When Sunlight Isn t Enough - The Growing Edge

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When Sunlight Isn t Enough - The Growing Edge Powered By Docstoc
					When Sunlight
Isn’t Enough

   Most indoor growers will see a boost in
   crop production and quality with artificial
   lighting. And they can use lights to manage
   a crop for best market results                 Photo co
                                                                               g Internat
                                                                    EYE Lightin
                                                                         ww w.e

A supplementary lighting system using
Eye Lighting lamps is utilized in Japan at
Jinnai Farm in order to supply produce,
non-stop, through the winter months.

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                                                                                                               Clockwise from top left: With rising energy
                                                                                                               costs many growers can afford to provide
                                                                                                               supplementary lighting only to higher
                                                                                                               value crops such as this hydroponic saffron.
                                                                                                               Light reflectors are required to give uniform
                                                                                                               distribution of light and direct light down
                                                                                                               onto the crop. Some seeds need or benefit
                                                                                                               from low levels of light for germination.
                                                                                                               Metal halide lamps from Eye Lighting
                                                                                                               are used to maintain optimum growing
                                                                                                               conditions throughout the year in this
                                                                                  Photo courtesy EYE           greenhouse in Japan. HPS lamps from Eye
                                                                               Lighting International/
                                                                                  Lighting are used to grow oba, a summer
                                                                                                               herb, at a Miyazaki Ltd. greenhouse in Japan.

                                                                                                                                                      Photo courtesy EYE
                                                                                                                                                   Lighting International/

This is the second part of a three-part series.   produced to go into developing fruits and              tables can take three times longer or more
By	lynette	Morgan,	Ph.D.                          hydroponic crops such as tomatoes, and                 under low-light, short-day conditions.
Short winter days combined with low-              strawberries can be low in sugars and over-            Seedling production can also to be slowed
intensity sunlight and long periods of            all flavor during this time of year. Also, leaf        considerably. While providing root zone
cloud cover can reduce incoming solar             cuticles are thinner under low light, and              and air heating can help speed production,
radiation to levels that crops may become         plants can become elongated or stretched,              low light is still a major limiting factor to
uneconomic to produce through low-light           particularly if densities are not adjusted for         winter production. Crops requiring mod-
seasons. Often, it is during the dull days of     illumination levels on a seasonal basis. This          erate to high light levels can be restricted
winter that prices for fresh produce, which       can predispose plants to certain diseases              even more—tomato yields and often fruit
can’t be produced out of doors, are high,         which are also promoted by cool, humid                 sugar levels as well tend to drop off con-
and hydroponic growers should take maxi-          temperatures.                                          siderably in winter, even with heating, in
mum advantage of greenhouse technology                                                                   regions where light levels fall significantly
                                                  Artificial Lighting
to fill this profitable market gap.                                                                      for several months of the year.
                                                  Perhaps the biggest concern for commer-
   Not only do plants grow much slower                                                                      Many hydroponic cut-flower crops
                                                  cial growers is the drop in production that
under low seasonal light, quality can be                                                                 also benefit from supplementary lighting,
                                                  occurs with a falloff in light levels heading
negatively affected as well. Low light means                                                             often used in combination with increased
                                                  into winter. Lettuce plants that took only
less photosynthesis and less assimilate                                                                  day length. In many carnation crops, for
                                                  three weeks in summer on NFT finishing

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Clockwise from top left: Strawberries, like many other
greenhouse fruiting crops, can have poor flavor under
low winter light. Supplementary lighting increases
assimilate production and sugar levels in the fruit,
resulting in higher fruit quality. Short-day plants such
as chrysanthemum respond to decreasing day length
to induce flowering, a process commercial growers can
manipulate to delay or promote flowering. Incandescent
lamps are being used for photoperiodic lighting in
this commercial chrysanthemum crop. Capsicum (bell
peppers) are prone to fruit and flower drop under low-
light conditions such as during winter.

