Greenhouse & Indoor GardeningLighting

Understanding the 4 Types of Grow Lights

Of all the hardware used in an indoor garden or greenhouse few compare in importance to horticultural lighting. For indoor gardens, horticultural lighting is the sole energy source for the plants. In greenhouses, artificial lighting supplements the sunlight and gives the plants enough light energy to perform up to the grower’s standards. Whether a gardener is looking for a primary or supplementary lighting source, he or she should take the time to examine the lighting technologies available for horticulture. When making a choice, gardeners need to take into consideration the space they wish to illuminate and the cost of operation and maintenance. Currently there are four technologies used by horticulturists: high intensity discharge (HID), light emitting diodes (LEDs), fluorescents, and induction lighting.

High Intensity Discharge (HID)

High intensity discharge lighting includes both metal halide (MH) and high pressure sodium (HPS). HID lighting consists of three components: the ballast, the reflector or socket, and the bulb. Ballasts can be specific to the type of bulb (MH only or HPS only) or they can be switchable, or “smart” ballasts, which can operate either type of bulb. Most ballasts are specific to wattage; although some of the newer, digital ballasts can operate different wattage bulbs. The ballast connects to the power supply and acts as a power converter so the bulb receives the proper current. The reflector, or socket, is the component that connects the ballast to the bulb and the bulb itself is the component that actually emits the light. After receiving the proper current from the ballast, the bulb converts that energy into light.

HID lighting is still the most popular choice for horticulturists for a few reasons. First, this technology has been around for a long time and is used in other industries. This means there are many manufacturers which helps lower the cost. Initial cost is not the only reason horticulturists continue to choose HID lighting. High wattage HID fixtures are very powerful and have the capability of penetrating a plant canopy along with covering a large area. For example, an indoor horticulturist who is using a 1000-watt HID light can adequately cover 25-50 square feet of garden space. Greenhouse growers looking for supplemental lighting can cover even more space with that same 1000-watt fixture. It is not uncommon for greenhouse gardeners to have a 1000-watt HID cover 100+ square feet for supplementary lighting purposes. Long story short, HID lighting has a low initial cost for a relatively large coverage area. This is the main reason HID lighting has remained so popular among horticulturists.

LED Grow Light
Light Emitting Diodes (LEDs)

Light emitting diodes, or LEDs, have been steadily gaining acceptance since their introduction within the horticultural community over fifteen years ago. Unlike other lighting technologies used in horticulture, LEDs are a solid state device which makes them extremely durable. LED fixtures usually consist of the panel of LEDs and a circuit board (generally housed within the lighting fixture). Many horticultural LEDs contain a heat sink and/or fan to help dissipate heat and increase the fixture’s life span. The biggest advantages of LED technology are longevity and the ability to customize the light spectrum. Plants have a heightened response to particular light wavelengths and LED technology has the ability to provide higher amounts of the particular wavelengths plants desire.

LEDs are the only technology that has the potential to manipulate the ratio of these wavelengths into the perfect ratio for photosynthesis. The first few generations of horticultural LEDs were somewhat of a disappointment because they lacked the intensity to compete with HID lighting fixtures. Many of the original LED fixtures contained only two wavelengths of light (red and blue) and utilized low wattage diodes. LEDs have made great strides since the first generation and today’s LEDs have the ability to compete with HID lighting in many ways. Many of the new LED fixtures contain more advanced wavelength ratios along with higher wattage diodes which allow for a greater penetration into the plant canopy. When comparing this technology to HID lighting, coverage is still somewhat of an issue. LED fixtures, unless they contain a special lens, are generally light-directional and the light will not spread out and cover a large area like a HID light. However, LEDs’ higher energy efficiency combined with a lower heat signature has many indoor horticulturists, at the very least, giving LEDs serious consideration. There are also many greenhouse gardeners who have had success using LEDs for supplemental lighting.


