Episode 287: light and light meters with House Plant Journal

A light meter is a valuable tool when growing houseplants. Photograph: Darryl Cheng

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TRANSCRIPT

[0:00] Music.

[0:16] Jane Perrone This week's On The Ledge podcast is here to lighten your load, quite literally.

[0:23] Music.

[0:29] I'm Jane Perrone, host of podcast On The Ledge, your one-stop shop for all info and chat regarding houseplants and indoor gardening. And in this week's show, I'm chatting to Houseplant Journal aka Darryl Cheng all about the topic of light how it affects your plants and how we've been talking about it all wrong all this time plus i answer a question about spikes and how to protect yourself from them in the gap between this episode and the last one on the ledge celebrated its its seventh birthday.

[1:17] I did an Instagram live talking about what I have learned in those seven years, which is worth going and checking out. I'm @j.l.perrone on Instagram, but yay me, made it through seven years and here's to many more episodes. Thank you to Julia for becoming a legend and for everyone who supports my Patreon. And if you're a Patreon subscriber at the legend or super fan level there's a treat waiting for you there in the form of a video podcast of an extra leaf where i talk to Darryl Cheng about light spectrums and it's really fascinating stuff so do check that out if you are eligible for that content if you want to find out more about getting access to my Patreon you can do that at the show notes at jane perrone.com And there is a free trial period. So if you just wanted to join up to check out Darryl's wise words about light spectrum, you can do that too.

[2:16] Darryl Cheng is the author of two books, The New Plant Parent and his new book, The New Plant Collector, which comes out later this month. And when it comes to science-based houseplant care, Darryl is the GOAT He knows his stuff and light is something that he particularly majors in. And that's why he designed and produced his very own light meter.

[2:43] I've got mine sitting here. I bought this from his website and just for transparency purposes I bought Darryl's light meter a couple of months ago ahead of this interview - I paid for it with my own money and I did think I think I might have got a discount code but I did buy it rather than being given it as a gift and I wanted to do that so I could properly assess it and give an honest opinion of what I thought so this isn't a puff piece for Darryl's light meter it's a really in in-depth look at light. We do talk about his light meter because in my opinion, it is the only light meter that I've come across that is really aimed at people like you and me. So not professionals working in production greenhouses, but people who've got to assess the light in their living room and their plant room and their basement and under their grow lights. So there's lots of advice here about how to use a light meter generally, what Darryl's light meter can offer and how to assess your light if you don't have a light meter and just want to understand the principles of improving light for your plants. We cover it all in this episode.

[3:57] So let's dive straight in to my chat with Darryl Cheng.

[4:01] Music.

[4:06] We're here to talk about light because i think this is sort of a mantra that you have really truly moved forward in the houseplant community which is that you know light is everything so let's get into this tell us Darryl - why is light so important for houseplant care why is it such a central tenet of what we're dealing with.

Darryl Cheng My background is engineering so I tend to look at things as though, you know, how do they work and what's going on inside them? And if we think of a plant, we can think of it like it's a solar powered sugar factory and that little factory makes the carbohydrates and those carbohydrates sustain the plant in the same way that humans have to eat food in order to have our sustenance. So that means that if a like if a plant is in a poor light situation we need to think about it if we want to really want to empathize with the plant we had to think about it as this plant is not producing a lot of food for itself in other words its sustenance is very poor and so it's not just a matter of 'oh IU think it looks nice in that corner', it's that this plant is starving there because it can't produce carbohydrates for itself.

Jane Perrone That's a really interesting way of thinking about it because that does change your perspective. That said, there is a lot of, I don't wish to use a really rude word here. It's a lot of baloney, if I can use an American phrase, spoken about light. And I'm sure I've fallen into these traps in the past of talking about light in a way that's pretty fuzzy. Why is that?

Darryl Cheng You know, it is an interesting thing because from again from my background of engineering one of my things that i used to do a lot was I would write training materials and so it's like how do you train someone to use - I don't know - like a software properly. You have to give them the right foundation and and the right i guess mental model of how this thing works and the thing with light is that because humans we our experience of light is just, okay, I can see versus I can't see, then the wordings that we use for light tends to be just very broad strokes that sort of make sense to you as your sense of light, but it doesn't actually translate into what's important for the plant. And so one very key key thing about it is that we often talk about light from a sense of, from a definition of brightness, right? So whether it's bright or dim or whatever, but plants really need us to understand it in terms of total amount of light received. And if I'm even going to replace that word in that second sentence to be photosynthetic active radiation, that's what the plants care about, which is PAR photons. We tend to confound all these definitions together in just one word, light or sun or no sun. But for a plant, there's a lot more subtlety and detail that needs to be unpacked. Otherwise, we'll think that.

[7:17] 1ft from a window is the same as 2ft from a window, which by the way, it's not the same, right? And without that detailed understanding, then our sense of what's okay for our plants indoors just becomes very fuzzy. And we don't really link the problems that our plants are having to light. And instead we just go to the behavioral, like our personal behavior problem, which is, oh, is that I water it correctly? Am I watering too much or too often? Am I giving it fertilizer or not? It turns towards the things that we do, but we're not thinking about how much light is it receiving. And we can't because we don't have a good concrete objective sense of light.

