Seedlings under a fluorescent light. Photo: D. Pettinelli

The reasons for providing artificial light for your indoor plants can vary greatly depending on what you’d like to grow. Sometimes a light source is provided temporarily to help a plant get through the winter or to give seedlings a head start before spring plantings. Other times artificial light is added to create a permanent growing environment for ornamental houseplants or indoor crops such as microgreens and hydroponically grown vegetables. Lighting technology has come a long way in the past few years, enabling hobbyists and professionals to get even more out of their light source than ever before, often at an affordable price and reasonable rate of energy consumption. Listed below are the most common types of light used for indoor plantings along with some considerations for each type. Choosing grow lights is not a one size fits all scenario and selecting the right one can be a daunting task; use the information below to help make a more informed decision before making the investment.

One of the first things to consider is whether or not, a grow light is actually necessary to achieve the desired outcome. Being able to recognize the symptoms of insufficient lighting will help determine if adding artificial light is the right decision. Some of the symptoms that you  may begin to see when plants are not given enough light include: stretching, loss of lower or even all leaves, slowed growth, diminishing leaf size as the plant grows, and chlorosis or yellowing of the leaf surface. Seedlings that are straining to reach for more light will flop over as they grow overly tall. Plants that are not photosynthesizing will use little to no water, causing them to sit in wet media for too long, resulting in root health issues. Conversely, too much light can result in burning of the leaf and stem. This is something that typically occurs when a high-powered grow light is added to a space without first acclimating the plants. When a new light source is added, monitor for symptoms of burning and add light incrementally if possible, either by using a dimmer switch, keeping the light further away to start, or keeping it on for shorter intervals of time. All these techniques can help plants acclimate to increased light levels.

A fluorescent light fixture with a mixture of bulbs. Photo: C. Johnson

Fluorescent: Fluorescent tubes have been the standard for indoor plant lighting for several decades. They are versatile and have become more energy efficient with the invention of new bulbs and improved ballast designs. These lights are great for giving your seedlings a head start or growing microgreens year-round. Fluorescent light does not have the ability to penetrate a lush plant canopy like some of the higher-powered light sources, but it does provide an excellent source of gentle light for young plants. Fluorescent lights give off a relatively low amount of heat and have become more energy efficient as bulb technology has progressed. One of the more exciting inventions in the world of fluorescent lighting has been the CFL (Compact Fluorescent Light) bulb. These bulbs act similarly to the traditional tube style with one major difference; they can be screwed into the standard light bulb outlet used on most lamps and light fixtures. This means that a desk lamp could be turned into a small grow light with the use of a CFL bulb. Most of the standard size bulbs are of a lower wattage and would require multiple fixtures to provide the same amount of light as one fluorescent tube. Standard fluorescent bulbs and CFL’s both come in a variety of light spectrums, which are sometimes referred to as color temperature. It is important that you purchase bulbs that give off the proper spectrum for plant growth. This is key in purchasing any grow light.  Some growers will mix fluorescent bulbs of different spectrums to achieve the correct light spectrum. Bulbs that mimic the full spectrum of sunlight are available but tend to be more costly than mixing regular bulbs of varied spectrums.

Incandescent bulb in a protective housing. Photo: C. Johnson

Incandescent: The classic incandescent lightbulb is not commonly used as a grow light due to several reasons. One of the primary reasons is that they give off too much heat while producing a limited amount of PAR (photosynthetically active radiation) light waves. To get noticeable plant growth from this type of bulb, you would have to place it so close to the plants that they would be at risk of injury from the heat. These bulbs also tend to be rich in red spectrum light but poor in blue, which is what makes them not ideally suited for photosynthesis. In recent years, incandescent bulbs have become less common as they have been replaced by CFL bulbs. As described above, CFL bulbs make for much better grow lights and are much more energy efficient than their incandescent counterparts.

Purple light emitted from a mixed spectrum LED grow light. Photo: C. Johnson

LED: Newer to the houseplant lighting scene are LED lights. LED stands for light emitting diode. The number of diodes in an LED fixture can vary greatly depending on the size and wattage of the light. Some research is required when selecting an LED light that is both functional and safe. A quick search will reveal that there are many companies producing this style of light but not all bulbs are well built or designed; this can cause a lot of variation in the output of the light. Reputable sellers will provide a chart showing the amount of light that will reach plants at varying distances. The light amount is measured in a unit called PPFD (photosynthetic photon flux density). This is usually a better indicator than wattage and will provide a more accurate idea of how powerful the light is before it is purchased. Like other lights, LEDs come in a variety of spectrums; with warmer temperature lights to promote flowering and cooler temperature lights to promote vegetative growth. A blend of the two, resulting in a bluish/purple light spectrum is a popular and effective option.

HPS grow light in a production greenhouse. Photo: C. Johnson

HPS (High Pressure Sodium) and HID (High-intensity Discharge): These grow lights use gas filled bulbs that create a powerful amount of light. They have been the standard grow light in production greenhouses for many years; only recently have LEDs begun to takeover. While these lights are extremely effective at encouraging plant growth, there are a few factors that make them less practical for your home lighting needs. One factor is that these lights produce a lot of heat. This can be hard to manage in a home and may even become dangerous if not properly vented. Another is cost, both for operating the light as they are not very energy efficient and for replacement bulbs and hoods. The bulbs have a shorter lifespan than LEDs and the hoods wear out over time making the light less efficient. Cost aside, this is the most high-powered option when it comes to grow lights. The light that these bulbs put out is able to penetrate the canopy to a higher degree than other lighting options. In a production setting, where plants are being produced for profit, this light is still considered a practical choice.

Mechanical outlet timer. Photo: C. Johnson

How long to light: A helpful item to purchase alongside a grow light is an on/off timer. Trying to remember to turn your grow light on and off each day can become tedious and impractical. It can also result in an irregular day length for your plants; this can cause irregular growth patterns. Changes in day length, as well as temperature in some cases, triggers physiological responses in some plants. This is known as photoperiodism. Notable changes in plants that respond to variations in day length include fruit production in citrus, or color change in poinsettias. Keep this in mind when setting the light timer. Learn about the lifecycle of the plant being grown and how changes in day length will affect it when deciding how long to light its grow space. In general, it is recommended that one supplement any natural light with artificial light to create a 12-hour day for most plants. This will keep them actively growing throughout the short-day winter season.

UConn Home & Garden Education Center, 2021

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