How much lux do plants need?
That’s probably one of the most frequently asked questions in indoor gardening.
But is lux a useful light measurement for plants?
Let’s discover the answer together.
In this article, we’ll talk about lux for plants and look at other more important light measurements in indoor gardening.
Let’s get started!
Lux for Plants: What Should You Know?
Lux is the measurement of lumens over a particular area.
So, technically, it’s the quantity of lumens that your plants receive from a light source in a specific period.
But lumen is the measurement of light that the human eye can perceive.
Plants don’t see light the way we do.
A lamp that has a very high lux rating can be useless for plants.
In other words, a light that dazzles your eyes might be too dim for your plants.
That said, you can still use lux to measure the light requirements of your plants, but the results won’t be very accurate.
Using Lux to Measure Light for Plants
The lux requirements of plants vary depending on the type of light.
For instance, the quantity of lux that plants need from sunlight differs from the one they need from an HPS lamp.
That has to do with the brightness, duration and spectrum of light.
The sun is brighter than an HPS lamp, but the maximum number of hours of sunlight that plants can get daily is around 8.
Whereas you can keep your grow lights on for around 14 to 20 hours per day.
For that reason, high-light full-sun plants need at least 60,000 lux from the sun.
But when the light is coming from an HPS lamp, they need at least 40,000 to 50,000 lux.
Here’s a list of the lux requirements of plants from different sources of light:
From sunlight (at least 6 hours a day):
- High-light or full-sun plants need at least 60,000 lux
- Medium-light or partial sun plants need at least 35,000 lux
- Low light or partial shade plants need at least 15,000 lux
- Very low-light plants or heavy shade plants need around 1000 to 5000 lux or less
From an HPS lamp (at least 14 hours a day):
- High-light or full-sun plants need at least 40,000 lux
- Medium-light or partial sun plants need at least 20,000 lux
- Low light or partial shade plants need at least 12,000 lux
From an MH lamp (at least 14 hours a day):
- High-light or full-sun plants need at least 35,000 lux
- Medium-light or partial sun plants need at least 17,500 lux
- Low light or partial shade plants need at least 10,000 lux
From a fluorescent grow light (at least 14 hours a day):
- High-light or full-sun plants need at least 37,500 lux
- Medium-light or partial sun plants need at least 19,000 lux
- Low light or partial shade plants need at least 11,000 lux
We didn’t mention LED grow lights in the above list because their light spectrums vary greatly.
As a result, we can’t establish one lux measurement for all LED lights.
How to Use the Above Lux List?
You can use the above list of lux requirements to find out if your plants are getting enough light during the day.
But you need a lux meter to find out the quantity of lux your plants are receiving.
Using a lux meter, measure the light intensity at the level of your plants’ leaves.
Remember to take several readings during the day.
Once you get the readings, compare them to the values found in the lux list.
If you find that your plants aren’t receiving enough light per day, you need to get them a supplemental grow light.
You can also use the above list of plants’ lux requirements to compare and select grow lights.
The problem here is that most grow light manufacturers don’t give the lux measurements of their devices.
Instead they provide PPF and PPFD measurements, which are much more useful than lux and lumens.
PPF and PPFD
PPF and PPFD are used to measure light that drives photosynthesis in plants which is known as photosynthetically active radiation (PAR).
PPF is the quantity of useful light (PAR) that a light source emits.
And PPFD is the measurement of PPF over a defined area and at a certain distance from the light source.
So, in some way, PPF is equivalent to lumen and PPFD is equivalent to lux.
But PPFD is a much more accurate measurement of light for plants than lux.
The PPFD requirements of plants are the same for all types of grow lights (but different for sunlight).
The PPFD of sunlight is between 900 and 1500 μmol/m²/s.
Most grow lamps have PPFD measurements lower than 900 μmol/m²/s.
For that reason, high-light plants need at maximum 8 hours of sunlight, but they need at least 12 hours of light from a grow lamp.
The lower the PPFD of a grow light, the longer you need to keep it on.
Here are the minimum PPFD requirements of plants (at least 14 hours a day of artificial light):
- High-light or full-sun plants need at least 400 to 600 μmol/m²/s
- Medium-light or partial sun plants need at least 250 μmol/m²/s
- Low-light or partial shade plants need at least 150 μmol/m²/s
You can use PPFD to choose the best grow lights for your plants.
Most grow light manufacturers give the PPFD measurements of their devices in addition to information about their coverage area.
Using that data, you can choose a grow lamp that suits your plants’ light needs and perfectly fits your growing area.
Lux, lumens, foot-candles, etc. are the most common light measurements, but they’re not the most accurate ones for plants.
Sometimes, you can still use these units to measure light for plants.
But in other instances, you need to use more precise light measurements.
Those are PPF and PPFD.
If you’re growing plants under lights, you need to make yourself familiar with PPFD and you need to forget about lux, lumens, and other inaccurate measurements.
Enjoy growing your plants and don’t forget to share your thoughts in the comments below.
Also, you can contact me if you have any questions.