Artificial lighting for indoor plants
Once the preserve of professional growers, lighting systems can now be used to good effect in the home and greenhouse to grow plants. They can be inexpensive and do not need an in-depth understanding of the science and mechanics, so anyone wishing to experiment with growing plants under artificial lighting can have a go. Here is our beginner’s guide.
- Winter salad vegetables can be successfully grown under lighting to provide fresh leaves through the winter
- Tomatoes can be started earlier in the season under lights to extend the cropping period during the summer
- A terrarium/aquarium with lights is a simple way to start
Why use artificial lighting
Many plants grow well in a bright location in the home. However, the use of artificial light can be of further benefit in the following circumstances;
- For plants that need more intense light levels than are naturally provided indoors in the UK, especially during the short winter days
- For starting seedlings early in the year, rather than waiting for natural light levels to increase, resulting in stronger, more advanced young plants to plant out in spring
- For growing plants in a dark corner, windowless garage, where natural light would be too low
- For creating a decorative feature in a room, such as an illuminated terrarium full of foliage plants
What artificial lights to use in the home
Domestic light bulbs are unsuitable for growing plants as the light intensity is not enough and they are less energy efficient. For these reasons alone, it is best to choose specialised horticultural lighting.
Here is a summary of the options available for the home.
T5 High Output (HO) fluorescent tubes
Readily available from horticultural suppliers, T5 HO fluorescent tubes have the following advantages that make them the current top choice:
- The intensity of light produced is generally strong enough to enable good growth and flowering when positioned correctly
- They are long-lasting (about 20,000 hours)
- They are relatively inexpensive
- They are generally economical to run, but not as energy efficient as LED (Light Emitting Diodes) lighting
- They give off comparatively little heat making them safe to place close to plants
LED (Light Emitting Diode) horticultural lighting is a viable alternative for indoor use. They benefit from low operating temperatures and are longer lasting and more energy efficient than T5 HO fluorescents. At this point in their development, however, they can still be more expensive than T5 HO fluorescents to install. Specialist light meters may also be required to measure the light output of these systems, which can add to the expense of the set up.
HID (High Intensity Discharge) lights, which include metal halide and high-pressure sodium systems, give off a tremendous amount of heat. This makes them less useful in the home as extraction equipment is necessary to remove the hot air. The bulbs cannot be placed close to the plants because of the potential damage from the heat generated, meaning more growing space is required.
How to use artificial lights in the home
Plants absorb mostly blue and red light (think of the colours present in a rainbow, plants use two particularly) in order to grow and flower. Blue light generally stimulates growth, while red light is important for growth and flower production.
T5 HO tubes are available that produce colour values based on the Kelvin scale (K). Tubes with a 4000K value tend to produce light with a reddish tone, while those with a value of 7500K produce bluish light. If using fluorescent tubes to grow flowering plants, a combination of red and blue tubes will provide the best light for growth and flowering.
Having the lights on for a sufficient number of hours is also an important factor – see the guidelines below for each set up.
Simple lighting system
There are a variety of kits available which include an electronic ballast (that starts up the tubes) and reflectors to direct the light downwards onto the plants.
- In the home a basic set-up could be a unit containing two 60cm (2ft) T5 HO fluorescent tubes suspended approximately 60cm above for example, a small group of houseplants, or crops such as leaf salads for fresh leaves during the winter months
- The lights can be on for around 16 hours a day
- Plants often require different light intensities. The foliage colour is a good indicator to determine whether the plants are receiving enough light and the intensity can be adjusted to suit them (See: 'Problems' below)
These benefit from predominantly blue light to promote good growth and compact plants. The T5 HO tubes should be placed around 60cm (2ft) above the seedlings for around 16 hours a day.
Tomatoes for example can be started earlier in the year under lights when natural daylight is still too weak. This enables plants to be more advanced by the time they are planted in their final positions in the greenhouse or border. Again adjustments can be made regarding light intensity requirements. (See: 'Problems' below)
Phalaenopsis (moth orchids) and African violets (Streptocarpus syn Saintpaulia) can be grown under lighting very successfully and are tolerant of a variety of lighting conditions. Leaves should be a mid-green.
Plants from naturally low-light habitats, such as ferns and many of the smaller tropical foliage houseplants that in nature inhabit shady forest floors, can be kept healthy long-term under simple artificial lighting. Use an aquarium tank as a terrarium for these plants and position the T5 HO lights in the hood. Not only will this provide a decorative focus in a room but the terrarium will help retain the high humidity which these plants love.
Other plant genera
Different plants will vary in their lighting requirements to grow and flower well. It is best to research the light requirements of your chosen plants and ensure the output of the lighting system is sufficient to ensure success. This can be done by carefully measuring the intensity and quality of light provided using a quantum light meter.
Generally a mid-green colouration to the foliage indicates the plants are receiving the correct amount of light. However, there are a number of problems that can be encountered and rectified.
Too much light:
Seedlings that take on a bleached appearance or become stunted after growing under lights. Similarly, yellow-green foliage on plants nearest the tubes can indicate the light intensity is too high for the plant in that position. The intensity of light can be lowered by:
- Reducing the number of tubes
- Repositioning the tubes further away from the plants
- Reducing the number of hours the lights are on (photoperiod)
Too little light:
Plants grow poorly and foliage is dark green. This can be corrected by:
- Increasing the light intensity by moving the plants closer to the tubes
- increasing the number of hours the lights are on
- Increasing the number of tubes
Too much blue or red light:
This can also create problems with plant development, but balancing the number of red and blue tubes should correct the following problems:
- Too much blue light with very little or no red light, results in stunted plants with dark green leaves, thick stems and few flowers
- Too much red or far-red light promotes stem elongation and tall spindly growth
The Royal Horticultural Society is the UK’s leading gardening charity. We aim to enrich everyone’s life through plants, and make the UK a greener and more beautiful place.