example, the time taken to produce flower                  on average, supplementary lighting would        M J-1 of solar radiation falling on the crops
buds depends both on light intensity and                   be beneficial. Many commercial growers          from the onset of harvest. It was also found
day length: more flowers are produced in                   quickly learn to recognize the signs of low     that there was a yield loss of 0.5-3.1% for
long days with higher light intensity. Roses               light or light deficiency in a crop. These      each 1% reduction of light.
too are a flower crop which responds to                    might include flower and fruit set drop,           With this sort of data and armed with
light intensity and duration, with greater                 small fruit size, slow growth and develop-      values for lamp energy usage, cost of elec-
numbers of flowers forming after exposure                  ment of a larger leaf area on the plants.       tricity and returns from winter-grown
to longer days and with high photosyn-                     Tomato and other plants may become              produce, tomato growers should be able to
thetically active radiation (PAR) levels.                  elongated, thin, and pale. Flowers take         determine the economics of supplemen-
   Assessing the requirement for additional                longer to form and flower trusses may be        tary lighting. However, for many other
greenhouse lighting is usually not a dif-                  weak.                                           crops, it can be difficult to determine how
ficult process. Light meters used inside and                                                               much of an increase in growth rate and
                                                           Economics	of	lighting
directly above the crop canopy in different                                                                yield can be obtained from a supplemen-
                                                           As well as slowing photosynthesis, low-
positions around the growing area give                                                                     tary lighting system investment. If this is
                                                           light intensity can also affect fruit set and
an accurate indication of incoming solar                                                                   the case, or if a grower is not convinced
                                                           fruit development in many hydroponic
radiation. However, growers should assess                                                                  that the use of such lighting is going to be
                                                           crops. During periods when light intensi-
data for total accumulated solar radiation                                                                 profitable, a small-scale trial area is worth
                                                           ties are low, winter greenhouses can have
over a week or month period for previous                                                                   installing and assessing. By comparing
                                                           an increased percentage of misshapen fruit
years to determine for how long the crop is                                                                growth rates, yield and produce quality
                                                           and abortion of the flowers and fruitlets.
likely to be below optimal light levels. Spot                                                              under the lit vs. unlit crop areas a good
                                                           This lowers marketable yield and often
measurements of light fluctuate consider-                                                                  idea can be obtained of the profitability of
                                                           gives a much higher percentage of second-
ably depending on cloud cover, time of day,                                                                a supplementary lighting setup. Such areas
                                                           grade and third-grade fruit. Researchers
position of the light sensor in the green-                                                                 can also be used to experiment with light-
                                                           have found that fruit yield of greenhouse
house and canopy and other factors, and                                                                    ing levels, lamp types and layout and light
                                                           tomatoes is directly dependent on the solar
don’t give a complete picture of total daily                                                               duration to determine how to get the most
                                                           energy received and on the total hours of
radiation as day length also must be taken                                                                 efficient use out of supplementary illumi-
                                                           bright light, a 10% reduction in sunshine
into account.                                                                                              nation at different times of the year.
                                                           hours results in 10% less yield.
   By knowing the optimal light range for                                                                     For growers in areas where energy costs
                                                              One study reported that yield is accumu-
the crop being grown and comparing this                                                                    are high and winter light allows an accept-
                                                           lated in direct proportion to solar radiation
to monthly radiation figures, it’s possible to                                                             able level of production, using a small
                                                           received, so that 2.01 kg of fresh weight of
determine for how many weeks of the year,                                                                  amount of supplementary lighting in the
                                                           tomato fruit was harvested for every 1000

