Greenhouse & Indoor PlantsLighting

How and When an Indoor Gardener Should Change Grow Light Bulbs

Light energy is the most important controllable factor in every indoor garden. For indoor gardens, the light energy is the driving force behind all plant growth; therefore, it is of the utmost importance for a gardener to maximize the efficiency of that light energy. The inverse square law states that light energy dissipates exponentially. This means the farther the plants are from the light source, the less energy that is available. Horticulturists implement light reflectors, various cooling techniques, and specialized configurations within the garden to ensure the plants are receiving as much light energy as possible. In fact, serious indoor horticulturists focus on always maximizing light energy throughout all stages of growth within their gardens. One simple way to help maximize light energy is to better understand the life expectancy of a given lighting technology. There are different types of horticultural lighting, and each has its own guidelines for maintenance and replacement. A closer look at the life expectancies of the most popular lighting technologies used by indoor horticulturists can help gardeners determine when to replace bulbs or lighting fixtures and keep their light energy levels as high as possible.

Light Bulb Longevity for Horticultural Applications

One of the most confusing aspects facing new indoor horticulturists and greenhouse hobbyists is how long light bulbs should be used for horticultural purposes. This is particularly confusing because the longevity of a lighting system depends on its application. For example, a HID light used in a streetlight will do its job effectively much longer than the same HID that is used to grow plants. As a light bulb is used over time, the light energy slowly decreases. This is due to the way the light’s internal components break down. This happens with all lighting technologies; however, some light technologies degrade at a faster rate than others. As the light output diminishes, it provides less photosynthetically active radiation (PAR) which is the particular wavelengths used by plants for photosynthesis. As the light source’s PAR output diminishes so does its ability to effectively provide energy for plant growth. With most lighting technologies, the PAR output will decrease to inadequate levels far before the light bulb burns out. Therefore it is so important for a horticulturist to understand the longevity of his or her particular light technology as it relates to plant growth.

Lighting Longevity for Indoor Gardens vs Greenhouses

As previously stated, the longevity of a horticultural lighting source will depend greatly on its intended application. This also holds true when comparing horticultural lighting fixtures used in indoor gardens and greenhouses. Within an indoor garden, the artificial light fixture is the sole energy source providing light to the plants. In a greenhouse, artificial lighting is generally used as supplemental lighting and the sun is the actual primary light source. When a horticultural light fixture is the primary light source for plant growth (as it is in an indoor garden), it is more important to always maintain a high level of PAR. What this means, in a nutshell, is that bulb changes for an indoor garden will be required more often than in a greenhouse.

High Intensity Discharge (HID)

High intensity discharge lighting includes both metal halide (MH) and high-pressure sodium (HPS). HID lighting is still the most popular choice for horticulturists because HID lighting has a low initial cost for a relatively large coverage area. Unfortunately, the low cost of HID lighting systems is directly related to the relatively short life of the bulbs used in these systems. Of all the horticultural lighting systems, HID light systems will require the most frequent bulb changes. Although a manufacturer will give its recommendation for bulb replacement, it is not always taken into consideration that the bulbs will be used for horticultural purposes. In other words, the recommended longevity of HID bulbs is usually based on the bulb being used for a standard lighting application, such as lighting a warehouse. If a HID system is the primary light source in an indoor garden, a good rule of thumb is to replace the bulb annually. This is assuming the bulb will be operating for roughly 12 hours every day. Although an HID bulb will continue to emit light long after one year’s time, the PAR output will have diminished significantly, and the bulb will need replacement to maintain a high level of growth in the garden. Greenhouse growers using HIDs for supplemental lighting can plan on replacing bulbs once every two to three years (sometimes even longer) depending on the number of light hours it is used per day.

There has also been a recent rise in the popularity of DE (double ended) lighting systems. DE lighting systems utilize HPS bulbs which are connected at both ends as opposed to a single mogul base attachment. The two largest advantages of this technology are increased light output and increased longevity. The reduction of harmonic distortion results in less physical wear on the internal components which creates an overall increase in lamp longevity. In other words, DE bulbs will not degrade in PAR as quickly as a bulb in a standard HID fixture. Gardeners who use DE bulbs as a primary light source should plan on bulb replacement every one and a half to two years. Again, these bulbs will most likely emit light long beyond that point, but the reduction in PAR will make the system much less efficient. For supplemental lighting applications, DE bulbs can be used for four to five years before needing replacement.

