Project title:
Bedding plants: the use of spectral filters to regulate plant growth
Project Number:
PC 150
Project Leader:
Dr Simon Pearson
Location:
University of Reading,
Whiteknights,
Reading RG6 6AS
Project Co-ordinator:
Mr Stuart Coutts,
Nightingale Cottage,
Felhampton,
Church Stretton,
Shropshire SY6 6RJ
Date commenced:
1 April 1998
Date Completed:
31 December 1998
Final report:
April 2000
Keywords:
Spectral filters, bedding plants, cladding materials, far-red light,
UV-light, infra-red radiation, Solatrol ®, Luminance THB ®,
Visqueen Anti botrytis film, XL Super Green
Whilst reports issued under the auspices of the HDC are prepared from the best available information, neither the
authors nor the HDC can accept any responsibility for inaccuracy or liability for loss, damage or injury from the
application of any concept or procedure discussed.
No part of this publication may be reproduced in any form or by any means without prior permission from the
HDC.
© 2000 Horticultural Development Council
The results and conclusions in this report are based a series of experiments. The conditions under
which the studies were carried out and the results have been reported with detail and accuracy.
However, because of the biological nature of the work it must be borne in mind that different
circumstances and conditions could produce different results. Therefore, care must be taken with
the interpretation of the results especially if they are used as the basis for commercial product
recommendations.
© 2000 Horticultural Development Council
CONTENTS
Page
PRACTICAL SECTION FOR GROWERS
1
EXPERIMENTAL SECTION
Introduction
Materials And Methods
Results
6
6
7
9
CONCLUSIONS
24
REFERENCES
24
© 2000 Horticultural Development Council
PRACTICAL SECTION FOR GROWERS
Background and Objectives
There has been a great deal of recent interest in the use of spectral filters for the production of a
range of protected crops. This interest has been a consequence of a number of significant
advances in the production of plastic cladding materials, in particular the ability to manufacture
materials which absorb discrete wavebands of light.
This new technology exploits many years of fundamental research into how plants measure
light quality for ecological gain in a natural environment. For example, plants are known to
detect the presence of neighbours by monitoring the ratio of red (660nm) and far red light (700
to 800nm). As red light is absorbed heavily by leaves and far red light is transmitted, plants in
competitive environments with lots of neighbours, will be irradiated with relatively high levels
of far red radiation compared to red (the latter absorbed by competitors leaves). This
proportionately high level of far red light is converted by the plant into a message to grow taller
and gain more light.
The absorption of far red light by spectral filters removes this signal the plants do not elongate
thus providing a potential commercial method of growth control.
Filtering of ultra-violet radiation also has commercial potential, since this is thought to regulate
the activity of a number of commercially important pests and diseases. For example, a number
of studies have shown that Botrytis is reduced in ultra-violet deficient environments. Work in
Israel also suggests that thrips, aphids and white fly can be partially controlled under ultraviolet filters, since these insects use light at these wavebands for sight.
Other materials are also being developed to control the level of heat inside greenhouses. These
absorb infra-red radiation, which if removed beyond 700nm should have no overall effect on
photosynthesis.
Although spectral filters appear to have potential application, initial studies suggest that
responses are species dependent. Some species respond well, others not at all; potential side
effects also need to be assessed. Therefore, in the latest HDC funded work at Reading we have
examined the potential of spectral filters for bedding plant production. The potential of five
films to regulate the growth of bedding plants was assessed.





A light diffusing heat control film (Visqueen Luminance THB ®)
A heat control (red absorbing) film (XL Super Green ® or ‘Greek Green’)
A fluorescent UV absorbing / blue emitting anti-botrytis film (Visqueen Anti-Botrytis film)
A commercial control (Visqueen UVI/EVA)
A far red absorbing material (a prototype Visqueen Far red filter)
© 2000 Horticultural Development Council
1
Summary of results
Light Transmission
The diffusing heat control material (Luminance THB) absorbed most of the infra-red radiation.
