Lighting for your Garden

by Professor Hydro

Light is one of the five major ingredients needed to support plant life (water, nutrients, oxygen and CO₂ are the others). If you are going to garden indoors (or supplementing the sun outdoors) you need to replace the sun. This can be done with several different kinds of lighting incandescent, fluorescent, H.I.D. (High Intensity Discharge) and the new sulfur lights. These are the most popular types of lighting that are recommended for gardening purposes. There are many other kinds of lighting, but they don't produce a spectrum that would be suitable for gardening.

Incandescent lighting is the least efficient method of lighting your garden. These bulbs will screw into any ordinary light socket. They are usually a spot-light type design and have a blue coating on the glass to increase the blue spectrum of the light (incandescent lights normally produce light in the red end of the spectrum). These lights are a very poor choice to light your garden because of their inefficiency.

Fluorescent lights are a better choice than incandescent bulbs because they are about twice as efficient (lumens of light output / watt of electricity used). However these lights still aren't all that efficient because the effective light they produce will not penetrate more than about 6" to 8". For most plants fluorescent lighting is not a great choice for lighting your garden, vegetables and flowers usually need full sun and the light from fluorescent bulbs just isn't intense enough. It is, however, perfect for use with seedlings and cuttings as well as for orchids and other plants that need lower light conditions.

H.I.D. (high intensity discharge). These lights are by far the most efficient lights that are commonly used for gardening.There are two kinds of H.I.D. lights that have suitable light spectrums for plant growth, MH (metal halide) and HPS (high pressure sodium). Metal halide lights are rich in the blue spectrum which is very close to full summer sun. High pressure sodium are high in the red and yellow spectrum which imitates the fall sun.

HPS lighting usually costs about 10% to 20% more than comparable MH systems, however, the bulbs have a longer life span and also have a higher lumen output than any other H.I.D. lighting. MH bulbs lose about 15% to 20% of their light intensity after one year whereas the HPS bulbs lose only about 5% of their brightness in the same time frame. MH systems produce between 70-115 lumens per watt of electricity used, HPS systems produce between 97-150 lumens pr watt.

Regardless of which system you use for best results you should change out your bulbs before the end of their rated life. MH bulbs should be changed after 12 to 18 months and HPS bulbs after about 24 months.

Gardeners continue to debate over which of these types of H.I.D. lighting is the preference for general indoor gardening. I will not take sides in this matter, my job is to supply you with information, not opinions. But, the old rule of thumb is that metal halide light is the best for good vegetative growth and the high pressure sodium is best suited for flowering. As to which is the best overall choice for general gardening?........I'd say it' too close to call, so I'll leave it up to personal preference. Some people have been known to have a MH and a HPS lighting system, using the MH for the vegetative cycle and then switching over to a HPS system when it's time to flower. Another option is to use conversion bulbs. These are bulbs that allow a gardener to switch a MH system to a HPS system (or vice versa) by changing the bulb only. These conversion bulbs are more expensive and less efficient than the normal bulbs but they are much cheaper than buying an entire separate system.

Below is some specific information about both MH and HPS lighting.

METAL HALIDE - (MH)

Metal halide bulbs are very efficient and produce between 65 and 115 lumens of light output per watt of electricity used. MH bulbs produce a light that is very close to full summer sun, with a spectrum rich in the blue end. This promotes fast vegetative growth and compact, stocky plants with short internodal leaf spacing.

Metal Halide bulbs create light by passing electricity through an clear inner arc tube that is enclosed in the vacuum of an outer clear glass tube. This inner arc tube contains mercury and other metals in iodide form. When electricity is applied to these metal iodides they give off very intense light and heat. The outer casing can be either clear or phosphorus coated. Most gardeners prefer the clear bulb as it produces the brightest white light available.

MH bulbs come in sizes from 70 to 1500 watts with the 250 w, 400 w and the 1000 w being the most popular sizes for gardening. All the bulbs need to run with a ballast (basically a transformer to step up the voltage to the proper amount), that are designed to run a specific size bulb.

The bulbs themselves need to be burned in a specific position. They come in three types: Vertical (marked BU or BD), Horizontal (marked HOR) and Universal (marked U). The universal bulbs can be burned in any position, but they are more efficient when burned vertically.

The bulbs also come in a variety of styles, among them are regular MH bulbs and Super Bulbs (which produce 10% - 12% more light but use the same amount of electricity). The super bulbs cost slightly more, but they are more efficient, making them a good choice in the long run.

Medal Halide bulbs (except the 1000 w Super Bulb) should be replaced about every 9500-10000 hours or aprox.18 months (assuming an 18 hour / day on cycle). The 100 w super bulbs do not last as long as any of the others and should be replaced every 6500 hours, which is about 12 months if you run the bulb 18 hours a day.

HIGH PRESSURE SODIUM - (HPS)

HPS bulbs are the most efficient bulbs that are commonly used by the hobby gardener. They are high in the red and yellow parts of the spectrum and low in the blue, this imitates the fall sun. Because of this spectrum some plants that are grown with HPS lights only can grow elongated and rather leggy, while many other plants are not effected by the light spectrum. For plants that normally bud and flower in the fall this is usually the light of choice because the light spectrum promotes flower production.

There are now available color corrected HPS bulbs such as the Son Agro and the Planta bulbs. These are designed specifically for indoor horticulture, and have a more balanced color spectrum.

HPS bulbs are made out of a translucent ceramic arc tube containing a mixture of sodium, mercury and xenon gas. This arc tube is suspended in a outer glass shield (bulb). HPS bulbs range in power from 35w to 1000w, with the 250w, 400w, 600w and 1000w being the most popular for indoor horticulture.

HPS bulbs should be changed out after about 24 months. The bulbs slowly lose their brightness over time and to maintain good light quality the bulbs must be changed before the end of their rated life.

Sulfur lighting is the new kid on the block. I have had no dealings with this type of lighting. At this point in time sulfur lighting is extremely expensive when compared to H.I.D. lighting. There have been a lot of fantastic claims about the efficiency and brightness of these lights, however from what I have heard lately the claims were greatly exaggerated. There are companies working on a marketable sulfur lighting system for the hobby gardener and I will keep you posted when I learn more.

LIGHT MOVERS

Using indoor grow lights is a less than ideal substitute for the sun. Grow lights are obviously less intense and don't cover a very big area. Grow lights lose intensity very rapidly as you get further away from the bulb, which means that plants need to kept as close to the bulb as possible for the best possible growth. There are two major problems with putting the light very close to the plants, the coverage area of the light is diminished and the plants can burn if kept too close to the light. Keeping plants under a stationary light can also cause another problem. Plants that are directly under the light will grow better than those that are just slightly off to one side, resulting in a pyramid effect. To combat this problem you can move the plants around or you can move the light around. Moving the plants can be difficult if not impossible, so moving the light is the only option in most cases.

There are basically two kinds of light movers, linear and circular. Linear movers move the light back and forth in a straight line (most models have a pause momentarily at each end further reducing the pyramid effect), these work well in a rectangular shaped garden. The circular light movers move 2 or 3 lights around in a circle and are designed for use in a square or round garden.

The use of light movers allows you to bring the lights closer to the plants without fear of burning them and they allow for more light coverage (you can increase coverage by 25%-35%). Light movers also give your garden better light distribution, and sends light to the plants from many angles instead of only one as with a stationary light.