Nico’s Nuggets: How Many Lumens Do I Need for My House Plants?

Greetings to you, Nico!
I have searched the web and noticed that you are the guy to ask about indoor lighting for cannabis horticulture. My question is rather simple, I think: In terms of lumens, what light output do I need to grow a plant or two successfully in my home? I have a small bedroom and am thinking of using a 4’ x 4’ or 4’ x 8’ grow tent? Any suggestions on bulb types and strength would be helpful! Many thanks & good luck to you! — Jonathan V. via the mailbag at [email protected]

Thank you, Jonathan, for reading and writing in with an excellent question!

Deciding on light setups for any indoor garden can be a daunting task. The horticultural lighting industry almost feels like it is purposefully trying to confuse us sometimes, possibly in the hopes of getting consumers to purchase more items—or more expensive items—than we really need.

A good example of this is the fact that many bulbs are still marketed using “lumens” as a selling point. While lumens are a measurement of light quantity, it is not an appropriate term for horticultural purposes.

Lumens would be used to describe the strength of light if you were trying to light a stage or illuminate an object for viewing purposes. However, for photosynthetic purposes, the proper quantification of light uses PAR values, which stands for “photosynthetically active radiation.” This measurement of light provides information on both the quantity and quality of light being emitted by a source in relation to its effectiveness and efficiency for the plant’s photosynthetic processes. It takes into account not only strength, measured by photon count, but also spectrum, measured by color or wavelengths.

This 4′ x 8′ grow tent uses two 1,000-watt HPS bulbs, but must air cool the lamps to keep temps down. (Photo by Nico Escondido)

However, rather than go too deep into PAR, I will answer your question in the more basic form, as lumens do have a place in this discussion, since you specifically ask about lighting a specific size of space. Using a 4’ x 4’ grow tent as the example of the space we want to light, we must first understand how much light is optimal for plant growth and development. To do this, we examine the process of photosynthesis, where light energy (photons) is converted to plant energy (glucose or sugar).

Most plant leaves do their best conversion with sunlight between 3,000 and 4,000 lumens per square foot. Let’s assume, on average, that a 1,000-watt high-pressure sodium (HPS) bulb has an output of 100,000 lumens. At a leaf’s peak efficiency of 4,000 lumens per square foot, that leaves an optimal coverage area of 25 square feet—or a 5’ x 5’ footprint—for a 1,000-watt bulb.

Of course, bulb manufacturers and garden shop sales people will tell you a 1,000-watt bulb covers anywhere from 6’ x 6’ t0 8’ x 8’, but that is not based on photosynthetic processes but rather on physical light coverage (hence the use of lumens and not PAR).

Still, even with a more scientific view, it appears that a 1,000-watt HPS would be plenty sufficient to light a 4’ x4’ tent—or just 16 square feet. However, there is still another factor often not considered when choosing the correct lamp: heat byproduct.

A 1,000-watt bulb produces a staggering amount of heat for a garden, especially a small garden space like a tent. In fact, when you break down photosynthesis even further and analyze the process, we find that photosynthesis has ideal temperatures as well. To be exact, 68°F to 74°F is the “gold zone” when it comes to photosynthesis. At 84°F and above, plant photosynthesis begins to slow down significantly.

In a smaller home grow tent, the best light solution is a 400-watt HPS set-up. (Photo by Nico Escondido)

So while a 1,000-watt bulb may seem plenty sufficient for a 4’ x 4’, it will be too much light. In a 4’ x 4’ x 8’ tent with no exhaust, a 1,000-watt bulb has the power to raise the temperature nearly 25 degrees in no time. That means 68°F can climb to over 90°F! Even with ample exhaust, you’d be hard pressed to keep temps below 80°F.

The solution is simple: a 400-watt or 600-watt will serve you and your plants much better. A 400-watt HPS puts out approximately 50,000 lumens. With a 16-square foot footprint in a 4’ x 4’, that’s 3,125 lumens per sq. foot. A 600-watter is roughly 80,000 lumens, giving 5,000 lumens per sq. foot in the same tent.

So the smarter choice in a 4’ x 4’ is going to be the 400-watt HPS due mostly to the heat factor, which should never be underestimated. That said, if you’re cramming more than four plants in a 4’ x 4’—and you plan to have excellent exhaust and maybe even some cooler air being pulled into the tent—then you could consider the 600, but it would be risky. If you don’t mitigate the heat properly, the 600 might cost you more in quality than you would gain over the 400 in yield.

LEDs could pose an alternative in the future, but right now, LED companies like this one are going belly-up after coming to market with a $1,400 price tag. (Photo by Nico Escondido)

Thanks for reading everyone and remember: Grow… And help the world grow, too!

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Don’t miss the previous Nico’s Nuggets: Common Solutions for Wilting/Drooping Leaves