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propagation area can often speed up the          tomatoes, cucumbers and roses have the          days with low light, the supplementary
rate of crop turnover. This is particularly      highest optimum light ranges of 550-750         lighting would then be triggered to switch
useful where large numbers of seedlings          micromoles per m2/s, while smaller cano-        on for the full 16 hour photoperiod.
need to be raised year-round and for plants      pied crops such as lettuce have optimum             Usually such systems in commercial
which are slow during the early stages of        ranges of 300-450. However, since natural       operations are computer controlled with
growth. Lettuce and herb growers often           light levels vary throughout the day and        light sensors inside the greenhouse deter-
use additional lighting during propagation       from day to day, even in the dull conditions mining when supplementary lighting
in winter to speed up this phase of crop         of winter it becomes difficult to design a      needs to be switched on, thus preventing
development. Supplementary light com-            lighting system that will provide just the      any wasted energy running lamps when
bined with suitable levels of heating can        right amount of supplementary light when natural light levels are sufficient. Such auto-
nearly halve propagation times in many           natural light conditions are changing fre-      mated systems are particularly useful for
low light regions, while the compact nature      quently. For this reason, supplemental light saving energy as the crop moves into times
of the seedlings and high density means          systems should be designed to provide           of the year when light levels naturally start
only a few lamps are required for the pro-       a certain light integral during a 24 hour       to increase and supplemental lighting
duction of many young plants.                    period, so that even on the darkest days        is only of benefit for the odd few days a
                                                 the sum of the supplemental integral and        month.
Types	of	Greenhouse	lighting
                                                 the natural light integral meet the plants’         Since supplementary lighting using
Greenhouse lighting can be roughly
                                                 requirements for optimum growth.                high-output lamps does require reasonably
divided into two categories: low-intensity
                                                    The light “integral” is a combination        large amounts of electricity which can be a
light used to extend photoperiod and
                                                 of the intensity of the light and the dura-     considerable expense in winter in commer-
higher-intensity lamps for daylight supple-
                                                 tion of the light period. Many commercial       cial operations. Many growers use a system
mentation and boosting photosynthesis
                                                 growers use supplementary lighting to give of only running lamps when they can take
during winter or low-light times of year.
                                                 the crop a much longer day length than          advantage of off-peak electricity rates.
Many potted plant or cut-flower nurseries
                                                 natural light provides. This allows more
use low-intensity photoperiodic lighting
                                                 hours of photosynthesis to occur and
to extend the day length in those species
                                                 will thus increase growth in many plant
which respond to increasing day length.
                                                 species. This type of approach allows
This may be to promote flowering in some
                                                 lower intensity or fewer lamps to be
species and prevent it in others and to
                                                 used, but over a longer length of time
manipulate bloom production in green-
                                                 each day, making the light integral the
house crops.
                                                 same to a system that uses more, higher-
   Supplementary lighting is used to
                                                 intensity lamps but over a shorter day
increase light levels up above those pro-
vided naturally, thus boosting photosyn-
                                                    Using this system, the supplementary
thesis and overall growth and yields during
                                                 lighting can run in a number of ways. In
winter in many regions of the world.
                                                 regions were there are not consistently
However, it is often also used to extend day
                                                 very low winter light levels, i.e. when a   Above:	Tomato flower trusses can become weak and sparse under low
length in the short days of winter.                                                          winter light levels, with broken or bent trusses becoming more common.
                                                 few bright, sunny days occur, the supple-
   Many common hydroponic crops have                                                         Right: Low light means less photosynthesis and less assimilate to go
                                                 mentary lighting can be left off during     into developing fruits, resulting in lower fruit size and quality. Below:	
different light requirements for optimum
                                                 the daylight hours and only used to         Supplementary lighting for propagation is used to speed up the rate
growth and yields. Light levels which fully
                                                 extend the day length. This assumes that of seedling production in winter. Only a few lamps are required for
saturate a single level lettuce crop, may be                                                 propagation because of high plant density and small size.
                                                 the natural light on such days is suf-
too low for a dense fruiting tomato crop,
                                                 ficient for optimum levels of photosyn-
and shade-loving plants may actually be
                                                 thesis, and is a particularly good system
harmed by high-intensity lighting. Small
                                                 for lower light requiring crops such as
seedlings need less light than mature
                                                 lettuce. In this way, the supplementary
plants, and making use of supplementary
                                                 lighting during bright winter days would
lighting to boost seedling growth rates is
                                                 have not increased the rate of photosyn-
often economic in regions where lighting
                                                 thesis significantly and would thus have
the mature crop is not.
                                                 been a waste of energy to run. However,
   While factors such as plant spacing,
                                                 by extending the winter day length from
density, species and crop stage all affect the
                                                 8 to 16 hours for such a crop would be
optimal light level for a hydroponic crop,
                                                 the most efficient use of the supplemen-
data is readily available for most common
                                                 tary lighting on these types of days. On
greenhouse crops. Larger plants such as