The XL Super Green heat control material absorbed some infra-red radiation but also a
significant proportion of the red waveband, reducing the overall PAR (photosynthetically active
radiation) transmission by 20%. The Visqueen anti-botrytis material was highly effective in
removing ultra-violet light. The far red absorbing material, produced by Visqueen, removed
80% of the far red wave-band at 730nm, though notably this was a lower dose than previous
batches of materials we have examined.
Plant Growth Responses
Crops of bedding plants were produced under these materials, including petunia, pansy,
marigold, dianthus, fuchsia, lobelia, geranium, lobelia, alyssum and begonia.
In most species, the responses found were typical of those expected with far red filters; a
reduction in plant height and an increase in branching. Other responses noted included an
increase in leaf greenness and a slight delay in flowering (approx. 1 week). Not all species
responded to the far red filter. For example, in seed raised geranium plant height was not
affected, and the height response with marigold was only marginal. Greatest responses, in terms
of height reduction (10 to 25%), were noted with pansy, petunia and dianthus (see Table 1).
Poorest height control was noted with the XL Super Green heat control material, where some
stretch was observed. This is perhaps not surprising as it partially absorbs light at the red
wavebands.
© 2000 Horticultural Development Council
2
Table 1 - The effects of different cladding materials on the growth of bedding plants
Plant
Species
Alyssum
‘Snowstorm’
Begonia
‘Olympia Pink’
Dianthus
‘Raspberry Parfait’
Marigold
‘Perfection (yellow)’
Lobelia ‘Fountain
Select (mixed)’
Geranium
‘Century Rose’
Impatiens ‘Accent
Select (mixed)’
Fuchsia
‘Jungle Bells’
Pansy
‘Turbo Select’
Petunia ‘Express
Select (mixed)’
Control Film
Far red filter
XL Super
Green
Luminance
THB
Height
(mm)
174
FW
(g)
23.0
Height
(mm)
151
FW
(g)
19.1
Height
(mm)
188
FW
(g)
17.7
Height
(mm)
167
FW
(g)
25.0
Visqueen
Anti-Botrytis
Film (UV)
Height FW
(mm)
(g)
177
21.4
153
90.2
133
89.0
167
84.1
153
97.4
141
89.3
201
30.8
179
27.1
209
25.7
198
28.5
204
30.2
315
80.3
298
72.4
329
68.6
329
85.8
328
81.8
312
41.8
285
39.2
338
37.9
337
42.2
307
39.9
143
79.2
153
85.5
158
82.4
149
88.6
141
84.4
85
17.6
82
16.7
92
16.7
87
18.8
79
16.7
195
27.7
251
37.9
269
38.7
213
34.5
219
33.3
109
14.1
94
13.9
123
13.5
111
14.1
105
15.2
145
21.8
122
18.9
157
10.4
140
20.9
135
19.6
Bold figures and shaded cells indicate shortest height or greatest fresh weight (FW) for
each plant species
There were interesting effects in terms of overall growth, with some of the greatest plant dry
weights being noted under the light diffusing heat-control material (Luminance THB, see Table
1). This is not surprising as in theory diffuse light should be converted more efficiently
into dry matter than direct beam radiation. The tunnel also felt cooler to workers inside. Lowest
dry weights were noted with the XL Super Green heat control filter, which also had the lowest
transmission of photosynthetically active radiation (PAR).
Pest and Diseases
In terms of the level of thrips infestation and Botrytis disease, there were no differences noted
between the materials; however, levels were very low and further work is required where
controlled numbers of thrips are artificially released into tunnels before the value of spectral
filters for insect control can be determined.
© 2000 Horticultural Development Council
3
Greenhouse environment
Leaf temperatures were also recorded during the experiment. The XL Super Green heat control
material was most effective in reducing tissue temperatures, though it also reduced the overall
light transmission. The light diffusing heat control Luminance THB material also reduced tissue
temperatures but the effect was small with a reduction of 1 to 2oC found. In terms of overall
heat reduction, Luminance THB (diffusing material) reduced the infra-red heat load on the
tunnel by 20% (as measured by pyranometers). An improved formulation of this material has
now been released commercially.
