Famous last words heard by houseplants:

“Bright indirect light?” This spot seems bright enough.

Your interior light levels form the growth potential of the plant. Your care efforts realize that potential (watering, fertilizing, repotting). Good light is the PREREQUISITE for a plant to grow but the term “bright indirect light” fails to convey anything concrete.

The worst part is the term “bright indirect light” makes you think just any place the sun doesn’t shine is considered indirect light. And our eyes adjust to a wide range of light levels so you will NOT feel the difference.

Instead, those with huge, unobstructed windows and/or skylights are patting themselves on the back at how good they are with houseplants while those with smaller windows living between buildings are struggling to figure out why their fiddle leaf fig always ends up with 90% of their foliage lost.

After measuring light, I realized there’s no such thing as a ‘greenthumb’ – just people with large, unobstructed windows.

Step 1: Get a light meter and get to know how bright your indirect light actually is.

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

The LTH Meter

Light | Temperature | Humidity

* BUY NOW *

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!

This is the Dr. Meter LX1330B Light Meter that I used before. It’s not as compact as the LTH Meter, has limited measuring ranges (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 levels of indirect light 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.

Commercial Light Levels: most of our typical “houseplants” are grown in greenhouses with varying layers of shade cloth.  To give you a rough idea, “50% Shade” would measure to 5000 foot-candles (FC) and “90% Shade” comes to 1000 FC – this is the strength of the sun shining through different layers of shade cloth, which is a black net-like material.  These numbers are easily searchable on the internet – I’ve included the source links where applicable. ***Don’t expect to achieve these light levels indoors from your indirect light.***

Interior Light Levels: plants can technically survive in a wide range of light levels so do not take the numbers listed here as prescriptive – they aren’t strict requirements. You should think of them as guidelines for good growth. Another consideration is that “good growth” is subjective as any plant will take the shape of its light situation – up to a certain point, it’s not entirely under your control!  These numbers are gathered from my own observations and measurements. ***Use these as guidelines.***

How to measure natural light:

• Sensor placement: top of the plant
• Sensor orientation: should be pointed straight up but if you find that you have to turn the sensor to get a higher reading, it just means over time, the plant will tilt towards that direction
• WHEN to measure: the purpose of the light meter is to measure the strength of INDIRECT light so you want to get a sense of the average strength of this light while the sun is NOT in view (and also not too close to sun rise or sun set – when the overall sky is darker)
• DIRECT SUN: also take note of the total duration the sun shines directly on the plant

Analysis for natural light:

• If the average indirect light is above “Good for Growth”, then expect good growth
• If the average indirect light is at or below “Minimum for Maintenance”, then don’t expect strong growth
• If the duration of direct sun is above the “Tolerance of Direct Sun”, then block it with a white sheer curtain
• If the duration of direct sun is at or below the “Tolerance of Direct Sun”, then there’s no need to block the plant (just be sure to keep up with watering)

For white LED grow lights:

• For good growth: set the distance of the grow light such that the reading at the top of the plant is at or above “Good for Growth” and keep the light on for 12-16 hours a day
• For maintaining over winter: set the distance of the grow light to get a reading around the “Minimum for Maintenance” and keep the light on for 8-12 hours a day

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 either natural light or 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?

Find it here:

Get your LTH Meter HERE