Articles

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  Grow Light Strength Recommendations by plant

  Reference for grow light strengths by plant type

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  Schlumbergera Blooming: Most Accurate Tips

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  Review: Soltech Grove LED Bar Light

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  Is a grow light considered direct or indirect light?

  If you want to develop strong fundamentals in plant care, my book and online course will guide you in the right direction.

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  Review: A-Pot by Dowan Ceramics

  I’m excited to partner with Dowan Ceramics to introduce and test their A-Pot self-watering houseplant pot! This is a paid partnership between Dowan Ceramics and House Plant Journal I was immediately drawn to the design of the A-Pot because of its stylish combination of glass and terracotta.  While I’m generally on top of checking up […]

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  Grow Light Review: Soltech Solutions Vita Grow Light

  Disclaimer: this is a paid partnership between Soltech Solutions and House Plant Journal.  The products were given to House Plant Journal to provide an honest review. Plant parents of today should rejoice from two converging trends: (1) the fact that LED technology has come a long way – cheaper and highly effective; (2) light requirements […]

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  Aerial roots: can I cut them?

  If your growing conditions are good (adequate light) and you’re watering/fertilizing accordingly, then everything keeps growing – including aerial roots after they’re cut: A small Monstera deliciosa will eventually grow many aerial roots, creating a crazy jungle look – here’s the view from below my monstera: I personally like this look but if you’re looking […]

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  Misting: What is it actually useful for?

  In my book, I stated that misting is about as useful to raise humidity as it is to light up a match to warm up a room: the effect on ambient humidity is minimal and temporary. And I don’t use a humidifier anywhere – my ambient indoor humidity in the winter goes down to 30% […]

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Artificial lighting technology has come a long way in the last few years – most notably: white LED lighting is strong enough to grow any houseplant – agricultural plants need more specialized lighting but we’re not covering that here.

Using NATURAL LIGHT? USE THIS TABLE

The Parameters You Need

Most houseplant advice about using grow lights is missing the necessary parameters you need to know if you’re actually giving your plants enough light.

If you’re following a baking recipe, the parameters you need to know are (1) the oven temperature and (2) the duration. Similarly when using grow lights, you need to know (1) the MEASURED strength at the leaf and (2) the duration.

Step 1: Measure light at the leaf – for starters, you must have a light meter to be sure if you are using the grow light effectively because the strength changes drastically within just a few inches from the light. Imagine you tried a baking recipe but you had no idea what temperature your oven was set at – you just turn it on and hope it will work? Don’t guess with light – it’s the foundational factor for plant growth!

A light meter is so critical to understanding light that I designed and manufactured my very own:

The LTH Meter

Light | Temperature | Humidity

I designed and manufactured the LTH Meter with houseplant owners in mind. When you know your light situation, you will know where to put plants (and where NOT to put plants) – and you can start using grow lights properly!

Disclaimer: this article contains Amazon Affiliate links. Earnings from qualifying sales goes to support the work of House Plant Journal – thank you!

The Dr. Meter LX1330B Light Meter that I used before is not as compact as the LTH Meter, has limited measuring ranges (you need to manually select the range), and does not measure temperature and humidity. The pricing has been varying wildly on Amazon but it should be around $40-60 [US Link] [CAN Link]. An alternate model: [US Link] [CAN Link]

What about apps? Android devices do not have standardized ambient light sensor hardware and the iOS platform doesn’t give access to the iPhone light sensor, which means those apps are doing a rough calculation based on the camera brightness value. An app might be able to give you a rough idea, but a dedicated device will do the proper cosine correction for the angle of incident light (that’s what the white dome is for).

Step 2: Bookmark this page so you can look up the strength of light x duration necessary for various plants. I don’t have every possible houseplant but after reading a few of these, I think you’ll get the idea.

“Just a starting point…” – but unlike a baking recipe, where precision is necessary, light requirements vary a lot even for the same plant so it makes more sense to provide minimum strength recommendations from which you’re free to go higher to experiment and see how well your plants respond. Realistically, since all of these plants can handle a short duration of direct sun (which measures around 10,000 FC or 2000 µmol/s/m²) any white LED grow light you will buy won’t come close to this strength. So, for example, if the minimum strength recommended for pothos is 100 FC x 12 h, it will probably grow VERY WELL with 800 FC x 12h.

“Maintenance” vs “Growth”

This is not merely about “survive vs thrive” – there are two common scenarios where you would want to use grow lights and the numbers in these tables.

Use the “Maintenance” strength if you are keeping the plant alive for the winter while it waits for warmer months to spend outdoors (where the light will be much stronger).

Use the “Growth” strength if the grow light is the only permanent light the plant will receive (example: inside an Ikea greenhouse cabinet).

Daily Light Integral – DLI

The total amount of light received over a 24 hour period is called the Daily Light Integral (DLI). A simple analogy is how total distance traveled = speed x time. With respect to light, DLI = strength of light x time.

The strength of light is in µmol/s/m² (which is PPFD; which is roughly 0.2 x FC) and time is whatever the duration is within the day (24 hours).

This calculation is easy with a grow light because the PPFD is constant and the duration is whatever we set it to on a daily basis. So going back to the pothos example above, here’s how we calculate the DLI achieved with 100 FC x 12 hours:

1. PPFD = 100 FC x 0.2 = 20 µmol/s/m²
2. PPFD x time = 20 µmol/s/m² x 12 hours = 20 x 12 x 60 x 60 = 864,000 µmol/m²
3. DLI (convert µmol to mol) = 864,000 / 1,000,000 = 0.86 mol/day — or about 0.9 mol/day

So a pothos getting 100 FC x 12 hours a day receives about 0.9 mols of PAR photons per day.

Realistically, you don’t have to calculate the DLI – as long as you have both parameters: light strength and time, you can use these tables and make reasonable observations of how your plants grow.

Note on FC (foot-candle) vs PPFD (PAR Photon Flux Density)

After using a spectrometer, I realized that illuminance (FC and lux) and PPFD correlate nicely for natural light and white LED light (approximate PPFD = 0.2 x FC – for those two light sources). So as long as you’re using either of those light sources, you can use whatever light meter you have (traditional foot-candle/lux or PAR meter) and refer to the chart.

Discrepancies between FC and PPFD only arise when using these grow lights:

• Red/Blue (“blurple”) LED grow lights
• Ceramic metal halide
• High pressure sodium
• Fluorescent

So you can expect good growth results even if measuring with a traditional foot-candle meter as long as you’re using white LED lights.

Note: the full unit of PPFD is “µmol/s/m²” but, for this chart, it is shortened to “µmol”

Looking for a sensible and logical approach to houseplant care?

Learn here:

Get your LTH Meter HERE