Page 43 - Simply Vegetables Spring 2022

Lighting
I realise lighting is a specialist topic and will not be of interest to all members but as we have a number of exhibition growers, giant vegetable growers and very keen growers who possible use it for propagation purposes it is likely a number of members use lighting in their greenhouses. It is a while since we had anything on lighting so I thought I would update you on what is available and give further details which may be of use to some of you. Over the last five years there have been a number of advances in plant lighting and the cost of LED’s has reduced as well as the amount of light they produce has increased.
Looking back to the early days of lighting in greenhouses the early lighting was the use of the old fashioned incandescent
light bulb (the type that used to be
used in houses) which has been used
for many years to control the flowering
of chrysanthemums and help with the flowering of indoor roses. The use of the light basically delayed the flowering and the actual wavelength of the light was not particular important.
The sodium and metal halide bulbs were the next to be used followed by the daylight fluorescent tubes. The high pressure sodium light give out a very yellow spectrum but had its uses in greenhouse growing. The metal halide and fluorescent tubes have a bluer spectrum which had a wider range of uses. In the 1970’s (yes I can remember that far back!) the fluorescent tubes where used in the old fashioned growing rooms which were
like sheds with shelving
in and lighting above
each shelf, there were no
windows. These were used
to raise seedlings which
were then sold or pricked
out. Because the lights gave
off heat no heating was
required thus saving the
heating bill, the lights could
be left on for 12 to 16 hours
which gave long day lengths
so the seedlings grew
rapidly.
Lettuce in NFT troughs & ligjhting
etc.) are mounted in an array and the light emitted blends together to produce white light. There are some other treatments
that will produce white light as well. An advantage of LED lamps is that they radiate little heat compared to other lamps so can be used close to plants without scorching them. As with other lamps there is some wasted energy and this is given off as heat and the lamps will have some method of cooling them such as fans or water cooling.
In recent years the LED (light emitting diode) light has become popular with growers, these have now come down in price compared to the early years and a
lot of research work has been carried out on their uses for growing crops. They are fairly widely used in tomato, cucumber and pepper production as well as for propagation. With the newer types of LED lighting it is possible to grow salad crops and others vegetables in buildings without any windows, in tunnels under London, warehouses etc.
it is only economic with reasonable high value crops. If you decide to try it cost the operation out carefully but you could make use of
cellars, attics, garages, sheds or any other building you have available. If you don’t believe me ask the local marijuana grower!
Before looking at the actual lighting we need to consider what is required; plants need light for photosynthesis that is to produce the carbohydrates for respiration, energy and growth. Plants also respond to light by producing flowers, change their growth patterns and it is involved in the dropping of leaves in the autumn.
By light we need to look at:
Duration – the length of the day light, longer in the summer than winter.
Intensity – how bright or strong the light, higher on sunny days than dull.
Wavelength – plants need from 400 to 700 nanometres and some others not visible to humans
Light measurement – as humans we have a certain perception of light but it
is not the same as what a plant needs or perceives. Human’s measure light in lux or lumens which is roughly what we see but this is of limited use to plants as it does not allow for their requirements. So what should we consider?
Photosynthetic Active Radiation (PAR) this refers to the amount of light within the 400 to 700 nanometres wavelength which is what plants require for photosynthesis and that is in the visible range for humans. It helps if any lighting / bulbs are rated in the amount of PAR delivered to the plant leaves. At present two groups of light measurement exist with regards to using electric lights for plants:
1. Is the quantity of light emitted from the light / bulb which is a measurement of light given off by the source; usually measured in micromoles/J (μmol/j). This means for every joule of electric energy used a certain number of photon micromoles are produced. (A joule = watt * second). A good LED light will be from 1.5 μmol/j and this is improving all the time.
2. Is the measurement of the amount of light reaching the surface i.e. leaves? This is the more important of the two because it is this that affects the plants growth
Why does this matter? The light levels decrease greatly as the distance between
Apparently LED lamps
were invented in 1927 but
only recently have they
become more widely used especially
in horticulture. They work by passing electricity through a semiconductor which releases energy in the form of photons. They are being improved all the time and the efficiency and cost improvements have increased their commercial use.
The wavelengths emitted depends
on the chemical composition of the semiconductor. Usually LED’s emitting certain wavelengths (i.e. red, green, blue,
LED lamps were invented in 1927 but only recently have they become more widely used especially in horticulture
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