Action Points for Growers

Growers should be aware of the availability of new types of plastic cladding materials that
have the ability to selectively transmit different bands of radiation and thus regulate plant
growth and greenhouse temperature.

Plastic materials that selectively absorb far red light have the ability to control the growth of
plants and alter the branching habit (example, Visqueen Growth Control Film). These
materials offer growers the potential to reduce the use of plant growth regulators.

Not all plant species can be regulated by growth control (far red absorbing) cladding
materials. The most significant reduction in plant height can be expected with pansy,
petunia and Dianthus.

Some cladding materials can cause stretch in plants. These materials absorb the red band of
solar radiation and in these trials caused increased growth of Begonia and Impatiens,
(example, XL Super Green Film).

UV films may be useful for the control of Botrytis and insect pests in crops. UV films may
also influence plant growth, in particular where they transmit a high proportion of blue
light. In these trials, the Visqueen Anti-Botrytis Film (UV absorber) reduced plant height in
Impatiens and Petunia.

Films that reduce infra-red radiation can reduce the heat load in the greenhouse. XL Super
Green is effective in reducing air and leaf temperature but also reduces growth as it
transmits a lower preparation of the light needed for plant growth (PAR, photosynthetically
active radiation).

Heat absorbing materials such as Luminance THB can reduce plant temperatures and the
light scattering effects of Luminance THB caused increased plant weights in the trial, in
particular for Alyssum, Begonia, Geranium and Marigold.

Before selecting a cladding material from the new range of spectral filters, check in detail
with the manufacturer/supplier about the light transmitting properties of the films. Be
aware that different plant species will respond differently or perhaps not at all.

Check with the manufacturer/supplier about the longevity of the spectral properties of the
filters – do they decline over time and how rapidly?
© 2000 Horticultural Development Council
4
Practical and financial benefits
Thus, these experiments have shown that far red filters have considerable potential for
controlling the growth of bedding plants, with most species responding positively to the
material. This may reduce the heavy reliance of bedding plant growers on the use of chemical
growth regulators. Further work is now required to optimise material design, including
investigations into how much far red has to be removed for the optimal response. Diffusing
materials (Luminance type) also seem to have potential in that overall plant growth can be
increased, and infra-red load reduced by 20%.
© 2000 Horticultural Development Council
5
EXPERIMENTAL SECTION
Introduction
During the last four years, the University of Reading, in conjunction with Visqueen Ltd and
Fordingbridge Ltd (major polyethylene tunnel film and structure manufacturers, respectively),
have developed the use of spectral filters to control plant height and heat and plant pathogens,
in particular Botrytis, in greenhouses. This work was conducted in a DTI/EPSRC LINK project
and included the development, design and specification of the materials, as well as primary
testing. Such materials have commercial potential as internal glasshouse screens or as direct
cladding materials.
Spectral filters selectively filter specific wavebands of light that can elicit certain biological
responses. For example, there is considerable evidence which shows that far red light enhances
stem elongation and reduces branching. Therefore, removing far red light may help produce
higher quality, more compact plants. Such technology has considerable commercial potential
to reduce the use of chemical plant growth regulators, thereby protecting the environment, and
to design crops with new specification (i.e. unique architectures). Studies carried out on an
individual plant basis at Reading University showed that height could be reduced by 15 to 45%,
depending on the plant species examined. However, initial experiments were conducted on a
small scale with a limited number of species (petunia, pansy, salvia, antirrhinum and
chrysanthemum) and the materials need to be tested on a crop scale basis. One potential
problem with this technology was that in long day plants such as petunia and pansy, although
growth was controlled, far red filtering led to a small delay in flowering (up to 10 days).
Further work at Reading University has examined the potential benefits of a new range of heat
removing materials for use in greenhouses (Luminance materials). These materials have been
developed to filter a high proportion of the incoming infra red (700 to 1800 nm) radiation
entering the greenhouse. This forms 50% of the solar heat load incident upon a greenhouse.
