Grow Lights Explained

There are several different types of grow lights for short day plant growers to select from. LED lights have become very popular and are becoming more affordable alternatives to high-intensity discharge lamps. In order to select proper lighting, growers need to consider PAR, PPFD, DLI, and other factors that describe a light’s electrical use and lifespan. When it comes to grow lights, remember that your goal is to mimic the sun. That means trying to imitate the sun's light intensity, light color, and the time period that plants naturally rely on for sunshine.

Short-day plants that thrive in ample light and will begin flowering when exposed to a threshold level of red light. Naturally, this is a cue that days are shortening and dusk is coming sooner, which tells the plant it is time to flower. 70 watts per meter PAR (photosynthetically active radiation) minimum should be maintained 18-24 hours a day to keep short-day plants in a vegetative state. Dropping light exposure to 12 hours of light and 12 hours of dark will trigger budding so lighting control is vital to successful greenhouse cultivation of short-day plants.

The “whiteness” of a light is measured by its color temperature in units of Kelvin (K). Light with a lower Kelvin rating has a yellowish tint, and light with a higher Kelvin has a bluish tint. In photomorphogenesis, plants respond to colors in the light spectrum by altering their growth patterns. Seed germination and the switch from vegetative growth to floral growth are photomorphogenic reactions. Natural daylight has a color temperature of 5000 K to 5800 K. Natural visible light color perception can vary based on weather and season, but this is not to be confused with the electromagnetic spectrum light colors which do not change. A light bulb that is identical in color to natural daylight would be rated as a 100 on the color rendering index. 

*not recommended

Fluorescent tube lighting is available in a variety of color spectrums but is not generally used for photoperiod control. They are most often used in growth chambers and germination since they do not emit much heat. Fluorescent lights come in a variety of spectrums. “Natural sunshine” fluorescent lights have a broad spectrum of peaks, where yellow and conventional fluorescent lights have just a few wavelength peaks around 540 nm and 630 nm. Full-spectrum lights will emit all colors of the visual light spectrum from 400 nm to 700 nm.

Tubular fluorescents are labeled T5, T8, and T12. The “T” indicated that the bulb has a tubular shape, and the number indicates the diameter of the bulb. T5 is the thinnest type of fluorescent bulb. T5 bulbs make good grow lights because they are very efficient and come in a wide range of colors. T5 bulbs cover a grow space around 2x4-ft, additional coverage can be gained by the use of multiple bulbs. PLL T5 bulbs are “U” shaped and can provide the efficiency of a tube bulb in a more compact space.

Fluorescent Grow Light Ballasts

Compact fluorescent (CFL) bulbs are a more energy-efficient option than incandescent bulbs for supplemental lighting but are not as effective for controlling the long-day plant photoperiod effect. CFL bulbs are ineffective for photoperiod control because they have blue and red light wavelengths, but do not emit much far-red light. This far-red light wavelength is responsible for rapid flowering in many long-day plants, so some growers choose to alternate incandescent and CFL bulbs. Compact fluorescent grow lights are more efficient than traditional fluorescent bulbs and they can be screwed into a regular (E27) light socket. No additional ballast is required for CFLs. CFL bulbs come in a range of brightness, from 13 watts to 300 watts. They can also be purchased in different colors such as warm spectrum bulbs (2700K) or cool spectrum bulbs (5000K to 6500K).

Incandescent Bulbs and Halogen Lamps

LED grow lights have gained popularity because they can be customized to emit only the most productive wavelengths for plant growth. They can be placed closer to the canopy of a crop because they do not emit as much heat as high-pressure sodium lamps or metal halide lamps (HID lamps). Heat sinks in the LED light housing help them dissipate heat well. LED lamps consist of several small lights in a grid, called an array. The LED driver functions like a ballast and regulates the input power, the driver also protects the light-emitting diodes from voltage fluctuations as they heat up and age. 

The diodes of an LED grow light are covered and sealed in by a lens. The qualities of the lens can influence the viewing angle of the light. Viewing angle refers to the pattern the light disperses in. High-intensity discharge bulbs and fluorescent bulbs have a viewing angle of 360° but LED grow lights typically have a 120° viewing angle. 

Full-spectrum LED lights can come with 2 settings, a red setting, and a blue setting. The blue setting will support vegetative growth, and the red setting will support flowering. Full-spectrum LED provides 730 nm (IR), 660 nm, 630 nm, 610 nm, 580 nm, 460 nm, 430 nm, and 410 nm (UV) light wavelengths. Growers should select lights with 32 watts per square foot of space, or about 32 watts per plant.

High-intensity discharge (HID) lamps include high-pressure sodium (HPS) and metal halide (MH) lamps. HID lamps may be used to provide photoperiod control by suspending them over the plant canopy, mounting to a boom, or using them with an oscillating reflector. HID lamps emit photosynthetically active light and are also used for supplemental lighting on short winter days and cloudy days. High-pressure sodium (HPS) lamps and metal-halide (MH) lamps are two popular types of high-intensity discharge (HID) lamps used in horticulture.