How to Replace HID Bulbs

Unlike many of the other lighting technologies used for indoor horticulture, HID bulbs require special handling when replacing. HID bulbs should never be handled with bare hands. The oils from your skin can cause hot spots on the bulb. This may cause the bulb to eventually malfunction or burst. It is very important to always use rubber gloves or a protective fabric layer when handling HID bulbs. Disposal of used HID bulbs is another consideration. HID bulbs should not be thrown out with the regular garbage. HID bulbs need to be disposed of at a waste management center or recycling facility. It is a good idea to call your local waste management center and ask how they handle HID disposal.

Light Emitting Diodes (LEDs)

Unlike other lighting technologies used in horticulture, LEDs are a solid-state device which makes them extremely durable and practically immune to the degrading of light energy that is found in other technologies. 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. Most horticultural LED fixtures will not need replacement for ten or more years. At that point, the entire fixture will need to be replaced (growers do not generally replace individual LED bulbs). Horticulturists using LEDs as the primary light source can usually bank on ten years before a new fixture is needed. If the LED system is used for supplemental lighting in a greenhouse, the system can be used for 15 to 20 years. All in all, LEDs outperform most other lighting technologies when it comes to longevity.


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. Like HID lights, fluorescents have internal gases and components that break down and cause the light output to diminish over time. For primary lighting, T5 fluorescent bulbs should be replaced every one and a half years. For supplemental purposes, T5 fluorescent bulbs can be replaced once every three to five years.

How to Replace Fluorescent Bulbs

Fluorescent bulbs contain mercury and should be handled with care. Using two hands, the long fluorescent tube should be gripped at both ends to reduce potential twisting of the bulb during removal. Although fluorescents are not as affected by the oils in your skin as HID bulbs, it is still wise to wear gloves when replacing fluorescent bulbs. As with HIDs, fluorescent bulbs will need to be disposed of properly. Ask your local waste management center how to properly dispose of or recycle unwanted fluorescent bulbs.

Induction Fluorescents

Induction lighting does not use electrodes to bring electricity into the bulb like HID and standard fluorescent lighting systems. Instead, induction lighting utilizes microwave or radio frequencies to pass through the sealed bulb and excite the metals and gases sealed within. This allows induction fluorescent bulbs to retain a high CRI (color rendition index) and high amount of PAR (photosynthetically active radiation) for much longer than other technologies. For primary lighting, induction fluorescents will last for roughly 10-15 years. Much like a LED, once the life of the bulb is over, the entire fixture is usually replaced. For supplemental lighting applications in a greenhouse, induction fluorescents can go 15-25 years before needing replacement. 

Maintenance for all Horticultural Lighting Systems

Aside from bulb changes (where applicable), keeping the light fixture clean is an important procedure which all indoor horticulturists and greenhouse hobbyists should implement. Dust, dirt, and other debris can quickly accumulate on the surface of a light source and cause a significant reduction in available light energy. Horticulturists who wish to have the most consistent light levels as possible should routinely wipe down the light bulb, reflector, or lighting system to remove unwanted filth. Always be sure to let the lighting fixture cool down before wiping it clean. It is also important to use a dry, clean cloth when wiping down lighting systems so as not to leave any residue or other substance behind.

Keeping light levels consistent in an indoor garden is imperative for continuing dependable yields. Light energy is the direct input that influences plant growth above all other factors. The horticulturist who makes the most efficient use out of his or her lighting system will always be the most successful. Aside from reflecting and maintaining a proper distance above the plant canopy, the easiest way an indoor gardener can ensure his or her light levels are adequate is to replace bulbs when necessary and to keep the lighting system clean. Growers with multiple lighting systems should log data to keep track of their bulb replacement schedules. For an indoor horticulturist, maintaining a high level of PAR is the most influential factor to continuously producing bountiful yields harvest after harvest. Routine bulb changes and cleaning the lighting system ensure a high level of PAR in the garden and a high return on investment.

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

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