Removing this heat load may produce better controlled summer environments, increased yields
via prolonged carbon dioxide enrichment and reduced water use/loss. Plant quality may also be
increased via reductions in thermal stress. However, the potential of these materials has only
been examined using simulation energy balance models, and needs to be tested on experimental
crops and greenhouses. Luminance also absorbs a high proportion of far red light suggesting it
may have some potential as a growth control material, but this needs testing.
Work at Reading University and Visqueen Ltd has also lead to the development of a new range
of ultra-violet filters, which removes radiation up to 400 nm (glass removes only up to 320 nm).
UV light between 320 and 400 nm is known to elicit biological responses, including fungal
sporulation. Research at Reading has shown that these UV filters have the potential to reduce
disease such as Botrytis by up to 40% (as tested on two semi-commercial primrose crops).
Greatest control was achieved on crops grown under Visqueen UV block film. While UV
block films offer the potential for reduced disease infection, their effects on plant morphology
and flower colour need to be assessed.
© 2000 Horticultural Development Council
6
Objectives of the work
1.
Test the efficacy of spectral filters on the growth control of bedding plants. Five materials
will be tested; the Visqueen far red filter, Luminance THB, Visqueen UV (Anti-Botrytis
Film), a green heat control film (XL Super Green) and a commercial control.
2.
Test the effects of the materials on plant growth on a wide range (10) of bedding plant
crops on a semi-commercial scale.
3.
Test the efficacy of Luminance THB to control greenhouse temperatures and reduce heat
loads.
MATERIALS AND METHODS
Plastic Cladding Materials
The experiments were conducted in a suite of 10 small experimental and purpose built tunnels
at Reading as for the Visqueen/DTI/EPSRC LINK project. The tunnels were 4m x 8m in
dimension. The efficacy of five materials were tested;





Viqueen Far Red Filter; removes 80% of far red radiation at 730nm
Visqueen Luminance THB; removes infra-red radiation
A green heat control film (XL Super Green); absorbs some infra-red but also a significant
preparation of the red waveband (also called Greek Green)
An ultra-violet absorbing material (Visqueen Anti-Botrytis)
A commercial control material (Visqueen UVI/EVA)
Experimental Design
There were two replicate tunnels per treatment, each tunnel sub-divided so that half of the
plants were given a night break lighting treatment (4 hours light centred at midnight using
tungsten lamps (@ 5 µ mol m-2 s-1). Each half of the greenhouse was screened with black
plastic to prevent light spillage.
The experiment therefore comprised a two way factorial combination (in a split plot layout) of
spectral filters * night break treatment.
Plant Varieties Tested
Plants were bought in as plugs trays from WJ Findons and Sons and grown on in six packs
according to standard commercial practice. The varieties and potting dates are shown in Table
2.
© 2000 Horticultural Development Council
7
Table 2. Bedding plant types, cultivars and potting dates as used in the trial
Bedding Plant Type
Cultivar
Potting date
Alyssum (Lobularia maritima)
Snowstorm
8-5-98
Begonia (Begonia semperflorens)
Olympia Pink
12-5-98
Fuchsia (Fuchsia hybrida)
Jingle Bells
12-5-98
Geranium (Pelargonium hortorum)
Century Rose
13-5-98
Impatiens (Impatiens wallerana)
Accent Select (mixed)
9-5-98
Lobelia (Lobelia erinus)
Fountain Select (mixed)
11-5-98
Marigold (Tagetes patula)
Perfection (yellow)
10-5-98
Petunia (Petunia x hybrida)
Express Select (mixed)
11-5-98
Pansy (Viola x wittrockiana)
Turbo Select
12-5-98
Dianthus (Dianthus barbatus)
Raspberry Parfait
11-5-98
There were either 60 packs per species per treatment, or 60 x 9cm pots for geranium and
fuchsia. Of these, and as a side trial, 20 packs were sprayed with Cycocel (1000ppm plus
wetter at 1ml per litre) to test whether the quality of plants grown under spectral filters was
better than plants grown using PGR's. Plants were grown according to standard commercial
practice, spaced pot thick and irrigated as required with a nutrient solution (Sangral 1:1:1)
diluted to a conductivity of 1500 µ S, pH 6.4. No significant pest or disease problems occurred
in any of the tunnels through-out the experiment.
