In Part 1, we talked about what type of light carnivorous plants need, but what about how much light they need? In the context of helping us make better decisions for our grow setups, this question is really three questions:

  1. How is light quantified for plants?
  2. How much light do different types of carnivorous plant need?
  3. Which bulb/fixture will produce enough light for my carnivorous plant?

Let’s start with the first question by laying out some definitions. Don’t worry about actual numbers yet, we’ll get to those. Just focus on the concepts for now.

 

Understanding light quantity

Photon – A photon is a single particle of light and therefore the smallest unit of measuring it (Source). Plants absorb photon energy to conduct photosynthesis. Since plants use photons rather than brightness (lumens and lux), our goal from here on out is to quantify all measurements of light in terms of photons. This will include conversions from lumens, lux, and watts.

Photons

Micromole (abbreviated μm or μMole) – Since photons are so small, listing an actual photon count for plant lighting would result in astronomical numbers. To help simplify things, we refer to them in micromoles of photons instead, or just micromoles (μm or μMole). 1 μMole consists of 602 QUADRILLION photons (Source).

Photosynthetic Photon Flux (abbreviated PPF or µMol/S) – PPF is the number of µMoles produced by a light source per second within the PAR range of 400-700 nanometers (Source). Think of this like the shower setting on a garden hose. If you squeeze the handle for 1 second, a certain number of water drops will spray out. The PPF is somewhat helpful to know, but nobody turns on a grow light for only 1 second and since light spreads out the farther it gets from the source (becomes dimmer), it doesn’t tell us anything about how many µMoles are actually hitting our plants. Hmm, sounds like we still need more information.

Note: Sometimes light manufacturers will provide the PPF of a grow light but there can be some inconsistencies of how it is measured.

Photosynthetic Photon Flux

Photosynthetic Photon Flux Density (abbreviated PPFD or µMol/M2 S) – PPFD is the number of µMoles produced by a light source per second over a square meter within the PAR range of 400-700 nanometers (Source). This is the same as PPF but now we’re adding in an area over which the PPF is spread out, or the density of the photons in a given area. This is the idea behind the Inverse Square Law.  Again, think of this like the shower setting on a garden hose only now you’ve marked out a square meter on the floor. After squeezing for one second, how many drops of water fall within the square meter?

Photosynthetic Photon Flux Density (PPFD)

Knowing the PPFD is very helpful because not only does it tell us how many photons are hitting a given area, but it also means there’s a distance associated with it. Now we’re getting somewhere! Still though, nobody turns on a grow light for only one second and that’s where Moles/Day comes in.

Moles/DayThe number of µMol/M2 S (PPFD) produced over a 24 hour period. It is the daily accumulation of photons over a square meter. Time to pull out the garden hose again. Now, instead of squeezing the handle for one second, keep squeezing it for several hours over a square meter bucket (better have good drainage because that’s a lot of water). The longer you squeeze, the more water drops will collect in the bucket. It could also be said the higher the water pressure (the stronger the light), the faster the bucket will fill up.

Quite a few µMoles can be produced in a 24 hour period, so to make it a more manageable number, we convert it to Moles/Day. There are 1 million µMoles in a Mole.

Knowing the Moles/Day a light produces is the ultimate goal because then we can compare it with the known needs of certain types of carnivorous plants. See where we’re going with this?

Moles/Day (a.k.a. the ball pit)

Moles/Day (a.k.a. the ball pit)

 

The Moles/Day needed by carnivorous plants

To estimate the Moles/Day needed by different types of carnivorous plants, we referred to data in this document from Purdue University for plants with similar light requirements. The Moles/Day required by a plant is known as its Daily Light Integral (DLI). The updated table below provides an overall breakdown by genus but the numbers can vary somewhat from one species to the next.

Daily Light Integral (DLI) for Carnivorous Plants

Note: Toward the end of June on a clear day in the United States, the sun emits 40-60 Moles/Day depending on the exact location. These numbers fall in winter as the days become shorter.

