Originally published in Maximum Yield (April 2005)
In nature, plants depend on the energy of the sun. Through a process called photosynthesis, sunlight is converted into sugars to provide fuel for the plant’s growth. These sugars are utilized as needed in a process called respiration, and excess sugar is also stored for later use.
Photosynthesis is made possible by chlorophyll, which is contained within the leaf cells. Chlorophyll gives vegetation its characteristic green color. Light is trapped by the chlorophyll, activating the process of photosynthesis.
Inside the chlorophyll, light energy is combined with carbon dioxide and water to produce oxygen and sugar. The sugar is then oxidized (or metabolized) through the process of respiration, producing carbon dioxide, water, and energy for growth. Excess oxygen and water are transpired by the leaf into the air.
Plant growth, therefore, is directly affected by the color, intensity and duration of the light the organism receives.
High Intensity Discharge (HID) Lighting
Nothing beats the Sun when it comes to growing, however, new types of High Intensity Discharge lighting have made growing indoors a viable alternative. Many of you are familiar with fluorescent “grow” lights designed to grow plants indoors. These products are fine for low-light plants where limited results are expected.
But what if you want to achieve the ultimate growth potential of your favorite plants indoors or, supplement sunlight in your greenhouse?
Your answer is to use High Intensity Discharge lighting, or HID for short. These lighting systems consist of a lamp, reflector and power supply and are designed to provide the maximum output of photosynthetically active radiation (PAR) for the amount of power consumed. HID lighting systems can illuminate your garden with the right quality and quantity of light to make for impressive results.
Horticultural HID lighting is used by the world’s premier growers to provide many benefits simply unattainable with conventional fluorescent and incandescent lamps. HID lighting allows commercial growers to increase crop yields, bring crops to market on schedule and produce crops when out of season, making them even more valuable to the consumer market.
HID lighting is so efficient and powerful that many indoor growers turn a healthy profit even after the initial investment and the monthly electric bills have been paid. Until recently, HID lighting for horticulture has been prohibitively expensive for everyday gardeners due to a limited market and the costs of production. But thanks to the ingenious new lighting products introduced over the past couple of years, lighting costs have been reduced to the point where everyone can enjoy their benefits.
Light intensity is commonly measured in power (watts) per square foot. For optimal photosynthesis to occur a general rule of thumb is 20-50 watts per square foot, with 20 being best for low-light plants and 50 best for light loving plants.
Maintain 250W HID lamps 12-14″ from plants, 400W lamps should be from 16-24″ and 1000W lamps a minimum of 24″ from plants unless your lamps are suspended by a circular or linear light mover in which case you may decrease the lamp to plant distance by 25-50%.
To increase light effectiveness, paint your growing area with a semi-flat white paint sometimes referred to as an eggshell finish. The minimal gloss in this type of paint will provide maximum diffusion while still allowing you to wipe clean any smudges or stains that may appear in time. Other wall treatments include; Mylar 90-95% reflective, Flat white paint 75-80% reflective, Gloss white paint 70-75% reflective, Yellow paint 65-70% reflective, Aluminum foil 60-65% reflective, Black <10% reflective.
Most plants grow best when exposed to 16-18 hrs of light per day. Additional hours of light during the day have not been found to increase growth by any significant amount. Plants that exhibit photoperiodism, the trait that causes day length to trigger flowering, should be exposed to 12-14 hours of light once flowering is desired. Total darkness is required during the darkness cycle for flowers and fruit to form correctly. Select a timer to control the duration of HID light. Some popular plants that are frequently grown indoors and exhibit photoperiodism are Chrysanthemums,
Poinsettias, Bromeliads, Pansies, Gibsofilia, Fuschi, Petunia, Gladiolia and Roses. These plants will flower when their photoperiod is 12hrs of light and 12hrs of darkness. Using indoor lights and a timer, you can force flower them during market peaks to increase yields and provide on-time delivery.