Jane Perrone And I guess our eyes are playing a role here in that the human eyes are doing a completely different thing to a plant leaf in terms of we are designed to mitigate uh excess or you know our eyes adapt right so that makes it quite hard for us to actually measure this stuff and to tell when something's we don't have that same measurement that the plants got?

Darryl Cheng Right exactly and that's actually what makes it so that, our into our intuition about light is completely wrong because if it were correct and more aligned to the intuition, so to speak, of a plant's sense of light, then we would feel literally hungry on very dark and cloudy days the way a plant would is hungry on a cloudy day and we would feel very like very full on a nice bright cloudy or bright sunny day for like that's the intuition right versus a human intuition is just oh I can see easily I can see clearly or oh it's a little harder for me to see that that's all we can say about about light because it's just our sense of whether we can identify the objects in our surroundings, not our sustenance or metabolism.

[9:19] Jane Perrone And our terminology for light is really quite limited in terms of the, obviously, if you're an engineer or somebody who deals with this stuff, in terms of light, you have a lot more jargon and terms that you can use. But us regular folks, houseplant folks, as you say, it's just a sunny place, bright, indirect, that old saw, you know what I mean? We've gone into this. that word so i mean i think this is where we get to the point where we realize that a light meter is a good thing um well we're going to get on to your light meter which i have purchased um uh so I've just for full disclosure I've bought this with my own money I did get a discount code but I have bought it and it's important to me to do that because that way I can genuinely give it an honest review and um it's a really useful piece of kit I do already have uh I did already have a light meter but comparing the two i've found yours to be uh smaller easier to use it's great we'll get on to light meters and what their purpose is but what is the way through this this light problem that we have? Is the answer, get yourself a light meter, start to understand these different terms, these different measures that we can use for light. Do we need to go down that road? What are the alternatives?

Darryl Cheng So I would actually say that we already are living through that alternative, which is most people don't think of light in this way. They don't have a light meter and they just grow their plants, some successfully and others maybe not. If we think about all the people who you think have a green thumb, I would say that those are simply the people who have the largest windows.

[11:11] You're onto something there you are onto something and that is to say that you know every person's individual so-called skill to water a plant to fertilize you can follow all the instructions but the difference in how the plant will grow is more the thing that has the biggest impact is just simply the size of their windows and when you start using a light meter, then it tells you why. So what I'm saying is the reason why we're living through this with people with no light meters is that they are attributing the satisfaction of their plants to, oh.

[11:49] You know, oh, maybe I overwater, maybe I underwater. Oh, I tend to underwater. I don't have enough humidity. Then we start, like once we start ruling out some of those things, we get to the bottom of the barrel, which is superstition. Oh, I just don't have a green thumb. But I'm saying that superstition can be replaced with my windows don't provide high enough DLI for this plant to grow in a way that I like.

Jane Perrone Absolutely. I mean, just to say that before we talk about DLI, interestingly, I posted something on my newsletter about how fiddle leaf figs, Ficus lyrata, are like the worst figs for vast majority of people because they're just, you know, they need so much light. And it was really funny because I got a few emails from people like well I actually find this a really easy plant and every single person lived in Florida or California I was like well that's why because you've got really good light you've got that that's the thing this plant really needs you know you're probably living not like in the UK or somewhere where there's like small dark windows but somewhere where there's a lot of light and the architecture is such that you you probably have very large windows. Anyway, so yes, I would totally back that up. Tell me about DLI.

Darryl Cheng So DLI stands for Daily Light Integral. And so earlier when I talked about the distinguishment of brightness versus total light received, total light received. The unit or the way that we quantify this is called Daily Light Integral, which very simply just means means the total number of photons received over a 24 hour period. And of course, there's a lot more detail in there, such as when I mentioned earlier, photosynthetic active radiation, it's just to say, it's not just any photons, we care mostly about the photons between 400 to 700 nanometers, which is also our visible spectrum, right? So that means, here's a complicated thing, which I'm sure you can now guess, Is that natural light through a window, the levels will change all the time, right? Because the sun is sometimes shining, like you can see right now sun's shining on my window creating this nice little rainbow here behind me you've got a literally surrounded by a rainbow daryl it's absolutely talking about light and it's like you're some kind of uh light um light being yeah yeah it's like i know that you know right now during the day because the sun is hitting the window pane which has little like little diffraction things that make a nice rainbow uh but then other times of the day the sun will move away and it's getting you know i'm just looking at the brightness of that patch of sky sometimes there'll be clouds rolling in and so right away in order to in order to actually capture this DLI for natural light setting it requires something that logs the data over time and then you even have to look at your excel spreadsheets which is what i've done before so that's always going to be a very highly changing quantity but and a light meter only only tells you that moment at that moment how much how much light is hitting it at that moment right and that so brightness here's a little like analogy to help you think of it brightness is to total light received as speed is to total distance traveled. So what i'm saying is in order to understand like the total light received we have to measure it but that only tells us the in the immediate brightness but then we have to also now think of like I guess a, context that will then like when you sample that moment that will give you an understanding of the total amount of light received and what i'm saying is the light meter is just the sort of basic fundamental tool, but there's a whole body of knowledge that I've tried to, I guess, write about in order to help you understand total light received based on just sampling different scenarios.

