In this article we explain the potential benefits of using grow lights for your indoor Bonsai, and we help you pick the right setup. But first, let’s investigate the importance of light for different types of plants.
Does my Bonsai need extra light?
Trees from temperate climates need to be kept outdoors to thrive, as they need a period of winter dormancy each year. Deciduous trees don’t need light when they have shed their leaves, but even evergreen tree species do not need much light during their winter dormancy. Normally they don't need any supplementary lighting.
However, lack of light can become a life-threatening problem for tropical and subtropical plants. This can happen when they are kept indoors too far from a window, if there is only a place at a a north-facing window available, or if there is too short daylight and little sunlight in winter where you live.
Some tropical tree species can cope with little light if they are positioned cool enough in winter to take a rest period. At the same time, however, it must not get so cold that the plants are damaged. So it is often very difficult to create optimal living conditions for indoor bonsai of different species indoors without special aids. If the tree lacks sufficient light, perhaps in combination with dry, warm heating air, it will lose an unusually large number of leaves and become susceptible to pests such as spider mites, scale insects, etc. The tree will grow large, pale leaves and spindly shoots with long internodes, which significantly impair the appearance of the bonsai.
Using grow lights for Bonsai
A good measure to help indoor bonsai trees in unfavorable locations or in warm places to become healthier and more vital over the winter is to use grow lights to supplement or replace natural daylight. However, there are a few things to bear in mind to ensure that the costs and benefits are in reasonable proportion.
Duration and time of day for artificial light
Tropical and subtropical bonsai that are placed directly at a south- or west-facing window receive a few hours of natural light per day during sunny weather in winter. It may then be sufficient to switch on the grow lights for a few hours in the morning and after sunset, controlled by a timer, to give the bonsai trees a total of 12 - 15 hours of light (natural plus supplementary). If you observe your trees and their reaction to such a time-limited supplementation of daylight well, you will be able to determine for yourself whether it is sufficient. Depending on the room temperature and plant species, there may either be some healthy growth even in winter, or growth may pause but the plant will retain its leaves and their healthy dark green colour. Both scenarios are acceptable, but if the trees lose a lot of leaves, get unhealthy leaf colour or pests, the lighting may still not be sufficient. On a side note, very often something should perhaps be done to increase humidity as well.
If the winter weather is overcast and gloomy, or if the bonsai are placed more than half a metre away from a large south or west facing window, it is advisable to switch on the grow lights continuously for 12 - 15 hours a day. A timer is also ideal for this task. It is not advisable to light the plants overnight, as they need their natural day and night rhythm. The dark season, when additional lighting is necessary or helpful, is roughly from the autumnal equinox to the spring equinox. So when the length of daylight falls below 12 hours, it is time to put the plant lamps back into action. So in the northern hemisphere this is roughly between September and March, in the southern hemisphere between March and September.
| Light intensity | Illumination per day | |
|---|---|---|
| Plants from temperate climates | 700 lx | 9 hours |
| Subtropical Plants | 1.000 - 2.500 lx | 12 - 15 hours |
| Tropical Plants | 1.500 - 5.000 lx | 12 - 15 hours |
The values given in the table are to be regarded as minimum values. For comparison: On a sunny summer day, the light intensity outdoors can be up to 100,000 lx.
What kind of light does a Bonsai need?
It is important to pay attention to the correct distribution of the light spectrum. Red and blue light promote photosynthesis, growth and the development of flowers and fruits. The light source must have sufficient illuminance (measured in lux) and must not generate too much heat in order not to damage the plants. The light from grow lights should be as similar as possible to natural sunlight, which has a colour spectrum between 5,000 and 10,000 Kelvin. For lamps, this light colour is called "daylight white" or “full spectrum”.
The colour spectrum of natural light is composed of green, red and a high share of blue colour component. The green part of the light is not needed by plants and is therefore reflected, which is why we see their foliage in green. Plants, on the other hand, use blue and red light. The right ratio of these light components is important. Too little light or too small a red component will result in unattractive growth with pale shoots with few leaves. The red colour spectrum promotes the growth of fruits and blossoms, while the blue supports shoot and foliage growth.
How to use a grow light
The artificial grow light should illuminate the bonsai trees from above. A distance of 30 - 40 cm between lamp and plant is considered optimal. If the lamp is hung higher above the plant, the light intensity it reaches will be lower.
Choosing your grow lights
The ideal plant lighting contains a high blue component and at the same time a certain red component to promote healthy plant growth. Plant light should be as similar as possible to sunlight, so lamps with a full spectrum light colour are particularly recommended. The label 1A or 1B also means that a lamp can be used as plant light. What follows is a list of the many options available for grow lights.
LED - Modern LED bulbs or LED tubes are ideal for plant lighting. Here, too, care must be taken to choose the right colour temperature for the plants. LEDs are somewhat more expensive to buy than conventional light sources, but they make up for this in a short time thanks to their extremely low energy consumption. With a recommended lighting time of up to 15 hours a day (see table), LED plant lamps are always worthwhile.
Full spectrum fluorescent tubes - These are also well suited for plant lighting, especially as their heat radiation is so low that they can even be hung at a distance of 20 - 30 cm above the plant. Fluorescent tubes hung higher above the trees usually spread their light too widely and thus do not achieve the required illuminance. In contrast to LEDs, however, fluorescent tubes consume considerably more energy and do not last nearly as long.
“Modern LED bulbs or LED tubes are ideal for plant lighting.”
Sodium vapour lamps - These were once considered a particularly good solution for illuminating plants. Although they do not achieve the ideal light spectrum for plant light, they have a particularly high luminosity, because sodium vapour lamps reach wattages of 1,000 W. However, the lamps produce a lot of heat when generating light, which is why they have to be hung at a greater distance from the plants. Sodium vapour lamps are expensive to buy and also cause very high running costs due to their high energy consumption. Therefore, their use is less recommendable for the bonsai hobbyist.
Energy-saving lamps - Due to their low heat generation, full spectrum energy-saving lamps are also well suited as grow lights. Their compact design makes them ideal as a light source for individual plants. However, they are inferior to an LED solution in terms of power consumption and service life.
Normal incandescent lamps are not suitable as lighting for plants. They have too high a red component in their warm white light, usually between 2,300 and 2,500 Kelvin. Plants need red light to grow, but too much red light leads to unhealthy growth, and what's more, incandescent lamps generate an extremely high amount of heat, which can be harmful to plants.
Conclusion for Bonsai grow lights
Good quality LED plant lamps offer the best ratio of ideal plant light and energy-saving lighting. There is an enormous variety of suppliers and brands of grow lights, including many from the Far East. Not all products are of satisfactory quality, which is why it is a good idea to first check out test reports from consumer protection organizations on the internet or ask on our Bonsai forum. Supplementing light for your indoor Bonsai in winter will improve their health and vigor substantially.
