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Grow Lights

Since the grow light is often the most expensive item in the indoor garden, it is important to choose the right one for your space and application. You should measure the square footage of your garden area, and think about the plants you will grow. Different plants have different light requirements. In general, the taller the plant, the more light required in order to reach the lower sections. Plants that flower or fruit (such as roses or tomatoes) typically require more light than those which do not (such as lettuce, kale or most herbs). However, if you are gardening indoors under an artificial light, you have much more control over plant height. Many gardeners try to keep plants shorter in order to maximize light efficiency--light intensity fades rapidly as it moves further from the bulb. When possible, choose compact or dwarf varieties of common plants. If the plant begins flowering in response to a shorter day length, cut your light hours back to 12 on/12 off when the plant is 1/3 to 1/2 as tall as you want it to be. It will continue vegetative growth for several weeks, and in the end it will reach the desired height.

Types of Grow Lights

Incandescent Grow Lights like those sold in nurseries or hardware stores are not powerful enough to grow most plants. Incandescent bulbs also have short life spans and are not energy efficient.

Fluorescent Grow Lights are good for starting seeds or cuttings, but typical fluorescents are not powerful enough to light an area more than 8-10" below the bulb. Fluorescent bulbs are more efficient than incandescents, but much less efficient than HID light systems. However, there have been some exciting recent developments in fluorescent technology that deliver much more light than regular fluorescent bulbs. Hydrofarm's Compact Fluorescent Light System, and Sunlight Supply's New Wave and Tek-Light T-5 products, are so powerful that they can take plants through at least part of their vegetative growth. The advantage of these fluorescents is that they produce so much less heat than metal halide or high pressure sodium lights. With heat eliminated or greatly reduced, lights can be placed much closer to plants for maximum efficiency.

HID (High Intensity Discharge) light systems have revolutionized indoor gardening in the last 20 years. They are the most energy efficient grow lights available, so they produce much more light for the amount of power consumed. The two most common HID lights for plant growth and maintenance are Metal Halide (MH) and High Pressure Sodium (HPS). Metal Halide bulbs emit a balanced light spectrum which appears blue-white to the human eye, and High Pressure Sodium bulbs emit a spectrum which is more concentrated in red/orange light.

All HID light systems require a ballast (transformer) and a special socket to operate. The type and wattage of the bulb must match the ballast; for example, a metal halide 400 watt ballast will not operate any other wattage metal halide bulb, or ANY high pressure sodium bulb. Smaller wattage systems (150 to 270 watts) are mini systems which feature the ballast and socket built into the reflector. These lights are ideal for starting seeds or clones, or for growing adult plants in small spaces. Larger wattage systems (400 to 1000 watts) will cover larger growing areas, and since these bulbs produce much greater light intensity they are superior for growing taller plants such as tomatoes.

Coverage Area

Most gardeners use at least 25 watts per square foot of garden space. You may need less than this if your light is used to supplement natural sunlight, or if you are growing a plant that does not require as much light (such as lettuce). On the other hand, many gardeners prefer to double or even triple the recommended wattage to achieve faster growth rates. These gardeners also must pay close attention to the garden environment due to the intense heat produced by HID lights. There is really no such thing as too much light, but using a big light in a small space will sometimes result in high temperatures that are difficult or impossible to control. Keep an HID bulb about 18-24" above the top of your plants. (This is an average distance...lower wattage bulbs may be placed closer, while higher wattage bulbs may need to be placed further away). Higher bulbs provide less intense light to a larger area, while lower bulbs provide more intense light to a smaller garden area.

Average Coverage Area by Wattage

These figures are average coverage areas. Different reflector designs will spread light differently, and the height of the reflector above the plants will also affect coverage area. Another thing to keep in mind: light is most intense in the center of the garden area, and becomes less and less intense as it moves away from the bulb.

  • 150/160/175 watts...2' x 2'
  • 250/270 watts...2' x 3'
  • 400/430 watts...3' x 3'
  • 600 watts...4' x 4'
  • 1000 watts...5' x 5'

Power Consumption

HID lights WILL increase your electricity cost from $5 to $50 per month--the exact amount depends on the size of the system and the number of hours operated. However, since these grow lights are so energy efficient, you are getting huge amounts of light (and growing power) for your money! Before purchasing an HID light, make sure your grow room's power circuit can handle the power draw. For safety reasons, do not exceed 75% of the rated ability of the fuse (for example: use no more than 15 amps on a 20-amp circuit). Amps and estimated daily power cost are listed below.

Wattage Amps Cost of
18 hr day
Cost of
12 hr day
These figures based on input voltage of 110, and electricity cost of 9¢ per kilowatt hour. To calculate your cost, multiply the bulb wattage x hours of operation and divide by 1000. This figure is the number of kilowatt hours of electricity consumed. (Example: a 400 watt bulb running for 18 hours will use 7.2 kilowatt hours). Check your power bill for the cost of each kilowatt hour.

Types of Reflectors

Horizontal Reflectors

Bulb mounted in a horizontal position. These reflectors produce the most downward light intensity, especially with a specular aluminum reflective finish.

Vertical Reflectors

Bulb mounted in a vertical position with the base up. These reflectors usually have a cone or "umbrella" shape. This design spreads light out further than a horizontal configuration, but the overall light intensity under the bulb is much lower.

Air Cooled Reflectors

Horizontal reflectors with an added glass shield and duct adaptors. Vinyl or aluminum ducting runs from the reflector to a squirrel cage blower fan which is mounted in a nearby wall or ceiling. The blower pulls air through the reflector and exhausts the heat outside the growing area. Air cooled reflectors provide the most efficient means of heat control because the heat is exhausted from its source, before it has a chance to spread through the entire room. Hydrofarm's Radiant, Valubrite, and Valubrite 6" systems, and Sunburst mini system, can be easily adapted for air cooling.

Light Intensity Definitions

Grow lights and their relative intensities are measured in several ways. The watts are the amount of power that the system consumes in order to operate. The lumen output of a lamp is a measure of the intensity or brightness of the light. (One lumen is equivalent to the light emitted by one candle). Footcandles are another way to measure footcandle is the amount of light provided by one candle to an area of one square foot at one foot away. The light output of a lamp is usually expressed in lumens, while the light requirement of a plant is usually expressed in footcandles.

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