Plant grow lights, what do I need to know?

The first factor is to choose the colour of light required, whether you need a warm or cool light source. The temperature of light is expressed in degrees Kelvin and is known as the correlated colour temperature or CCT, and relates to actual thermal temperature. High intensity discharge (HID) lamps and fluorescent light sources all have CCT ratings and this indicates how warm or cool the light source is, for instance a lamp with a CCT of 2700 Kelvin is considered warm, 4200 Kelvin is considered neutral and 6000 Kelvin is considered cool.

CCT ranges of the plant light

5000-7000 Kelvin: Strong Blue Light
Promotes bushy growth. Ideal for the rapid growth phase of plants. Greatly enhances all round plant growth when used with super high output, high pressure sodium or 3K warm metal halide lamps.

4000 - 4200 Kelvin: Cool White Fluorescents
Can be used to supplement blue lighting. Ideal for propagation.

4000 Kelvin: Neutral Metal Halide
Best single light source for plant growth, producing shorter, bushier growth than 3700 Kelvin and colour rendition. Used in general plant lighting.

3700 Kelvin: Softer Metal Halide (coated)
This coated lamp is used for general plant lighting and for more rapid growth than 4000 Kelvin produces.

2100 - 2700 Kelvin: High Pressure Sodium Lamps
Redder colour mix, ideal for the fruiting and flowering stages as well as supplementary greenhouse lighting.

hat types of plant lamps are there?

There are two main types of lamps used in horticulture today:

High Intensity Discharge (HID) lamps

Simple to use and covers a large area for almost any type of plant.
HID lighting can be divided into two categories: Metal Halide (MH) and High Pressure Sodium (HPS).

Metal Halide (MH)

Metal Halide (MH) lamps give off an abundance of blue light imitating the light of spring and summer making them the best light for propagation and vegetative growth, promoting short internodal length.

High Pressure Sodium (HPS)

High Pressure Sodium (HPS) lamps produce 'redder' light and can be compared to the light of an autumn sunset. More yellow/red colour in the spectrum and less blue promotes a higher flower-to-leaf ratio in flowering and fruiting plants. HPS grow lights are widely used to extend the natural 'day length' that a plant is subjected to thereby imitating summer conditions.
Since the blue light provides all the blue light necessary, the HP sodium is used during lowlight hours and during total darkness. If you are growing indoors and without any natural light then a combination of the two lamps is ideal, especially when used with a light mover.

Fluorescent grow lights

Fluorescent lamps are very energy efficient and therefore ideal for the green grower. There are several categories: strip fluorescents, compact fluorescents (CFL) with a high wattage design, warm or cool light and high or low wattage. The fixtures for these lamps usually come complete with lamp holders and built-in ballast. Your local hydroponics dealer will show you some different models.

Fluorescent lighting has a lower intensity than metal halide or sodium lamps and due to their low heat output they will not dry out the growing media when placed close to the plant. This makes them ideal for propagation and early vegetative growth as well as for orchids and other plants that need lower light conditions. Because of their low heat output they can also be kept approximately 1 inch from the plants, and do not require ventilation to remove excess warmth. This means you are getting more usable light to your plant and maximising the output of your lamp.

For vegetative growth you should choose a 'cool white' lamp. This is also acceptable for flowering, but a 'warm white' light will be better as it is stronger in the red end of the spectrum which is more suitable for flowering. Use fluorescent lights only for seedlings and clones, for supplementing daylight and for vegetative growth. A HID Sodium Lamp is always recommended for flowering due to its high light output and it is more efficient when you look at what your plants produce for the number of watts you put in.

How much does a 600 watt grow light cost to run? 

The first thing that we will look at is the rate paid for electricity. This can be found on your bill and is expressed in cents per kWhr (kilowatt hour). For example, my basic energy charge is $0.079797/kWhr, but there is more. Added to the basic rate is a PSCR renewable energy, system access distribution, energy efficiency and securization charges plus a securization tax, which brings the rate up to $0.122109/kWhr.

The next thing I need to know is some information on the light fixtures, specifically the wattage of the bulbs. Let's say that I am using two, 4-foot fluorescent fixtures that contain two 40 watt grow lights each. The total wattage is 160 (four bulbs x 40 watts). Since we are charged using kilowatts, divide 160 by 1,000 and multiple by the rate, which for me would be $0.122109/kWhr = $0.0195374. The result is the cost to run the lights for one hour. Multiply $0.0195374 by the number of hours you plan to run the lights (14 hours). The cost to run the lights per day is $0.2735236 or $8.20 per month.

If I have an HID (high intensity discharge) light with a 600 watt HPS grow light in it, using the same method of calculation, it would cost me $30.90 per month.