As we all know, plants need light to grow. They use the energy within the light to produce their own chemical energy through the process of photosynthesis. It is possible to use artificial grow lights instead of the sun to supply plants with the light the need. Although not better than the sun itself, grow lights allow us to grow plants very well indoors, and even to provide light in the winter seasons to produce crop out of normal season.

Light Wavelengths Required for Ideal Plant Growth

Plants convert light into energy within their leaves. This is where the cholorophyll is stored, and is what plants use to convert light energy into chemical energy. Chlorophyll uses only very specific wavelengths of light to convert chemical energy. It doesn’t use green light as we might initially think. Rather, it uses red light that is 600-680 nm, and blue light that is 380-480 nm. Blue light is highest during the summer months and is responsible for growth of plants, while red lights is highest during the harvest months, and is responsible for the growth and development of flowers and fruit. Plant leaves absorb red and blue wavelengths of light, while reflecting green wavelengths. This is why leaves are of the colour green. This is shown in Figure 1. The height of the graph shows the wavelengths that are absorbed by plants in the process of photosynthesis. As can be seen in Figure 1, highest absorption is in the red and blue regions of the graph. The area between thewe two regions contains the green wavelengths, where there is minimal absorption. Thus when choosing lights to grow plants indoors or during night periods, it is important that they emit highly in the blue and red wavelengths in order to maximise plant growth.

 

Figure 1. Absorption of light by plants*

Types of Grow Lights

In theory any type of light can be used to supply plants with lights, as white light is a mixture of all light wavelengths / colours, and will contain the blue and red wavelengths required for hydroponics plant growth to some degree. However the intensity of blue and red wavelengths produced by standard light globes or fluorescent lights are not ideal for plant growth, and would only be suitable for growing lights that require low light conditions.

Grow lights that produce high intensity blue and red wavelengths have been developed. These are known as High Intensity Discharge (HID) lights. These lights have a globe and a reflector, which maximises the amount of light shined on plants, with the highest intensity produced being in either the blue or the red wavelengths. There are two main types of lightsavailable. Metal halide lamps emit mostly blue light (see Figure 1), and are most appropriate for the growth of leafy vegetables such as lettuce and herbs. At the other end of the spectrum, high pressure sodium lights emit mostly red light (see Figure 1), and are most appropriate for sprouting seedlings and for the growth of flowering and fruiting plants.

How to Use Grow Lights

Most plants grow well when grown in light for 16-18 hours per day. Some plants, such as those that require darkness to trigger flowering, require a more extended dark period. In these cases a light period of 12-14 hours is ideal. An electronic timer is ideal to regulate the light/dark cycle.

The distance grow lights are placed from plants is also an important consideration. Grow lights, as all lights, produce a certain amount of heat, and placing them too close to plants can cause heat stress or even death of leaves, flowers, and fruit that grow too close to the light source. The ideal distance to place lights from plants is determined by the power of the individual light. The instructions for each grow light should contain information that will assist in their setup and prevent undesired effects. When growing indoors, the temperature of the room the plants are in also needs to be considered, and if required, ventilation needs to be provided so that the environment doesn’t become too hot. This is particularly important in confined spaces such as in closets or under stair cases, where air space and circulation are minimal.

Summary

Grow lights are a great advantage for hydroponic gardeners that want to grow plants indoors or at times that are outside the normal growing season. Seedlings can be raised early, while the last frosts are still about, in preparation for planting into soil, or summer vegetables, herbs, and fruits can be grown year-round.

*Figure 1 from Roberto K. 2003, “How to hydroponics”, 4th edition.