Assessments
Final measurements included plant height, leaf number, branch number, time to first flowering
and overall quality (assessed through a photographic record). Observations were taken of the
incidence of any diseases (such as botrytis) and pests (such as thrips). In all tunnels, the air
temperature and solar radiation transmitted was monitored using a DataTaker DT500 logger in
conjunction with aspirated PT100 temperature sensors and tube solarimeters.
The spectral activity of the materials was monitored through-out the experiment to examine
whether the chromophores degraded to any extent. These spectral transmissions, between 350
to 1800nm, were measured on an optical bench using a Bentham Instruments
spectroradiometer.
© 2000 Horticultural Development Council
8
RESULTS
Spectral Transmission of Plastic Cladding Materials
Prior to the start of the experiment the spectral transmissions of the five materials was
determined (Figure 1).
Figure 1. Spectral transmissions of the experimental materials, determined at normal
incidence using a Bentham Instruments M300 double monochromator spectroradiometer
on an optical bench with a quartz halogen source
As expected, the Luminance material transmitted only 40% of the near infra-red radiation
between 800 to 1800nm. The prototype far-red absorbing material removed up to 90% of the
far red radiation at 735nm. However, this was less than previous batches used at Reading,
where the absorption has been equivalent to 97% of incoming far red radiation. The XL Super
Green film (Greek Green) absorbed a high proportion of the incoming red (600 to 700nm) and
blue (400 to 500nm) radiation. The control material transmitted a high proportion of all
incoming wavebands. The Visqueen UV absorbing material absorbed all radiation up to 400nm,
but fluoresced it in the region 401 to 450nm.
© 2000 Horticultural Development Council
9
Plant Growth Responses
Several plant growth and development parameters were studied during the course of the
experiment. The results were analysed statistically using analysis of variance and simple linear
regression, where necessary. Only significant responses are discussed.
Alyssum ‘Snowstorm’
Cladding Material
Plant Height (mm)
Fresh Weight (g)
Control
174
23.0
Luminance THB
167
25.0
Visqueen anti-Botrytis film
177
21.4
XL Super Green
188
17.7
Far red filter
151
19.1
SED (5%)
6.1
1.8
Significance
0.05
0.05
The shortest plants were found with the far red filter treatment, 13% shorter than the control
(P<0.05). These responses are consistent with a classical response to red:far red ratio. Greatest
plant fresh weight was found under Luminance THB, which was 40% greater than for plants
grown under the XL Super Green material (P<0.05).
© 2000 Horticultural Development Council
10
Begonia ‘Olympia Pink’
Cladding Material
Plant Height
(mm)
Branch No.
Flower
No.
Fresh
Weight (g)
Control
141
8.6
12.8
90.3
Luminance THB
153
9.2
14.8
97.4
Visqueen Anti-botrytis film
141
9.0
12.6
89.3
XL Super Green
167
8.6
13.3
84.2
Far red filter
133
9.2
14.3
89.0
SED (5%)
4.5
0.31
0.8
8.1
Significance
0.05
n.s.
n.s.
0.05
Plant height was significantly shorter (by 20%) under the far red absorbing filter, compared to
the XL Super Green material. However, compared to the control this difference was smaller,
with the far red material plants being 6% shorter. These results are consistent with a classical
effect of red: far red ratio on height of begonia. In terms of fresh weight, the largest plants were
found under Luminance THB, which were 8% greater than the controls. This suggests light
scattering can increase plant mass in begonia. Cycocel also significantly affected plant height in
begonia, with the treated plants having a mean height of 138mm compared to 161mm for the
controls (P<0.01) (Data not shown).
© 2000 Horticultural Development Council
11
Dianthus ‘Raspberry Parfait’
Cladding Material
Plant Height
(mm)
Branch No.
Flower
No.
