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Light

The most essential requirement for growing plants is an adequate source of light.

If you grow outdoors, an abundance of light is typically available depending on time of year and geographical location. If you are growing indoors; however, you must provide light at the right intensity, duration, and spectrum to optimize the growth of the type of plant you intend to cultivate. Light from lamps are measured in lumens which vary based on the wattage of the light and the efficiency of conversion of electrical energy into light energy (Efficiency= lumen output/wattage input).

Spectrum of Light

The spectrum of light for plant growth is known as photosynthetically active radiation (PAR) and ranges from 400-800 nanometers. At the 400 side of the spectrum, the light appears more blue which is favored by crops growing vegetatively. At the 800 side of the spectrum, the light appears more red and orange which is favored by crops growing to produce fruits and flowers.

Types of Lights

There are three types of lights that are commonly used to grow plants indoors or to supplement the natural light from the sun: Fluorescent, HID (High Intensity Discharge), and LED.

  • Fluorescent – compact, low-intensity lights that generate less heat than HID lights, but less lumens per watt than HID lights. Fluorescent lights are generally less expensive than HID lights and much less costly than LEDs. Sizes, wattage, and spectrum of bulbs vary. The bulbs used in plant fluorescent lights for vegetative growing is in spectrum the range of 6500K, while bulbs that produce light in the fruiting/flowering spectrum is in the range of 3000K. Fluorescent lights can use both spectrums of bulbs.
  • High Intensity discharge (HID) – There are two types of HID lights that are predominant in the plant industry: High pressure sodium (HPS) and metal halide (MH). High pressure sodium lights produce light in the red portion of the spectrum which is needed during fruiting and flowering. Metal halide lights produce in the blue portion of the spectrum which is favored by plants growing vegetatively to produce leaves and stems. Both HPS and MH lights, are available in 150, 250, 400, 600, and 1000 watts and in both coil-type and digital ballasts. The major disadvantage from HID lights is the amount of heat output from both the ballast and the bulb.
  • Light Emitting Diode (LED) – Although the technology has existed for many years, recent developments in manufacturing and production has brought LED lights to the world of horticulture. LED lights are very efficient in converting electrical energy into light energy, but do not have nearly the output of traditional HID lights. Like fluorescent light, heat output from LED light is minimal compared to HID lights. Unfortunately, the cost of LED lights has made them impractical for many growers, yet the bulb life of a LED lamp makes it a viable option for the future. LED bulbs are available in red, green, blue, and purple currently to mimic the particular spectrum of light that it is re-creating.

Duration of Light

The duration of light provided to a plant can play a role in the stage of growth that the plant is being pushed towards. Some plants are quantitative short or long day plants, meaning the plants will not fruit or flower until the photoperiod, or the number of hours of light, is increased or reduced to a certain amount. In short-day plants, the plant will not flower or fruit until the photoperiod is 12 hours or less. For quantitative long-day plants, the opposite is true. Each plant is different; however, so researching the type of plant that you are growing will help in this determination.

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