Monthly Outdoor DLI ranges for the United States

Image Source: Measuring Daily Light Integral in a Greenhouse by Ariana P. Torres and Roberto G. Lopez

Note: In addition to Moles/Day, it is important for many plants to receive 14-16 hours of uninterrupted light during the active growing season. Plants fall into one of three categories; long-day, short-day, or day-neutral where the length of time in light and darkness, despite the Moles/Day, can determine whether a plant goes dormant or not (day-neutral plants are unaffected). Short-day and long-day plants that receive 10-12 hour of light respectively will begin to transition into dormancy. This is called Photoperiodism.

 

What about Lumens and Lux?

As with Kelvin and CRI, the problem with using lumens and lux to measure light is that they are based on human perception. Humans perceive light in the yellow/green part of the spectrum as brightest. However, light in the blue and red ranges appears much dimmer. This means that while a bulb with a high lumen or lux value may appear intense, it could actually have very little usable light for plants. It all depends on the Spectral Distribution Curve (SDC) of the bulb. Lumens and lux can provide some useful information though and are still widely used, so we’ll take a look at them here and how to convert them to PPF and PPFD later on.

Plant Response vs. Human Perception

Lumen (lm, a.k.a luminous flux, or just flux) – A measurement of the brightness of a light as perceived by the human eye in a given angle or beam. Lumens are also known as foot-candles when they are measured within a given area and distance. The brightness of a candle 1 foot away from the source within a 1 square foot area (1 foot-candle) is equal to 1 lumen (Source).

Lux (lx) – Lux is the same idea as foot-candles but uses the metric system instead. It is the brightness of a light as perceived by the human eye that falls on a 1 square meter object 1 meter away. 1 lumen spread over 1 square meter equals 1 lux. The same 1 lumen spread over 1 square foot (foot-candle) equals a little over 10 lux because the same amount of light is concentrated in a smaller area (Source). Again, this brings us back to the Inverse Square Law.

 

Lumens and Foot-candles vs. Lux

If you remember from earlier, PPFD also measures light in a square meters at a given distance. This makes lux which measures brightness our closest equivalent to PPFD which measures photons.

 

Now let’s start applying all this with some numbers

Let’s say we want to grow Venus Flytraps which have an optimal DLI of 22-34+ Moles/Day. First, we need to hunt down a potential grow light and then make some calculations based on the data provided by the manufacturer.

Great! Most of the work is already done for you. If the manufacturer provides the PPFD, it’s important they also provide three things to help interpret the number:

  1. The distance from the light the PPFD measurement was taken (sets of PPFD measurements at varying distances are better).
  2. PPFD measurements at various points across the illumination footprint to show average intensity, hotspots, and limitations.
  3. The PPFD measurements in context of a Spectral Distribution Curve. A light with a broad-spectrum curve will have a more accurate PPFD than a light with narrow ranges of color. You can read more about why this is the case here.

MARS HYDRO TS 600W LED Grow Light PPFD

PPFD footprint of a MARS HYDRO TS 600W LED Grow Light at a 12″ height with intensity readings

SANSI compact LED PPFD

PPFD measurements of a SANSI Full Spectrum LED Grow Light at given distances with intensity hotspots

After evaluating the PPFD value(s) and averaging if needed, all that needs calculated is how long the light must be on (Moles/Day) at a given distance to meet the DLI of our carnivorous plant. Here’s the equation and calculator to do that.

Even after buying a light, it’s a good idea to test the PPFD for your particular setup rather than relying solely on the manufacturer’s claim. A quantum PAR meter can help.

If the manufacturer only provides lumens, we’ll need to calculate the overall area the bulb or fixture will light up at a given height. Then, convert the lumens into lux and PPFD before finally calculating the Moles/Day. To do this we need to know a few things:

  • The brightness of the light at its source (total lumens).
  • The type of light such as an incandescent or fluorescent bulb.
  • The angle the light is focused from the fixture (usually achieved with some type of reflector).
  • Our potential or preferred light height.
  • The length of the light fixture (if applicable).

Note: The following section breaks down each step with a separate equation and calculator. If you’d rather do everything at once, use this calculator instead.

 

Step 1 Figure out the area lit by using the height of the light and angle of the reflector or beam.