Stephen Albert
To grow vegetables and herbs indoors you will almost certainly need artificial light. Natural light preferably from a south-facing window will always benefit plants growing indoors, but sufficient natural light is not always available indoors. That’s when your plants will need the right artificial light.
Plants use light to grow. They convert light into food energy via photosynthesis. Plants need 12 to 16 hours of light each day for the best growth. When sunlight is not available, use artificial light.
Artificial light for growing plants is available in several forms. Here are the types of lights commonly used by plant growers:
- Incandescents
- Fluorescents
- Full-Spectrum Fluorescents
- Compact Fluorescents
- HID – High-Intensity Discharge
- LED – Light-Emitting Diode
Here follows a description of each of these light sources:
Incandescent Lamps
An incandescent lamp is the classic light bulb. An incandescent produces light by heating a wire filament to a temperature that results in the generation of light. The metal wire is surrounded by a glass bulb filled with inert gas or a vacuum. An incandescent light emits more heat than light.
- Pros: Incandescents can be used for low-light, heat-loving houseplants like ferns. They are smaller than other lights and can be used to grow a single plant or a small number of plants.
- Cons: This is the least energy-efficient choice for growing plants. Only about 10 percent of their energy is used for light, the rest is used for heat. They can “cook” a plant if not used properly. This is not a good choice for growing edible plants.
Fluorescent Lamps
A fluorescent lamp, or fluorescent tube, is a low-pressure mercury-vapor gas-discharge lamp that uses fluorescence to produce visible light. An electric current in the gas charges the mercury vapor which produces short-wave ultraviolet light which in turn causes a phosphor coating on the inside of the lamp to glow. These are the standard shop light or office lights.
Fluorescent lamps provide two to three times more total light than incandescent lamps using the same amount of electricity while producing less heat. They are more effective and energy-efficient than incandescent lamps. A fluorescent lamp will provide about 20,000 hours of light, about three years of light if it is on for 16 hours a day every day of the year.
Fluorescent tubes have two dimensions: diameter and length. Fluorescents are commonly labeled T5, T8, or T12. The T designates the diameter of the tube. T5 tubes are about the diameter of a dime (15mm); a T8 is about the diameter of a nickel (25mm). T12s are the largest (38mm). The smaller the diameter of the tube, the more efficient the lamp. A T5 is more efficient than a T8 which is more efficient than a T12. A 48-inch T12 bulb is most often used in an office and as a shop light.
Fluorescent tubes are available in full-spectrum light modes that are intended to replicate natural light.
T5 tubes—the most efficient fluorescent lamps—can be used only in ballasts designed to hold T5 tubes. T5s can be labeled HO for high output or VHO for very high output. HO tubes can be used only in ballasts designed to hold HO T5 tubes.
Full-Spectrum Fluorescent Lamps
Full-spectrum fluorescent lamps at a step up from fluorescent lamps. Not all fluorescent lamps are the same. There are fluorescent lamps for ambient room lighting and there are others designed for plant growth. A full-spectrum fluorescent lamp offers more of the light waves plants use to grow. To be clear, a standard T12 fluorescent tube will replicate natural light and can be used for growing plants, but a full-spectrum fluorescent lamp offers more.
Briefly, natural light can be split into different wavelengths of color: violet, blue, green, yellow, orange, and red light. The range of light is known as Photosynthetically Active Radiation (PAR). In the process of photosynthesis, plants use different spectrums or light wavelengths—these are particles of light–to produce the energy needed to produce the food plants use to grow.
Full-spectrum fluorescent lamps provide a better balance of light across the entire PAR light spectrum than standard fluorescent lamps.
Besides full-spectrum fluorescent lamps there are also narrow-spectrum fluorescents that provide specific spectrums of light—such as upper red, blue, pink, and green. Narrow-spectrum fluorescents can be used to trigger specific plant growth and development phases. A fluorescent fixture that holds four to eight tubes can hold a combination of narrow-spectrum tubes with different color spectrums; these fixtures often offer multiple on and off switches to turn on the lamp or light spectrum you need for a specific phase of growth, such as leafy growth or fruit growth.