Fresh
Weight (g)
Control
201
25.1
4.7
30.8
Luminance THB
198
22.7
4.7
28.5
Visqueen Anti-botrytis film
204
22.2
5.1
30.2
XL Super Green
209
20.6
3.8
25.7
Far red filter
179
22.2
4.0
27.1
SED (5%)
2.8
0.74
0.3
1.6
Significance
0.05
n.s.
0.05
n.s.
Again the shortest plant height was found under the far red absorbing material, in this instance
plants were 10% shorter than the control. For Dianthus there were no significant effects of the
materials on fresh weight, but flower number was significantly reduced under the XL Super
Green material (flower number was reduced to 3.8 compared to 4.7 with the control (P<0.05)).
© 2000 Horticultural Development Council
12
Fuchsia ‘Jingle Bells’
Cladding Material
Plant Height
(mm)
Branch No.
Flower
No.
Fresh
Weight (g)
Control
195
3.8
8.5
27.7
Luminance THB
213
3.7
10.7
34.5
Visqueen Anti-botrytis Film
219
3.8
11.4
33.3
XL Super Green
269
3.4
13.2
38.7
Far red filter
251
3.6
12.4
37.9
SED(5%)
19.5
0.15
2.1
4.6
Significance
n.s.
n.s.
n.s.
n.s.
For fuchsia, none of the treatments were found to have any significant effect on the measured
parameters. However, this reflects a high degree of plant to plant variability within each
treatment. Further work is required with higher levels of replication to determine any likely
responses for this species.
© 2000 Horticultural Development Council
13
Marigold ‘Perfection’
Cladding Material
Plant Height
(mm)
Branch No.
Days to
Flower
Fresh
Weight (g)
Control
311
13.5
53
80.3
Luminance THB
329
13.5
53
85.5
Visqueen Anti-Botrytis Film
328
12.8
51
81.8
XL Super Green
329
12.7
54
68.7
Far red filter
298
16.2
54
72.4
SED (5%)
3.4
1.8
1.0
5.0
Significance
0.05
n.s.
n.s.
0.05
For marigold, the shortest plant heights were found under the far red absorbing material; 12mm
shorter than the control and 20mm shorter than the XL Super Green material (P<0.05). This is
again consistent with a small effect of red:far red ratio on the height regulation of marigold.
Luminance THB also produced the greatest fresh weight, with a mass 24% greater than the XL
Super Green grown plants (P<0.05).
© 2000 Horticultural Development Council
14
Impatiens ‘Accent Select’
Cladding Material
Plant Height
(mm)
Branch No.
Flower
No.
Fresh
Weight (g)
Control
85.2
7.0
2.5
17.6
Luminance THB
88.0
7.2
2.5
18.8
Visqueen Anti-Botrytis Film
78.7
7.0
2.6
16.7
XL Super Green
92.3
7.1
2.4
16.7
Far red filter
82.5
7.1
2.4
16.6
SED (5%)
1.9
0.1
0.16
1.0
Significance
0.05
n.s.
n.s.
n.s.
In this instance, the tallest plants were found under the XL Super Green filter (92.3mm),
compared to 85.2mm under the control. However, the control plants were not significantly
different from the far red filter (82.5mm), and shortest plants were found under the UV
absorber, which also has the highest blue transmission. Given that the XL Super Green material
has the lowest blue transmission, the results may suggest that the response seen here may be
related to blue light, since highest blue transmission produced shortest plants and vice-versa.
However, further work is required to confirm these responses. Filters had no effect on fresh
weight, though largest plants were found once again under Luminance THB.
© 2000 Horticultural Development Council
15
Geranium ‘Century Rose’
Cladding Material
Plant Height
(mm)
Branch No.
Flower
No.
Fresh
Weight (g)
Control
143
5.1
29.0
79.2
Luminance THB
149
5.4
31.7
88.6
Visqueen Anti-Botrytis Film
141
5.4
27.8
84.4
XL Super Green
158
4.7
27.7
82.4
Far red filter
153
5.4
28.8
85.5
SED (5%)
7.2
0.3
1.6
1.3
Significance
n.s.
n.s.
n.s.
