Horticultural light bulbs designed for indoor gardening can have a tremendous amount of information listed on the product label, and not every bulb is created the same. There are multitudes of bulbs out there for every conceivable purpose. Understanding exactly what all the listed information means can allow the gardener to make a knowledgeable decision and purchase the bulb that is right for their plants.

What does HPS Stand for?

HPS stands for high-pressure sodium bulbs. These bulbs are primarily used during the bloom phase of plant growth, as the spectrum they emit is favorable to fruiting and flowering plants. Using HPS bulbs during the bloom phase will produce bigger, dense flower growth and an increase in overall yield.

What does MH Stand for?

MH stands for metal-halide bulbs. The spectrum emitted by MH bulbs is on the blue side, which keeps plant growth rich, squat and dense. MH bulbs are mainly used during the growth phase of plant development but are available in a neutral spectrum that can be used for all stages of growth. There are also MH bulbs suited solely for the bloom phase.

Don't Forget LEDs

LED stands for light-emitting diode lamps. These lamps are low wattage and can last as long as 10 years. Somewhat untested, they are likely the future of horticultural lamps.

Cool, Warm, and Neutral Lighting Temperatures

Many horticultural light bulbs are labeled as either cool, warm or neutral. This refers to the spectrum of the bulb and stage of plant growth it is best suited for. Cool refers to a spectrum that is more blue and can be accompanied by blue packaging. Cool bulbs are for the growth phase.

Warm refers to bulbs that emit light more on the red end of the spectrum and are often in red packaging. They are best used for the bloom phase of plant growth. Neutral refers to a more yellow spectrum that can be efficiently used for all stages of plant growth. Neutral bulbs are not as good as switching between cool and warm bulbs, but you do save money by only needing one bulb.

Watts

All bulbs list the wattage on the product packaging. Bulbs come in a multitude of different wattages. Some common wattages are 250 W, 400 W and 1,000 W. The wattage tells the consumer how powerful the bulb is, which will determine the size of the grow space it can effectively light. The wattage is also important because it tells us what ballast it matches up with. For instance, a 400-W MH bulb needs a 400-W MH ballast for it to fire, and a 600-W HPS bulb needs a 600-W HPS ballast to fire it. There are digital ballasts that can power both HPS and MH bulbs as long as the wattage matches the ballast.

Conversion Bulbs

Conversion bulbs are a neat innovation. They are bulbs that can be powered by the opposite ballast, which saves growers money because they only need to buy one ballast for both MH and HPS bulbs.

PAR Watts

PAR stands for photosynthetically active radiation. This has to do with the direct amount of light produced by the bulb that can actually be used and absorbed as energy by the plant. The PAR watts shows how efficient a grow bulb is. You can buy cheaper bulbs that were intended for industrial use, but the spectrum won't be designed for growing and the PAR watts will be low.

For the best results you want the highest available PAR watts for the bulb. Along with PAR watt listings there is often a visual description of the spectrum in the form of a colored graph. This will depict the spikes of light in various sections of the light spectrum. This may be accompanied by a colour rating in Kelvins. Cool bulbs will typically be around 6,500 K while warm bulbs lean towards 3,200 K.

Lumens and Foot Candles

Lumens and foot candles are units used to measure the exact level of light each bulb produces. On the packaging of any quality grow bulb will be listed the guaranteed starting lumens and foot candles. This amount will gradually decline over time relative to the amount of use the bulb gets. The spectrum will also change slightly over time. This is why I recommend replacing your bulbs every eight to 12 months to ensure optimal output and plant growth.

In Conclusion

There is so much information available on horticultural grow bulb packaging that it can be confusing at times. But this wealth of knowledge is meant to offer security to the gardener so they know exactly what each bulb can provide their plants.

Take the time to read the packaging carefully to make sure the bulb is right for you. And never be afraid to consult your local hydroponic retailer. They may have some great anecdotal advice that can lead you in the right direction when buying your next grow bulb.

Want more? Don't miss part one of this two-part series: A Beginner's Guide to Product Labels.