HPS lamp wavelength spikes at 632 nm, 605 nm, 589 nm, and 568 nm light wavelengths. MH lamp wavelength spikes at 674 nm, 630 nm, 583 nm, 564 nm, 540 nm, 497 nm, and 422 nm. Short-day plants respond better to HPS lamps, or a combination of HPS and MH lamps than to MH lamps alone. HID lamps produce a lot of heat, so it is important to have very good ventilation when they are used. HID lamps should have about two feet of space above the crop canopy to avoid burning the plants. 

HPS lamps are a grow light that uses sodium in an excited state to produce light. HPS lamps are a high-efficiency bulb that is preferred for flowering due to their large amount of red light. HPS bulbs replace MH bulbs after vegetative growth. HPS lamps have unbalanced full spectrum light with large outputs of green, yellow, and red light and very little violet, blue, and cyan light. For small-scale growing, an HPS lamp with 150W, 250W, or 400W is usually sufficient. For commercial growing, an HPS lamp with 600W, 750W, or 1000W is preferred. Be cautious with 600W HPS lamps, while technology has improved, many growers have issues with these not performing well with ballasts.

MH lamps are a type of HID grow light that produces light by running an arc current through vaporized mercury and metal halide gas. The efficacy of MH lamps ranges from 75-105 LPW. MH lamps require a minimum of 4-15 minutes to restart after turn-off (“restrike”). These lamps are popular for use in the vegetative growth phase due to the very high blue content of light. Common wattages for metal halide lamps include 250W, 400W, 600W, 750W, and 1,000W.

CMH lamps are a new variant of MH grow light that uses an extremely hot ceramic tube to ionize gasses and salts chosen for a specific spectral output. These lamps are becoming popular with indoor growers due to their very high CRI (up to 96). CMH lamps also have more red than a standard metal halide lamp. Sun System is a big producer of ceramic metal halide lamps, which they often refer to by their brand name "LEC." CMH lamps commonly come in 315W or 630W systems that have two 315W bulbs.

Mercury vapor lamps produce light by arcing electricity through mercury vapor. They are very old members of the HID family and are not commonly used anymore because they have very low efficacy and do not have the proper color spectrum for plant growth.

HID lamps receive electrical currents through a ballast that makes sure they run on the appropriate voltage from an outlet. The total wattage needed by the ballast and lamp are the input watts. Hoods often integrated with the ballast for HID lamps, some hoods have a reflective lining to help redirect light towards the plants. Reflective hoods are called reflectors or parabolic reflectors.

Magnetic ballasts have an iron core. They are heavy, inefficient, and produce a buzzing noise. Digital ballasts are more expensive than magnetic but are more energy efficient. Ballast with built-in cooling are helpful since HID lamps produce a lot of heat.

Air-cooled hoods are sealed reflective hoods that connect to ducts equipped with an exhaust fan to move out the heat produced by lamps. Water-cooled hoods are less common but are more efficient than air-cooled hoods and allow growers to move lamps closer to the plant canopy. Water in a chilled reservoir above the sealed reflective hood is pumped through, transporting heat away from the lamp.

The reflectivity of a material is its ability to reflect light away without absorbing or diffusing it, or altering its quality, intensity, or spectrum. Reflectors inside grow light fixtures maximize efficiency, but to truly maximize lighting efficiency growers often line grow rooms with reflective materials like mylar, visqueen, astrofoil, or foylon. Flat white paint with a high titanium dioxide content is also very effective. When light is reflected from all angles of the grow room, lower portions of the plant can receive more light. If lower branches of plants do not receive as much light as the top of the canopy, their growth will be reduced and their buds will take longer to mature. Placing plants on a table helps reflected light reach these lower branches and help the plant grow evenly with buds that will all be ready to harvest at once.

Light saturation is the limit to how much energy is able to be processed by a leaf. Lighting that is more intense than the leaf can handle can result in photoinhibition, where photosynthesis becomes inhibited by the excessive light. Bleaching from light can also occur when there is too much light. Bleaching causes chloroplasts to turn white, so leaves lose their green color. This effect is often seen in greenhouses and outdoor crops at elevations with high-intensity sunlight when growers are not using plastic with sufficient UV filtering. Occasionally you can also observe bleaching in plants that are under grow lights that are too strong. Using online calculators can help you determine the correct wattage of light for your cultivation space.

Grow lights can be hung inside greenhouses to ensure that plants receive consistent levels of light throughout the year. Lighting may also be utilized to control plant growth cycles and is especially important to control when cultivating short-day plants. If natural light is too intense, shades and mesh screens can be used to add protection. Blackout shades or shutters ensure that light pollution does not damage crops that are very sensitive to the timing of light exposure.