circulation of air. Finally,
as pre~iously indicated, perforations of a larger diameter,
for exa~ple 10 millimetres, may also be provided in the
sheeting in order to achieve complete admission of
incident rain in certain places.
In the abovedescribed arrangement for cultivation
on cold ground it is also possible to dispense with the
use of hoops. The sheeting then comes into direct contact
with the plants, and~as the plants grow~will in the
course of time be lifted higher off the ground; in this
case the sheeting used is called co-growth sheeting.
In figure 4 the same reference numerals are used
for the same parts as in figure 1.
A microperforation 3 has a diameter of its smallest
section 4; the thickness of the sheeting is 5; the height
of protrusion is 6 and the pitch of perforations is
given b~ 7.
In a specific example the sheeting has a thickness
1 of 40 ~m; the diameter 4 is 560 ~m on average; the
protrusion height 6 is 530 ~m on average and the pitch 7
" ~29Z874
- 13 -
is chosen such that the open area is 15o.
Figure 5 will be explained in the following example.
DESCRlPTIûN OF A PREFERRED EMBODIMENT
Example
In a greenhouse complex paprika's were cultivated on
a conventional substrate of mineral wool with use of a
conventional nutrition solutidn. Two completely separated
sections were present. One section was provided with the
conventional protective sheeting being polyethylene sheet,
thickness 40 ~m and provided with the conventionally used
anti-condensation agent. The other section was equipped
with protective sheeting as per the method of the invention;
i.e. low density polyethylene film, 40 ,um thickness
comprising 2~ ethylenevinylacetate, U~'-degradation inhibitor
and an open area of 15,o~ having microperforations with a
smallest diameter of 560 ~m in a concentration of about
60 microperforations/cm2 over the whole area over the
sheeting.
Heating of the greenhouse sections was carried out
with use of a conventional system comprisirlg polyethylene
tubing below the substrates, tube heating above ground
level and additional air-heating.
The protective foils were movably mounted in order
to be able to vary the protected area, as is also
customary in greenhouse cultivation.
Both heating and optionally cooling and movement of
the protective sheeting, as well as the addition of
nutrition liquid were computer-controlled; the control
being in accordance with the difference between predetermined
respective desired values of the different parameters and
the measured values of these parameters such as temperature,
humidity, luminance etc..
The test was carried out in the Westland-area of the
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Netherlands in the period from November 1985 to
November 1986.
In the accompanying graph is given the cumulated
yield in kg/mZ for section A provided with conventional
protective sheeting and section B with the protective
sheeting according to the invention. The increase in
yield is very marked, in week 42 B has reached about
22 kg/m2 whereas A has reached about 19 kg/m2, an increase
of about 15o. In the beginning of the period the results
in project B did la~ behind as a result of the increase
in number of buds and fruits to be nourished and a two
weeks later planting date; this retardations were made
up in the remaining season and the yield even increased
as a result of the good light transmittance of the foil
in the period with limited duration of daylight.
Humidity control appeared to be more easy in the
case of project B than in project A; the percentage open
area chosen, 15,o~ appeared to be somewhat at the high
side in circumstances of low humidity outside the
greenhouse.
By experimenting an optimum value of the percentage
open area has to be established; the optimum reached will
be depending on the type of vegetation; greenhouss
environment, sun-hours etc..
The sheeting used in project B did exhibit a very
good water retention; exper;ments showed that the sheeting
could absorb up to 30 g/m2 more water in a homogeneous
film without formation of drops than the sheeting of
project A was able to~ take up.
Whereas normal foil (A) showed already in an early
stagedrop-condensationat its surface, the sheeting of the
invention (B) did show practically no drop-condensationup
to high relative humidities. Drop-condensation has a
marked effect on the light-transmittance, the sheeting
of the invention therefore shows a constant transmittance
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~2~8'~
. - 15 -
which is substantially not influenced by relative
humidity of the greenhouse atmosphere.
Particularly in the early hours, when the outside
temperature is still low, the closed foil of section A
had to be opened to lower the unadvantageously high
relative humidity and to prevent absorption of light
by the accumulated condensation droplets on the lower
side of the screen, with a subsequent loss of heat.
The perforated foil of section B seemed to be
markedly in favour with respect to the foil in section
A, in that it could remain closed for a longer time
thereby providing heat insulation as well as good light-
transmission.
As a result of the presence of the microperforations
in the sheeting of project B a more uniform luminance was
observed in comparison to project A; the protruded rims
of the microperforations give the sheeting a scattering
foil effect.
The foil of section B therefore shows very good
possibilities to be used as a protective sheet against
sunburn, particularly in high-temperature regions.
The sheeting used in section B, finally, snouled a
very good ductility and softness in comparison to
conventional sheeting of project A, leading to a
favourably small diameter roll of foi~ when it is in
removed condition.
This condition is of importance because of the
fact that in view of shadow prevention the foil pack
is required to be as small as possible.