0.05
For geranium, there were no significant effects of material on overall plant height; however, the
tallest and shortest plants were found under the low blue (XL Super Green, 158mm) and high
blue transmitting films respectively (UV absorber, 141mm). These results are therefore
consistent with the effects of an independent blue photo-receptor response, and further work by
the authors has confirmed a direct effect of blue light on the height regulation of geranium.
Largest fresh weights were found under the Luminance THB material, where plants were 9g
larger than the controls. This species also showed a dramatic response to Cycocel, with sprayed
plants having a height of 118mm compared to 180 for the control. This response completely
outweighs any effect of light quality for this species.
© 2000 Horticultural Development Council
16
Petunia ‘Express Select’
Cladding Material
Plant Height
(mm)
Branch No.
Flower
No.
Fresh
Weight (g)
Control
145
11.3
3.9
21.8
Luminance THB
140
11.1
3.7
20.9
Visqueen Anti-Botrytis film
135
10.4
3.9
19.6
XL Super Green
157
10.4
3.3
19.0
Far red filter
122
11.5
3.5
18.9
SED (5%)
3.8
0.3
0.3
0.8
Significance
0.05
n.s.
n.s.
n.s
For petunia, the far red absorbing material had a very significant effect on plant height, which
was reduced by 16% compared to the control. Tallest heights were found under the XL Super
Green material which is again consistent with an overall response to red:far red ratio. Heaviest
plant weights were found under the control, though differences were not significant.
© 2000 Horticultural Development Council
17
Pansy ‘Turbo Select’
Cladding Material
Plant Height
(mm)
Branch No.
Flower
No.
Fresh
Weight (g)
Control
109
5.5
4.5
14.1
Luminance THB
111
5.4
4.6
14.1
Visqueen Anti-Botrytis film
105
6.2
4.4
15.2
XL Super Green
123
5.1
4.3
13.5
Far red filter
94
5.5
4.7
13.9
SED (5%)
3.0
0.3
0.3
0.8
Significance
0.05
n.s.
n.s.
n.s
For pansy the shortest plant heights were found under the far red absorbing material, 94mm
compared to 123mm under the XL Super Green (red absorbing) material. This confirms
previous studies that pansy is responsive to far red light in terms of height regulation. All other
parameters were not significantly affected by the materials.
© 2000 Horticultural Development Council
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Lobelia ‘Fountain Select’
Cladding Material
Plant Height (mm)
Fresh Weight (g)
Control
311
41.9
Luminance THB
340
42.0
Visqueen Anti-Botrytis film
307
39.9
Super Green
336
37.1
Far red filter
285
35.8
SED (5%)
10.1
4.0
Significance
n.s.
n.s
None of the materials had any significant effect on the growth of lobelia, though shortest plant
height was found under the far red absorbing material.
© 2000 Horticultural Development Council
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The Greenhouse Environment
Air temperatures and solar radiation transmissions were measured inside each tunnel. Figure 2
shows air temperatures inside three of the claddings measured on 8 representative days, starting
on the 30 May.
Figure 2. Air temperatures measured inside the tunnels covered with the XL Super Green,
Luminance THB and control materials.
© 2000 Horticultural Development Council
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Figure 3 demonstrates that air temperatures were slightly warmer in the Control tunnels than in
the Luminance THB and XL Super Green tunnels. However, for the Luminance THB tunnels
the maximum air temperatures were only reduced by approximately 1oC. The coolest
temperatures were found inside the XL Super Green tunnel, where maximum temperatures
were reduced by 3 to 5oC. However, this was not surprising, since this material also
significantly reduced the incoming PAR as well as some of the infra-red radiation. The UV
blocking material had more or less identical tunnel temperatures to the control material and the
far red blocking material showed a temperature profile between the Luminance THB and XL
Super Green material (data not shown).
In terms of leaf temperature, the Luminance THB treatment performed slightly better than for
air temperature, with a maximum temperature reduction of up to 2OC (see Figure 3).