3 0 	 T H E 	 G ROW I N G 	 E D G E 	                                                                                  W W W. G ROW I N G E D G E . C O M
Off-peak rates are typically from 10 p.m.        Photoperiod Control with Artifi-                  make use of less expensive lamp types such
to 5-6 a.m., which would be the natural          cial	lighting                                     as incandescent bulbs. This makes a pho-
dark period for plants. During winter, this      Photoperiodic lighting is a different system      toperiod-control lighting system cost less
may mean that the crop receives two light        of illumination to supplementary light-           to run than the HID systems used to boost
periods during each 24 hours. The natural        ing, although on some crops both may be           photosynthesis.
short winter light hours and an additional       used during production. The length of the            As with supplementary lighting, there
light period during the off-peak times.          daylight or dark period affects the physiol-      are a number of different photoperiod illu-
   While this process does increase the 24-      ogy of many crops, and manipulation of            mination methods used. Day length may
hour light integral significantly, the crop is   photoperiod is used to direct plants into         be simply extended by lamps automatically
receiving an unnatural light/dark period,        either continued vegetative or induction          switching on a few hours before dawn or
and not all plants will thrive under this sort   of flowering growth patterns. Long-day            at sunset to create the day length required.
of lighting regime. Some commonly grown          plants begin to flower when the day length        Often only 3-6 additional hours are
hydroponic crops such as tomatoes benefit        increases past a certain duration (or when        required using low-intensity lighting. This
from certain light/dark periods, often 16-       the night length is less than some critical       lighting level is just sufficient for the crop
hour days followed by 8-hour nights, and         value). Examples of common long-day (14-          to register that it is still daylight, which is
using the night-light method can disrupt         18 hours day length) greenhouse crops are         at intensity far lower than could be used
flowering and productivity. For vegetative       carnation, dahlia, many annuals, calceo-          for boosting any photosynthesis. Another
plants and many other crops this type of         laria and nasturtium.                             commonly used method of photoperiodic
off-peak lighting has no negative effect on         Short day plants (10-12 hours day              lighting is to switch the lamps on during
plant physiology and is a good method for        length) such as chrysanthemum, gardenia,          the middle of the night for a short period,
maximizing energy efficiency.                    poinsettia, cineraria and kalanchoe are           making use of off-peak electricity rates
                                                 induced to flower when the day length             and using less overall energy for the same
                                                 shortens and the night or dark period             effect. This method termed night break
                                                                increases past a critical level.   or night interruption has the same effect
                                                                   Growers use methods             as extending the natural day length for
                                                                   such as black out screens       several hours and has been proven to be a
                                                                   installed in the greenhouse     highly effective method of providing a long
                                                                   to shorten the day length       day response in crops. Another method
                                                                   if required to promote          is cyclic night breaks, which help reduce
                                                                   flowering in short-day          energy costs by only switching lights on for
                                                                   crops. Photoperiodic light-     a few minutes each hour during the night.
                                                                   ing can also be used to         This is just enough of a wake-up for the
                                                                   prevent flowering in short-     crop to register that long day lengths are
                                                                   day plants, keeping them        occurring and induce flowering in long-
                                                                   vegetative and holding the      day plants.
                                                                   plants back until flowering
                                                                                                   Greenhouse	lamps
                                                                   needs to be induced. This
                                                                                                   Important factors to consider when select-
                                                                   means growers can control
                                                                                                   ing a light source for a greenhouse opera-
                                                                   when flowers are produced
                                                                                                   tion are:
                                                                   for certain high-profit
                                                                                                   • Total energy emitted and lamp effi-
                                                                   times such as Valentine’s
                                                                                                   ciency. How much of the energy supplied
                                                                   Day, Mother’s Day and
                                                                                                   produces usable plant light in the correct
                                                                                                   wavelengths? A lamp that puts out a lot of
                                                                   Manipulation of photo-
                                                                                                   heat as wasted energy is not the best choice
                                                                   period is relatively easy to
                                                                                                   where excess heat is not wanted.
                                                                   carry out using greenhouse
                                                                                                   • Wavelengths emitted. Energy is wasted
                                                                   lighting systems. However,
                                                                                                   on wavelengths that plants can’t use. Check
                                                                   since photoperiod control
                                                                                                   the spectral output of lamps to see how
                                                                   only needs low-intensity
                                                                                                   much of the energy is converted into the
                                                                   light, usually in the range
                                                                                                   wavelengths that power photosynthesis.
                                                                   0.5-5 watts per square foot,
                                                                                                   • Life expectancy. Bulbs have different life
                                                                   the systems used for this
                                                                                                   spans. Most bulbs drop off in intensity over
                                                                   type of illumination can
                                                                                                   time and need regular replacement which
                                                                   be much less intensive and
                                                                                                   can be a considerable cost. Light fixtures