Fluorescent lighting has been used in horticulture for many years. Thanks to some of the newer fluorescents that are much more efficient than the shop lights from your dad’s basement, fluorescents still have a solid place in horticulture. High efficiency fluorescents, like the T5 style, are still very popular among indoor horticulturists for vegetative lighting or for seedlings and clones. Fluorescent tubes, like the T5, automatically disperse light evenly throughout the entire length of the bulb. This is ideal for keeping young plants even in growth. Young plants grown under HID lighting (or any light source that is emitted from a single focal point) need to be continuously rotated to remain uniform.

Unfortunately, as with the early generations of horticultural LEDs, fluorescents generally lack the punch needed to penetrate deep into a plant canopy for fruiting or flowering plants. Although some gardeners have had success fruiting under fluorescent lighting, HID lighting is a far more popular choice for fruiting plants. The light coverage of fluorescents is also limited to pretty much directly under the fixture which makes them less desirable for larger areas. Fluorescents can be a solid choice for supplementary lighting purposes in a greenhouse, especially when the grower is trying to limit heat in the greenhouse. When raised high enough above the plants, fluorescents can give adequate coverage in a greenhouse for supplementary purposes. However, if the garden requires more than four hours per day of supplementary lighting (like in a year round greenhouse) HID lighting may be a better fit.

Induction Lighting

Induction lighting is a technology that has more recently been making headway in the indoor gardening industry. Induction lighting is unique in that it utilizes a sealed bulb design. This differs from HID and standard fluorescents because these technologies rely on electrodes which bring electricity into the bulb. Induction lighting utilizes microwave or radio frequencies to pass through the sealed bulb and excite the metals and gases sealed within. There are two types of induction lighting currently used in horticulture: induction fluorescents and sulfur plasma.

Induction Fluorescents

Induction fluorescents have all of the advantages of standard fluorescents but will not degrade as quickly. This allows them to retain a high CRI (color rendition index) and high amount of PAR (photosynthetically active radiation) for a long time. What this equates to is more uniform growth in the garden and much less maintenance. The initial cost of these fixtures is high but the gardener will not have to pay for bulb replacement throughout the entire life span of the fixture (usually 10-15 years). Like the T5 fluorescents, induction fluorescents are a great choice for vegetative growth or clones and seedlings. Induction fluorescents also have a coverage limited to almost directly under the light source. To combat this problem, some indoor horticulturists will place multiple fixtures close together to get adequate coverage.

Sulfur Plasma

Sulfur plasma is an induction technology that emits all its light from a small quartz sphere. The most promising thing about sulfur plasma is that it has the intensity of HID lighting with a more suitable light spectrum for plant growth. In fact, sulfur plasma lighting has the closest spectral light output to that of the sun than any other artificial light source. As with HID lighting, coverage and plant canopy penetration are not issues for sulfur plasma. Sulfur plasma is the newest lighting technology to be introduced to the horticultural industry and is currently very expensive. As more manufacturers produce sulfur plasma lighting, there will be a reduction in cost and, hopefully, a continued increase in efficiency. Sulfur plasma lighting could be the ideal lighting source for both indoor gardens and greenhouses in the future. Another advantage of sulfur plasma is longevity. As with induction fluorescents and LEDs, sulfur plasma lighting can last up to 10 years and will lose very little PAR and CRI over the course of the fixture’s life span.

When purchasing a horticultural lighting system and facing all the choices, gardeners have their work cut out for them. As the old saying goes “there is more than one way to skin a cat”. This could not be more true when it comes to horticultural lighting. All of the previously discussed technologies work well to grow plants. Some are more efficient with a higher initial cost while others are inexpensive and lose more energy to heat. Personal preference, an individual’s budget, and the specific use for the fixture are all things that a grower must weigh heavily before making their decision about a lighting system. Some gardeners want to experiment with the latest and greatest in horticultural lighting technologies. Some gardeners want a reliable lighting system that will not break their budgets. In the end, it is ultimately up to the gardener which lighting fixture is the best fit for his or her garden.

Eric Hopper resides in Michigan’s beautiful Upper Peninsula where he enjoys gardening and pursuing sustainability.

Curious About CBD? Here’s What it Can and Cannot Do for You

How and When an Indoor Gardener Should Change Grow Light Bulbs

How LED Grow Light Patterns Work

Subscribe to our Free Email Newsletter

Comment here