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:

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.

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 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?

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/Day – The 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?

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.

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.

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.

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.

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.

Some other considerations

Bulb life and degradation

Consider the life of a bulb and the rate it degrades before investing in a grow light. Replacement costs can add up fast if a bulb has a short life span. A bulb that degrades quickly and no longer produces intended light levels will eventually impact a plant’s health. Here are some resources available on this subject for various types of bulbs:

Heat management

Heat from a light may be good or bad depending on the plant and should be taken into account when designing a grow space. Drosera from the Petiolaris Complex like lots of heat. Heliamphora and Darlingtonia on the other hand need cool conditions.

Lights typically run hotter or cooler depending on the type of technology. Incandescent bulbs produce a lot of heat while LEDs produce less. Many fluorescent lights tend to be somewhere in the middle. In addition to the type of light, other factors including ballast placement, heat sinks, and reflector design can influence the rate of heat dissipation.

Reflectors

Grow light reflectors are important for several reasons. The main one being that they help redirect light down onto plants from the sides and back of a bulb. They also help spread light as evenly as possible across the footprint of the grow space.

Reflector can have either a specular (smooth) finish or diffuse (hammered) finish. Specular reflectors may be more efficient at reflecting light into the grow space. However, they can create uneven hotspots in the footprint. In contrast, diffuse reflectors spread light more evenly but may bounce a portion of it away from the plants. To reduce light being wasted outside of the grow space, additional reflective materials such as Mylar can be positioned vertically around the perimeter.

Next steps

We know this is a lot to take in. We’ve not only covered how light is quantified for plants and how much light certain types of carnivorous plants need, but also talked about three different ways of converting light measurements. Sometimes the same information said in a different way can help make complex ideas easier to understand. Farmer Tyler’s article on supplemental lighting for plants covers many of these same topics, as does this article from Inda-Gro and this post by edman007 on FlytrapCare Forums.

And finally, let’s look at some grow lights currently available on the market along with specifics about the quality and quantity of light they produce: Part 3 – Which Grow Lights Are Best?