W W W. G ROW I N G E D G E . C O M 	                                                                                T H E 	 G ROW I N G 	 E D G E 	 31	
also have a limited life span, and replace-      cent lamps are useful additions to small        lamp.
ment cost should be taken into account.          confined areas such as directly above small     Greenhouse supplementary lighting often
• Cost and efficiency. Cost vs. energy           seedling benches, they are not widely used      differs from that used in grow rooms or
efficiency and life span need consideration.     for crop lighting where high-intensity dis-     growth cabinets. The reason being that in a
Efficiency of a lamp is the percentage of        charge lamps are more common. There are,        greenhouse situation, even in the depths of
electrical energy that is converted into         however, other types of fluorescent lamps       winter, there tends to be natural light pres-
light energy, particularly light energy in the   specifically designed for small scale plant     ent, albeit at a low intensity. This natural
wavelengths that are required for photo-         production in hobby setups which provide        light in winter provides sufficient illumi-
synthesis (400-700 nm).                          a greater light output and more wattage.        nation in the blue wavelengths to balance
• Fixtures required. Ballasts and reflec-           High intensity discharge (HID) lamps         out the use of the more energy efficient
tors are common additional fixtures, and         come in a range of different bulb types and     and longer life span high-pressure sodium
the cost of these needs to be taken into         are the most commonly used for supple-          lamps. In an indoor propagation area
account.                                         mental lighting in greenhouses. These           growing only seedlings for a commercial
                                                 lamps require ballasts and can sometimes        greenhouse operation, metal halide lamps
lamp	Types
                                                 generate unwanted amounts of heat. As           are a good option as they have an output
Incandescent lamps, also called tungsten
                                                 with grow room or hobby lighting systems,       spectrum more suited to propagation and
filament lamps, are mostly used in green-
                                                 supplementary greenhouse lighting sys-          vegetative growth. This is why commercial
houses for photoperiodic lighting to give
                                                 tems typically use reflectors to direct light   growers may use metal halides lamps in
certain crops an extended day length to
                                                 downward into the crop and create a more        a propagation area to push early vegeta-
promote or retard flowering. Since only
                                                 uniform lighting pattern. Selection of lamp     tive growth, but use high-pressure sodium
low illumination levels are required for
                                                 type, wattage, reflector, placement within      lamps or a metal halide/HPS combination
this purpose and since these lamps give of
                                                 the growing area and systems of moni-           system out in the main greenhouse for
a high proportion of red and far red wave-
                                                 toring and checking overall illumination        greater efficiency for the later stages of
lengths, they are commonly used for day
                                                 are all important aspects of greenhouse         plant growth.
length extension and night break lighting.
                                                 supplementary lighting systems.                    In a growing area with no natural light,
Incandescent lamps give off a large propor-
                                                    The most commonly used HID lamps             where sunlight is not available to supple-
tion of their energy usage as heat, so their
                                                 for greenhouse lighting are those using         ment in the blue wavelengths, combination
efficiency ratio is low, around 7%. Bulbs,
                                                 high-pressure metal halide or high-pres-        lamps are often used which provide a full
available in 40-watt to 500-watt sizes, are