Figure 3. The effects of the tunnel cladding on measured leaf temperature (of geraniums) on 8
August.
© 2000 Horticultural Development Council
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Luminance THB (far-red absorbing) showed a similar temperature profile to the XL Super
Green (red absorbing) material. This was perhaps not surprising since one of the key
differences between the Luminance THB material and the others is that it scatters radiation
amongst the canopy. This should in theory produce a more evenly lit canopy with a more
uniform and cooler temperature profile. The scale of the overall temperature reduction for
Luminance THB was somewhat disappointing, given that the material was shown by the
spectra-radiometer to reduce the incoming infra-red radiation by up to 60%, equating to a total
solar radiation transmission reduction of between to 16 to 20%. The relatively small effect of
Luminance THB on leaf temperature could be attributed to a number of factors, including the
fact that the measurements were made during the summer when the greenhouses were
ventilated at high rate (data collected with vents open). In such conditions the high rate of air
exchange may have dwarfed any effect of the material on overall air and canopy temperature.
Furthermore, the light scattering effect of the material may have actually led to the greenhouse
capturing extra radiation, since light at a low angle which would normally pass through both
sides of a tunnel may be scattered downwards onto the crop. This is partially supported by data
on overall light transmission by the materials (see Figure 4). In the morning, given the
orientation of the tunnels, the side of the tunnel faced the sun, but in the afternoon the sun was
directly overhead the tunnel. Figure 4 shows that in the morning the light received inside the
Control and Luminance THB tunnel were similar, but in the afternoon the Luminance tunnel
had a lower transmission. This difference in transmission between the morning and afternoon
could be explained by a scattering effect. However, further work is required to verify this, and
the data demonstrate, once again, the very complex relationship between greenhouse structure,
cladding material and the environment. The data did however confirm that the XL Super Green
material had the lowest overall solar radiation transmission. This is consistent with the lower
temperatures recorded under this material and with the data from the spectroradiometer.
© 2000 Horticultural Development Council
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minance
800
50
600
Lumina nce
40
Leaf Temperature
Solar Radiation (Wm-2)
Co ntro l
Gre e k Gre e n
400
200
30
20
0
19
21
0
2
5
7
10
12 15
Time
17
20
22
1
3
6
8
ntrol
10
19
Figure 4. The solar radiation received inside the control, Luminance THB and XL Super Green
tunnels on 8 August. Data are means of two tunnels.
© 2000 Horticultural Development Council
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CONCLUSIONS
This study has shown that different cladding materials can have significant effects on the
growth and development of a range of bedding plant species. In general, removing far red light
led to reductions in overall plant height and increased quality. However, this generalisation
cannot be applied to all species and, for some, responses of height to blue light (or no
responses at all) were found. Thus, when selecting a cladding material for a greenhouse careful
thought needs to be given to the spectral responses of the plants grown.
The Luminance THB film, which scattered light, generally led to the greatest plant fresh
weights, with the smallest plant weights under the XL Super Green material. The high fresh
weights under Luminance THB suggest that light scattering can be used to increase crop
growth, since this has a similar overall PAR transmission to the non-scattering control. The
response to the XL Super Green material is not surprising, since this has the lowest overall PAR
transmission.
Luminance THB also reduced the incoming total solar radiation entering the greenhouse,
though effects of the film on greenhouse air temperatures were disappointingly small. There
was however a small effect on leaf temperature. The small effects of Luminance THB need
further investigation and may reflect the high level of ventilation used in the tunnels throughout
the experiment.
REFERENCES
Smith, H., 1995. Physiological and Ecological function within the phytochrome family. Annu.
Rev. Plant Physiol. Plant Mol. Biol. 46: 289-315.
Van Haeringen, C.S., West, J.R., Davis, F.J., Gilbert, A., Hadley, P., Pearson, S., Wheldon,
A.E. and Henbest, R.G.C. 1998. The development of solid spectral filters for the regulation of
plant growth. Photochem. and Photobiol. 62: 119-128.
© 2000 Horticultural Development Council
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