Metal Halide Lights: Growing Bonsai My Way

by Jerry Meislik


This is a distillation of how I grow bonsai under lights. By way of introduction, I have been growing bonsai for over 30 years, and for most of those years I have grown indoor/tropical bonsai under varying conditions including windowsill, greenhouse, and artificial illumination.

While living in Michigan I started using metal halide lamps to help grow my indoor bonsai. Despite having the use of a greenhouse the addition of metal halide lights to supplement the natural light actually resulted in increase growth and vigor of my trees!

Since moving to Montana my new growing room is a conventional room with no skylights. Seven 1000 watt metal halide lights are hung from the ceiling. These lights and the windows provide the only light that my trees ever get. The plants never go outdoors even in summer, and must survive only on this light. At our location and 5000 foot elevation frost is possible any day of the year thus making the plants housebound.

The floor is waterproof and moveable food service carts hold the trees.


The plant room floor is waterproof so careful watering is not required, and a centrally located floor drain removes the overflow. Plants are all arranged on food service carts on wheels. This allows the trees to be rotated, repositioned and household chores to be done with ease despite the huge size of some of the trees.

Two metal halide lights shown hanging from the ceiling.

Visitors to the plant room are impressed with the health and vigorous growth that can be achieved with artificial illumination.

Light is critical
The number one factor in growing plants is water, since plants will survive only one week or less without it. Light is the second most important factor and it is the "energy" supply for your plants. Green chlorophyll in the leaf and stems transforms light energy, water, carbon dioxide, and minerals into sugars and everything else a plant needs to survive.

Under low light conditions most plants barely survive, and they do not grow enough to allow bonsai training. Compare the leaf density of two Ficus benjamina 'Kiki' that are the same age and were styled at the same time. One survives on a windowsill of one of my students and the other is growing under metal halide lamps.

Left is a Ficus Kiki grown on a windowsill. On the right is an identical tree grown under metal halide lights!


Many species of trees will not survive indoors under typical home illumination and they gradually starve and die. The list of bonsai materials than can be grown indoors without supplemental light is very short. David Fukumoto has pioneered the selection and training of bonsai materials that can be grown under normal home illumination. See for further information on specific materials and techniques for success in typical home low light situations.

Jack Wikle of Michigan, one of my bonsai teachers, is a pioneer in growing bonsai under regular fluorescent light. Jack's indoor trees are grown under inexpensive fluorescent lights available at any hardware store and his results are marvelous. Many plant species grow beautifully under this type of supplemental illumination as Jack's trees demonstrate. See Jack's article by clicking here.

My friend Cyril Grum, Michigan, also uses inexpensive fluorescent lights to grow his small sized indoor bonsai. The trees are beautiful, healthy and his home-made light set up is fairlly easy to duplicate.

I have conducted experiments with Light Emitting Diode, LED, lights to see how they will compare to growing under fluorescent lights. Initial cost for LED lights is quite hight but the cost to run them is relatively inexpensive compared to metal halide lights. You can see the results of 3 sets of experiments done with LED lights here. Basically, not all LED lights are the equivalent of fluorescent lights although they may use less electric power.

Cyril's set up is a fluorescent light placed on a homemade wooden stand.

Care Of Indoor Trees

My trees are watered once a day with reverse osmosis water. This is water that has no salts, or minerals in it, and is the equivalent of distilled or rain water. As long as your tap water is not too mineral laden it should do well for your indoor trees.


Soil for my bonsai is typically red lava crushed to about 1/8 inch size. Red lava is 100% of the soil mix or amended with an equal volume of small bark chips. Most of my indoor trees do not seem to prefer one mix or the other. Re-potting is done every two to three years for most trees.

With the highly inorganic soil mix, fertilization is quite important to my trees. I fertilize every day with a fertilizer injector that automatically adds a precise amount of fertilizer to the water. If a plant is newly repotted or sick it is not fertilized.

Insects and diseases can be a real problem in the confined spaces of a growing room or greenhouse. Bugs usually enter the plant room on newly bought trees, therefore I isolate all new trees for two weeks and spray them with insecticide several times before they are permitted into the plant room. Typical problems include white fly, scale, spider mites, and mealy bugs. The usual sprays are used to control the problems. Keeping room air circulating with fans also helps to discourage many insects.

The temperature range in the plant room is 65 to 95 Fahrenheit, and this seems to be reasonable for most trees.

Humidity levels are not controlled in the plant room. It stays fairly humid simply from the watering of the trees and evaporation. I find that most trees are not really particular about humidity. Plants with thick and waxy leaves tolerate dry air the best. Plants with thin leaves dry out in extremely low humidity and make increasing the room humidity necessary.

Plant materials
Materials that I successfully grown indoors under metal halide lights include Ficus, Wrightia religiosa, Buttonwood, Caesalpina ferrea, Portulacaria afra, Coccoloba uvifera, Nea buxifolia, Fortunella hindsii, Bougainvillea glabra, Brassaia actinophylla, Carissa grandiflora, Hibiscus rosa sinensis, Psidium littorale, Phyllanthus sinensis, Sageretia theezans, Schefflera arboricola, and Tamarindus indica.

All these plants grow, and their trunks thicken under the high light conditions and many will flower, and even set fruit!. Under low to moderate light these same materials merely survive without much growth.

Ficus microcarpa after two years under metal halide lights.


Ficus microcarpa grown indoors for 18 years.


Benefits of the metal halide lights
Metal halide lights provide high light output at moderately high efficiency levels. The lights come in many sizes, shape and types. Another type of light is sodium. Sodium lamps are yellow/orange in color and give the plants a peculiar appearance. The sodium bulbs also allow stem elongation, or etiolation which is not desirable in bonsai but the frequency spectrum does help in flowering/fruiting

Metal halides can be found in 250 to 1000 Watt sizes. Each light comes with a transformer that can often can handle only one bulb, and only the bulb wattage for which it is built. So if you buy a 250 watt transformer you can not later upgrade it to a 1000 watt bulb for instance. Buy the size light that you will need, as upgrading to a higher wattage bulb is not possible.

Some light fixtures will allow switching form sodium to metal halide bulbs at different stages of the growth process to encourage blooms vs. vegetative growth.

Reflector and bulb of metal halide light.

Using metal halide bulbs you can grow many high light materials. These plants will actually grow significantly and increase their girth and leaf density. Under moderate to low light situations these plants will be almost static and not show any real increase in their size or ramification.

Disadvantages of metal halide lights
These lights have the problems of producing significant heat from the bulbs. This means that the tops of the trees can not be placed closer to the bulbs than one foot or the nearest leaves will burn.

Leaves burned by close proximity to metal halide lights.

In addition each bulb has a transformer that makes an annoying humming sound. One transformer is required for each bulb. Transformer noise can be troublesome in a quiet home situation. My transformers are all located in a room below the plant room for this reason.

Four transformers on a shelf.

The bulbs and the transformer must not get wet. In addition a wet bulb may shatter so keep any moisture off the bulbs as well as the transformer box.

Bulbs lose their efficiency over a one to two year period and will need to be replaced, and they are somewhat costly to purchase. In addition, some of the transformer components burned out and I have had to replace them as well.
Light supplementation of any sort is advisable for the best growth and survival of indoor bonsai. Those who cannot supplement their indoor lighting must use low light trees. Supplementation with fluorescent light allows more plant varieties to be grown, while metal halide lights allow us to grow many moderate to high light requiring plants. For those who desire to really grow those "finicky" tropicals I would strongly suggest using metal halide supplementation.

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