                                                 sure sodium bulbs. Both these types of          spectrum of wavelengths to give balanced
not widely used to supplement natural
                                                 bulbs have a high concentration of their        growth and the correct ratio of vegetative
light for photosynthesis. Their cost, how-
                                                 emission spectrum in the plant-usable           to reproductive growth. Lamps for indoor
ever, makes them a good choice for nursery
                                                 range, with good energy ratings and bulb        gardening have evolved to the extent that
and growers of carnations and other plants
                                                 life. However, high-pressure sodium lamps       there is a huge selection of compact, full-
that require day length extended to induce
                                                 are the most used types in greenhouses as       spectrum lighting with bulbs that provide
flowering. They may be used in combi-
                                                 they have a longer life span and are more       just the right amount and quality of light
nation with other, more efficient higher
                                                 energy efficient. Metal halide bulbs have an    for a range of different crops.
output lamps if supplemental lighting is
                                                 average efficiency rating of 20% and a lifes-   Reflectors, Movers
required during winter as well.
                                                 pan of 8000-15000 hours. High-pressure          Reflectors are required to give uniform
   Fluorescent lamps are most commonly
                                                 sodium lamps have efficiency ratings of         distribution of light and to direct light
used for seedlings in propagation areas and
                                                 25% and a lifespan of 20,000-24,000 hours.      down onto the crop. Different shapes, sizes
in growth chambers or grow rooms. They
                                                 They come in a range of wattages from 250       and designs of reflectors are available.
are not often used as the main source of
                                                 up to 1000. For greenhouse supplementary        Large reflectors, however, can create shad-
supplementary lighting in greenhouses as
                                                 lighting, many growers prefer the 400-w         ing problems, blocking some of the incom-
a large number of lamps would be required
                                                 bulbs, although higher stud greenhouses         ing natural sunlight. For that reason, many
to achieve a sufficient level of illumina-
                                                 may use the 1000-w bulbs. Combinations          modern greenhouse lamps have become
tion. Fluorescent lamps are more energy
                                                 of both metal halide and high-pressure          available with internal reflectors. Improved
efficient than incandescent lamps (20-25%
                                                 sodium lamps can also be used in green-         reflector designs are continually being
efficiency) and provide more light in the
                                                 houses, with a ratio of one-to-one being        developed. Many of these newer versions
blue region of the spectrum, which is use-
                                                 common. This provides the benefits of           allow greater distances between lamps to
ful for photosynthesis. Cool white fluores-
                                                 both types of light—the increased propor-       be used while still obtaining a uniform
cent lamps have a long life span (12,000
                                                 tion of light in the blue-violet part of the    light distribution over the crop. Older
hours) with 75-watt bulbs being the most
                                                 spectrum from the metal halide and the          reflector designs used to limit the distance
common used. Higher output fluorescent
                                                 higher output, longer life and better energy    between lighting fixtures to approximately
lamps are also available but require correct
                                                 efficiency of the high-pressure sodium          1.5 times the mounting height. Newer
fittings and ballasts for use. While fluores-