T5 lamp fixtures are relatively small and can be mounted in small spaces such as countertops and under kitchen cabinets. Small tubes and fixtures are ideal for growing small edibles, micro greens, and herbs.
Compact Fluorescent Lamps
A compact fluorescent lamp (CFL) is a twisted or folded fluorescent tube that somewhat resembles a traditional light bulb but it is a bit larger. A CFL can be used in a standard light fixture such as a table or floor lamp; a CFL does not require a special ballast like the ballasts used for fluorescent lamps.
CFLs are commonly full-spectrum lights and are ideal for small spaces. CFLs must be placed close to plants so that they don’t lose light volume. They are a good choice for growing a small number of leafy greens or foliage house plants rather than fruiting plants such as tomatoes and peppers.
High-Intensity Discharge (HID) Lamps
High-intensity discharge (HID) lamps yield higher levels of light than fluorescent lamps. They are the standard for amateurs, professional growers, and large-scale plant growers.
HID lamps are more efficient than fluorescent lamps and twice as efficient; they are also much more expensive. Because they yield higher levels of light and the cost of operation is lower, growers who constantly use artificial light to grow plants favor them. HIDs are a good choice for growing large groups of plants in large indoor spaces. They are a very good choice for growing heavy-flowering and fruiting plants.
HID lamps are available in specific spectrums such as red/orange (called MH bulbs) or blue/green (called HPS bulbs). This allows the grower to tweak the growing conditions.
The color of HID lamps can distort the appearance of plants as well as the growing space. HID lights can have an eerie yellow-orange glow making people and plants look jaundiced.
Light-Emitting Diode (LED) Lamps
A light-emitting diode (LED) lamp is a semiconductor that produces light when an electrical current passes through it. LED light is solid-state lighting that does not use electrical filaments like incandescent or gases like fluorescent. LEDs are lightweight, more efficient, and produce less heat than other choices. Some LEDs do not require fixtures; they are placed inside panels that only require plugging in.
LEDs convert more watts to light than other light sources and produce only about a quarter of the heat. Less heat lowers transpiration in plants which means less watering. Less heat also allows LEDs to be placed closer to plants without the risk of scorching plant tissue or damaging young seedlings.
LEDs are often used as primary lighting for intensive food production. They are a good choice to use during the shorter days of winter. Dual-band LEDs are a mix of red and blue light that is used for continuous growth through plant vegetative and flowering stages. A single-spectrum blue LED can be used to bulk up vegetative growth; a single spectrum red band LED can be added to induce flowering. Dual-band LEDS produce a light that looks pink to purple in color. There are also three and four-band LED fixtures that include multiple bands of light with the PAR spectrum. Multi-band LED fixtures are zoned so that the growers can turn on the color or mix of colors depending on the plant’s stage of growth.
Small LED lamps are affordable. Some large LED units can cost as much or more than HID lighting.
How Many Lamps Do You Need
The amount of growing space a lamp will light can be ballparked using the lamp’s listed wattage.
- A 1000-watt lamp lights 36- to 40-square feet.
- A 600-watt lamp lights 20- to 36-square feet.
- A 400-watt lamp lights 12- to 16-square feet.
- A 250-watt lamp lights 6- to 9-square feet.
- A 150-watt lamp lights 3 square feet.
Many growers recommend 40 watts per square foot to properly light and grow tomatoes. If you are growing in a concentrated space with a reflective wall covering, you may find that a lower wattage is sufficient.
Light Placement
Lights should be placed close to plants. The further the light is from a plant the less efficient the light and the lower the plant’s growth or yield.
Seedlings should be placed within a few inches of lamps. As plants grow, the lamp should be raised. Lights on chains or ratcheted cords can be adjusted as needed.
Reflector hoods can increase the efficiency of a lighting setup. Reflective material or wall coverings such as Mylar can help bounce light around all surfaces of plants without increasing the number of lamps.
Light and Dark
Plants need a dark to rest from a day of growing. Sixteen hours of light is recommended each day. That means plants can rest in the dark for 8 hours as well. Plants that don’t get rest will be stressed. Turn the lights on in the morning and off at night. Use a timer, if you can’t be there to turn the light on and off.
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