Page 44 - Simply Vegetables Spring 2022

LED lighting in greenhouse
the bulbs and plant surface increases, but a more powerful light needs to be further away from the plant so that the light is delivered to a larger area. Also the height of the greenhouse can sometimes affect the light position. The above measurements are in micromoles or more accurately in micromoles per square metre per second stated as μmol/m2/s.
A couple of other terms you may come across are:
required etc. young plants need less light than more mature plants. The growing of young plants is a very good stage in which to use supplementary or full lighting as it is very cost effective and can affect a large number of plants at a time in a small area. It speeds up the growth rate and helps to produce stocky healthy seedlings / young plants. As the plants grow they need more light and this can be done by increasing
(produce a flower spike) making them bitter to taste. Tomatoes on the other hand will start to flower when they reach a certain stage of growth, but to keep them flowering and growing they need 6 to 8 hours of darkness otherwise they will produce malformed growth.
One factor to consider when using lighting is how much heat they will produce as this can get quite high at
times especially with the old fashioned types of bulbs. If the heat is high and the temperature gets too high the plant can shut down and ventilation is required. The newer LED types tend to produce less heat so are less of a problem, although many have some form of cooling system and its worth checking what this is.
Going back to wavelength, plants
mainly use the red and blue spectrum
for photosynthesis and the modern LED lights can be made or programmed to give these wavelengths so are very efficient.
If you have the old high pressure sodium lights these give mainly yellow and red spectrums, some of this can be used for photosynthesis but it does not give the plants a full range of wavelength so they need to be used in conjunction with natural light. They can be used to extend the day length as the plants get natural light during the day.
Photosynthetic Photon Flux (PPF) this measures the total amount of light produced by the lamp in micromoles per second (μmols/s), this tells you the full amount of light coming from the lamp.
If the heat is high and the temperature gets too high the plant can shut down
the light intensity with the newer LED lights.
Plants in their vegetative stage require less light than plants in their reproduction stage (producing flowers and fruit), this is especially true of tomatoes. Lettuce, salad leaves and herbs require less light as they are in the vegetative stage. Plant growth will slow in the winter conditions even
Photosynthetic Photon
Flux Density (PPFD)
measures the amount of
micromoles of photons
hitting a square metre per second μmols/ m2/s.
if temperature is high and it needs the additional lighting to improve the growth and yields. If they are plants that respond to shorter days by producing flowers then to keep them growing vegetativley the day length needs extending by lighting. This applies to many herbs which need longer days or shorter nights. Longer days and high temperatures will make lettuce bolt
The light intensity and duration must be high enough for the photosynthesis needed to keep the plant growing and producing. The amount of light plants require can depend on a number of factors including the age of the plant, size of plant, stage of plant growth, plant species, growth effects
44 Simply Vegetables