Choosing The Right Lights To Grow Plants Indoors

Using Lights to Grow Plants Indoors

In order to grow plants indoors efforts must be made to replicate a plant’s natural environment as closely as possible. However, growing indoors in a controlled environment also affords opportunities to manipulate the environment to maximize plant growth and yield, even beyond what is achievable in an outdoor garden. Plants have widely varied light requirements for optimal growth; some require full intensity and full spectrum sunlight, while other plants may grow best under low light conditions and/or with a narrow color spectrum. Not surprisingly, there is a dizzying array of grow light options on the market, but choosing the right light allows you to fine tune intensity and spectrum in order to optimize photosynthesis in different situations. The two main types of grow lights used are Fluorescent, and High Intensity Discharge (HID) lights such as Metal Halide (MH) and High Pressure Sodium (HPS). There are other forms of indoor grow lights such as LED and Plasma, however those are still in the developmental stages and are just beginning to gain notoriety.

In the spring and summer, the skies reflect more of the blue spectrum and it is during this period of long days that plants are in their vegetative state, producing leaves and stems and growing to maturity. As the year progresses, the spectrum of the sky begins to shift to the red during late summer and fall as the days get shorter.

The spectrum used for indoor plant growth ranges from blue to red and is intended to mimic the spectrum produced by the sun throughout the year. In the spring and summer, the skies reflect more of the blue spectrum and it is during this period of long days that plants are in their vegetative state, producing leaves and stems and growing to maturity. As the year progresses, the spectrum of the sky begins to shift to the red during late summer and fall as the days get shorter. This is when the plants begin their reproductive phase in preparation for the intolerable winter months when only the seed of their offspring can survive and begin the cycle anew the following spring. These seasonal influences have defined the life cycles of plants for eons. The color shift is measured in Kelvins (K), a unit of measurement of the light spectrum. The blue spectrum is in the range of 6000K to 7000K and the red is in the range of 3000K to 4000K, approximately. Grow lights of various types are engineered to emit light within ranges optimal for photosynthesis, based on spectral analysis studies done to measure photosynthesis at different spectra. Metal Halide bulbs emit a blue spectrum of light, High Pressure Sodium bulbs emit a red spectrum. Fluorescent lights can be made to produce either a blue or red spectrum. LED and Plasma have the potential to produce a particularly specific or more full spectrum encompassing a broad range of the visible light spectrum. Each of these types of lighting have a variety of benefits and drawbacks.

High Intensity Discharge Lighting: Metal Halide Blues & High Pressure Sodium Reds

Metal Halide (MH) and High Pressure Sodium (HPS) lights are considered High Intensity Discharge (HID) lights because the lumen output is very high or bright. The gases used in these bulbs burn very brightly and produce a fair amount of heat. Because of the high intensity, the light is able to penetrate more deeply into the canopy and at a relatively great distance from the light source. Metal Halide (MH) lights are commonly used for vegetative growth due to their blue spectrum while High Pressure Sodium (HPS) lights are typically used for flowering due to their red spectrum which encourages flowering. Due to the high heat output that accompanies the bright light, your lights must be kept far enough away from the plants to avoid burning the leaves from excessive temperature. ”Hot spot light diffusers” can be used to reduce hotspots immediately below a bulb, but are not always necessary. Because the light can travel further, HID lights can be used for larger plants where even the lower leaves can use the light for photosynthesis. The amount of electricity needed for these lights can vary from 150 watts up to 1000 watts and require a ballast to regulate the flow of energy to the bulb. Reflectors help disperse the light and direct it toward the plants, most typically grown below them. Some reflectors are also enclosed and outfitted with ducting in order to easily exhaust the heat produced by the bulbs. This is beneficial for an indoor garden as excessive heat build-up can reduce yields and even damage plants.

Fluorescent Grow Lights

Fluorescent lighting produces light and heat at much lower intensity than HID lighting. This low intensity means the light doesn’t penetrate the canopy very deeply, and can’t travel very far from the source before it is too weak for plants to use in photosynthesis. Standard shop/office fluorescent tubes come in warm (Red) or cool (blue), but are not used for growing indoors because they are too weak to produce significant growth. New T5 fluorescent lights are designed to be twice as bright as standard fluorescent shop lights and are available in both Blue (6500K) and Red (3000K) spectra. Fluorescents are desirable to grow plants indoors because they:

  • provide a narrow spectrum and Red or Blue bulbs that are interchangeable in the same light fixture
  • can be used in combination to provide both Red and Blue bulbs in the same light fixture
  • produce very little heat
  • small plants respond well to the spectrum and intensity
  • can be positioned almost touching the plants without burning the leaves

Use of fluorescents is typically limited to smaller plants because of the limited light penetration characteristic. It is common practice to use T5 lighting with Blue 6500K bulbs for early vegetative growth and smaller plants, until they grow too large or begin the flowering cycle. At that point, the fluorescent lighting is usually replaced with HID lighting, and most often, HPS lights with Red 3000K spectrum which encourages blooming.