3 2 	 T H E 	 G ROW I N G 	 E D G E 	                                                                       W W W. G ROW I N G E D G E . C O M
improved reflector designs have allowed           ther benefit to crop growth or yields. Light      not realize that illumination intensity has
this distance to be increased to as much as       sensors should be placed inside the green-        dropped to the point were plant growth is
4.5 times the mounting distance and still         house and at the top of the crop canopy. As       being negatively affected. Since human eyes
attain uniform lighting. These improved           the crop grows it is important to check and       can’t detect the wavelengths that plants use
reflector designs are particularly useful for     reposition the sensors so they don’t end          for photosynthesis, a fall off in PAR can
hydroponic bench systems, multi-tiered            up inside the canopy where light will reg-        only be measured with the correct type of
crops or low-stud greenhouses. There are          ister as much lower and trigger incorrect         light meter. Most lamp suppliers can advise
also light shields available for lamps which      responses. The light sensor also needs to         on a regular replacement program for a
redirect light away from non-planted areas        be in direct natural light and not shaded at      greenhouse lighting system that allows
of the greenhouse such as walls and path-         any time during the day by shadows cast by        staggered replacement of old bulbs before
ways, so that maximum use of supplemen-           greenhouse structures and other fixtures.         their output drops significantly. Growers
tary lighting is made.                            This again will give incorrect responses,         often chose to replace every second or
   While fixed lamps in greenhouses have          and lighting may be triggered to switch on        third lamp on a regular basis so that the
been the standard in the past, newer light-       when it’s actually not required, resulting in     overall lighting intensity provided to the
ing systems may incorporate movable               a waste in energy. Light sensors also need        crop is consistent while the cost of replace-
lamps designed to make more economic              regular checking, cleaning and calibra-           ment bulbs is spread.
use of the light provided and eliminate           tion to ensure accurate readings are being           In the next article in this series, I’ll look
uneven lighting patterns over the cropping        taken. Installing more than one sensor in         at the details of indoor and grow room
area. Lamps mounted on overhead tracks            a cropping area is advised. When two or           lighting equipment, how to get the best
can be moved continuously or intermit-            more sensors are used, the measurements           from artificial illumination and the future
tently over the plants as required, and these     from all sensors should be averaged to esti-      of crop lighting.
systems are worth investigating for high-         mate the overall income light intensity in
                                                                                                    Together with Simon Lennard, Lynette Mor-
value hydroponic crops.                           the entire greenhouse.
                                                                                                    gan is co-owner of SUNTEC Hydroponic
   Adjustable height lighting systems which          Photoperiodic lighting uses simpler
                                                                                                    Consultants, New Zealand (www.suntec.
allow lamps to be lowered and raised as           control methods since no feedback on
the plants increase in height are also worth      incoming solar radiation levels is required.
considering for some crops. Fixed lighting        Commonly used 24-hour on/off timers               References	and	sources
is more economic for small plants such            allow day extension and/or night-break            Greenhouse Engineering NRAES-33 1994
as lettuce and dwarf potted plants. How-          lighting to be used.                              By R.A. Aldrich and J.W. Bartock. Pub-
ever, for crops such as some cut flowers,            Since there are many factors to take into      lished by Northeast Regional Agricultural
tomatoes, cucumbers, peppers which grow           account when designing an energy and              Engineering Service, New York.
upwards at a rapid rate, adjustable-height        cost-efficient greenhouse lighting system,        Warren Wilson, J., Hand D.W. and Hannah,
lighting allows the lamps to remain at the        growers are advised to have such systems          M.A., 1992. “Light interception and pho-
optimum distance above the top of the             designed or at least assessed by a lamp           tosynthetic efficiency in some glasshouse
canopy to maintain maximum illumina-              retailer or manufacturer. Most lamp sup-          crops.” Journal of Experimental Botany,
tion at each stage of development.                pliers make use of computer programs or           Vol. 43 (248), pages 363-373.
Measuring, Controlling Greenhouse Light           specific formulas to determine the ideal          Kinet, 1977. “Effect of light conditions
Supplementary lighting systems inside             number of lamps, their wattage, type and          on the development of the inflorescence
greenhouses are usually automatically             placement, distance apart and height above        in tomato.” Scientia Horticulturae, Vol. 6,
controlled with a simple photoelectric            the crop, including selection of reflectors       pages 15-26.
cell system. As a rule, the lighting system       and other equipment. Since this is a com-
should be activated to switch on when the         plex process, it’s best left to the profession-
level of natural light falls to twice the level   als as a correct lighting setup can result
provided by the lights. Usually a time delay      in considerable cost savings over a poorly
is used so the supplementary lighting will        designed system.
only switch on after natural light has fallen        Greenhouse lighting systems do need
sufficiently for several minutes. This avoids     maintenance and monitoring. Bulb burn
light activation when sunlight is being           out, of course, and spare bulbs should
temporarily blocked by a cloud cover.             be kept on hand. A burned out bulb can
   Computer-controlled systems may have           rapidly result in non-uniform growth pat-
specific supplementary lighting programs          terns in the crop if not replaced after more
that give more precise control over daily         than a few days. Also, the light output of
light integrals to ensure energy is not           supplementary lamps falls over time. This
wasted on lighting when it provides no fur-       occurs gradually, so a grower will probably

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