Color (Photosynthetic spectrum)
Photosynthesis is most pronounced in the red (600-680nm) and blue (380- 480nm) wavelengths of light. Horticultural lighting, also know as High Intensity Discharge (HID) lighting is designed to cover these specific wavelengths, known as the PAR spectrum (photosynthetically active radiation). There are two types of HID lamps which emit different color spectrums.
Metal Halide lamps emit a white/blue spectrum. MH lamps are best used as a primary light source (if no or little natural sunlight is available). This type of lamp promotes compact vegetative growth. There are also MH to HPS conversion bulbs available which allow you to provide MH light during vegetative growth and then switch over to the HPS for fruiting/flowering stages of growth.
High pressure sodium lamps emit a yellow/orange spectrum. They are the best lamps available for secondary or supplementary lighting (used in conjunction with natural sunlight). This type of light promotes flowering/ budding in plants. HPS lamps are ideal for greenhouses and commercial growing applications. The Son Agro and Hortilux HPS lamps add an additional 30% blue factor to their spectrum, making them a better choice than straight HPS lamps for solo use.
There are also HPS to MH conversion bulbs available which can provide MH light during vegetative growth then let you switch back to HPS for the fruiting/flowering stages of growth. White light is actually a combination of all colors of light. Red + Green + Blue (and all colors in between). Blue light stimulates hormones that trigger growth and inhibit dormancy. Blue light powers photosynthesis causing tips to grow towards the source (phototropism).
Metal Halide lamps emit strong levels of blue light making them good for promoting the growth of leafy plants. Blue light also serves to keep plant growth compact and shapely by minimizing the distance between internodes (branches). Green light is reflected, that is why plants appear green, however some green light is required for growth.
HID lamps do not emit much green light, neither do high pressure sodium lamps. Red light also powers photosynthesis, aids in seed germination, helps to form pigments and aid flowering. Red light is also responsible for triggering dormancy in some plants. High Pressure Sodium bulbs emit red light and are generally better for flowering and fruiting plants. Far-Red light speeds up some full sun plants, reverses some red light effects. HID lighting usually doesn’t emit far-red except in the case of some high and low pressure sodium bulbs, more so in the form of heat rather than photosynthetic light.
Choosing A Grow Light
In choosing an HID lighting system, red and blue are the two primary colors of light you’ll need to be concerned with.
Blue light is most pronounced during the spring and summer months when the sun is highest in the sky. It is responsible for keeping plant growth compact and shapely.
Red light, such as when the sun is lower in the sky during the fall harvest months, is responsible for triggering reproduction in plants in the form of flowers and fruits.
Metal Halide (MH) lamps emit primarily blue light making them ideal for the vegetative growth stage. High Pressure Sodium (HPS) lamps emit primarily red light which causes exaggerated flowering and fruiting during the plant reproductive stage.
Thus, if you plan to grow mostly leafy crops such as lettuce and vegetative herbs, your best bet is an MH lighting system. If you want to grow flowering plants, then invest in a Son Agro or Hortilux HPS since it adds about 30% more to the blue spectrum than does a standard HPS.
Remember, lights emit heat which needs to be vented to keep indoor gardens within 65-80 degrees and 50-75% humidity. The primary benefit to employing a High Intensity Discharge (HID) horticultural lighting system is the control it gives you over your plants’ growing environment. In many areas, once fall arrives the growing season is over, and if you’re a hard-core gardener like me, you’ll miss it dearly!
Horticultural lighting systems allow us all to extend the growing season by providing our favorite plants with the light spectrum and intensity nearly equivalent to the sun. This is a great advantage for those of us who appreciate having a year-round supply of fresh flowers, veggies and herbs!
HID lighting is also a great way to jump-start spring by starting your seedlings months ahead of last frost. Another great advantage of indoor horticultural lighting is your ability to control the length of daylight thus empowering you with the ability to force flower your favorite strain even when completely out of season. Vegetative growth photoperiods are from 16 to 18 hours/day. More than 18 hrs. is minimally advantageous and not worth the cost in electricity. Flowering photoperiods are usually between 10 and 14 hours per day. Remember, to grow perfect plants, the secret to the right light is Intensity, Duration and Color!