Lighting in horticulture

There have been significant improvements in horticultural lighting technology in recent years. Ordinary commercial or domestic lighting is less appropriate and manufacturers are offering more specialised lighting products.

This page summarises some of our research into the use of LEDs in horticulture to help you decide the best lighting system for your crops and production system.

Benefits of LEDs

LEDs (light emitting diodes) offer good energy efficiency. You can also control the intensity of the output as well as match the colour to the requirements of the crop.

In fact, one of the major advantages of LEDs is the ability to tailor the light spectrum. This is the balance between the various colour wavelengths a plant needs for photosynthesis and to control growth development.

• More energy efficient
• Last longer
• Only the parts of the light spectrum needed by the plant can be produced
• Easier to control heat output
• Lights can be close to the crop as there is little radiated heat
• More even light distribution

Impact of light spectrum on plant growth

There is no ‘ideal’ light spectrum, but you need to consider the effect you want on your crop.

Red LEDS are the most efficient and red wavelengths drive photosynthesis. However plants grow best with some blue light present. It is recommended to have LED modules that emit at least 10% in the blue spectrum. If people are working under LEDs, include some white light too.

Research on petunia has shown that when no far-red light is present, flowering can be delayed by two to three weeks.

To encourage flowering in short-day plants, you need to include red light. For long-day plants, you also need to include far-red.

You will need to check what the right spectrum for your crop is.

LEDs as plant growth regulators

Research at LVG Ahlem has shown that short-duration red light (660nm wavelength) applied just after sunset can control poinsettia growth on a par with plant growth regulator treatments.

The ‘dark red’ counteracts the effect of far-red in the sunset. The high-intensity light was applied around 60 minutes after sundown at 40-60 µmol per sq m per second.

What to consider when discussing your lighting system

Determining light requirements

Average light intensity is normally used for determining light requirements. It is measured in µmol·m-2·s-1, although W·m-2 is also commonly used in horticulture.

Plant responses

You need to think about specific plant responses to light frequency profiles, such as far red and UV.

Consider the coverage that might be achieved with different fittings. A 10-20% variation in light intensity is generally ok.

Practicalities

Think about the practicalities of the fittings. What are the expected lifetimes? Are workers safe under the light fittings? What about shading, weight, the number of fixtures per driver and the whether your electricity supply is large enough for significant lighting?

Using lighting in different systems 

Photoperiodic lighting

Photoperiodic lighting uses low-intensity lighting to create long days, when the days are short.

Compact fluorescent lamps are often used in horticulture, but they are not suited to all plant species. LEDs used for photoperiodic need to have the right light output and also be as energy efficient as possible.

Some growers now have LEDs on moveable mounts and only bring them down to crop level at night.

Growers in the USA are increasingly interested in mounting lights on spray or irrigation booms for photoperiod control under short-day conditions.

Supplementary lighting

Additional lighting makes up for the lack of sunlight in the UK during late autumn, winter and early spring months.

HPS (high pressure sodium) lamps are most commonly used. However, problems include a lack of blue and red parts in the light spectrum. They also radiate heat that is difficult to put to good use.

It is difficult to get the light from HPS lamps mounted above the crop to penetrate into the canopy of tall vine crops, like tomato. Inter-canopy lighting, using additional HPS lamps, alongside lamps above the crops could be used. Alternatively, replacing or supplementing HPS with LEDs for vine crops may be useful.

• Improve leaf growth

With inter-lighting, you help the growth potential of leaves in the plant canopy, and with LEDs you avoid overheating the parts of the plant closest to the lamp.

• Reduce heat waste

The intensity of the overhead light can be reduced by moving light into the canopy. This reduces waste heat from HPS lamps in the roof space and reduces the need for ventilation and screen gapping. 

• Match spectral needs

LEDs used in the plant canopy can be designed to give the best spectral output to meet the needs of the crop. 

Tiered production systems

LEDs are often considered for mutli-layered production systems such as growing rooms, tiered benches and vertical farming. These applications use two major advantages of LEDs; uniform lighting and the ability to put the light source close to the plant.

The light spectrum can also be tailored to the specific needs of the crop. This means any absence of solar radiation or natural light can be engineered by the lighting system.