[16:01] Jane Perrone Absolutely. And it is worth, I mean, I've done this with and without a light meter. If you spend a whole day in a single room and on a fairly bright day where you can see the shafts of light coming in it is really fascinating to see the way the light moves around the room and i've certainly done that in my south facing room where i've sat there the whole day working i've been able to see the light move from you know one side of the room it gradually then it hits the monster and then it's on there for a while and then it you know it does actually hit the in that particular room because it's a large bay window you know we have to look at the size of the window as well right it hits the back wall where the aspergistra is um and then it comes around and hits the sofa and the dog wants to climb on the sofa to get the sun so you can you can start to get a sense of that combined with the light meter getting a sense of that um overall picture as you say which is so interesting this is where the light meter comes in i've got your light meter in front of me tell me about light meters can you guide us for people who've bought a light meter and maybe they're not quite sure how to use it?

Darryl Cheng First of all, back to the whole thing about basic tool versus the full knowledge, right? I want to preface this to say, you know, in the context of building a house, let's say, you have a measuring tape, that's the basic tool, but building the entire house requires a lot of other skill, but you still have to have the measuring tape. So in the same way, you have a light meter it will tell you how bright is it right now but you need there's a lot of extra knowledge that then says uh to answer the question is this enough light for the plant, right so that's what i'm going to kind of explain right now which is that we have to make sure that we break out and separate the context of natural light through a window because earlier i talked about different scenarios right so one scenario or context is natural light through a window That's one context. And I'll explain that in a moment. But the next context that you're most likely to deal with is purely using grow lights.

[18:14] And then the other two contexts, which are more for geared towards professional growers, would be like greenhouse light settings and lastly, outdoor gardening. So what I'm saying is those four contexts, you need to keep them separate in your mind. And what I mean by that is when you see the words "full sun" written on little tag, don't take that to mean in your house where the sun is shining on you. That is not full sun - full sun is outside no obstruction, as in like no clear obstructions like a window and six or more hours in a garden that's the context of full sun so that anyway that's the gardening one. But now we're going to go back to the indoor context - the context of the indoors is there's two parameters that you care about right there's when the sun is shining directly at the spot that you're evaluating, then you care about how long that lasts. It could be one hour, it could be two hours, it could be zero hours if you just happen to have obstructions that don't let you see the sun. So the first parameter you care about is how many hours of sun does that spot get? Okay. And then once the sun moves away, then you need your light meter to tell you how strong is that indirect light because when we say indirect light what we're saying is the sun is not shining directly at me i'm just looking at a patch of sky.

[19:39] That's where I care to use a light meter to tell me how strong it is. Now that in effect is basically saying how big is your window, because clearly the bigger your window and the sun is not shining directly through that window, then you're seeing more of the sky and that translates into a bigger number on the light meter and if you just want to kind of prove it to yourself - if you stand right up against the window and even as you move farther away, the angle of the visible sky changes, and even just that subtlety of change changes the number on your light meter a lot. So that's where the light meter is critical.

[20:26] So that's why now, back to this, remember, still the ultimate question is, is this enough light for my plant or is my plant going to do well here? So on my website, houseplantjournal.com, there is a big table that the title of the table is called Bright and Direct Light Requirements by Plant. And there are three columns. The first two columns, one of them says minimum for maintenance and next one is good for growth. These are levels of indirect light. So that means when the sun moves away and you take your light meter to that spot, the number you see, let's say it's 200. Then I go to the entry for I want to know if I can grow an aglaonema here.

[21:09] And it says, good, or minimum for maintenance is 100 foot candles, and then good for growth will be 200. And, you know, I measure at that spot, maybe for a couple days in a row, at the same time of day, and I'm getting, you know, fluctuating numbers between 100 to 200 foot candles. So then I can say, okay, well, if I'm somewhere, let's say 150, as an average, then I can expect my aglaonema to grow pretty well here. Then that first parameter that I talked about, which is now how many hours is the sun actually hitting it? And let's say it's roughly one hour. Then there's another column on that table that says tolerance of direct sun. And I say one to two hours should be fine. So then that means I can keep my aglaonema here and I don't need to, let's say, block it with a curtain or move the plant farther back.

[22:05] Now, I know that I've just said a whole mouthful, like a big long paragraph here. But the problem is, if I just said 'bright indirect light' and left everything else to interpretation, then a lot of times the interpretation is, oh, I should never let the sun shine on this plant. And then you'll end up putting the plant really far from the window. But then go back to the objective measure. Measure the light the indirect light when you're so far from the window and you'll see it's not going to be close to 200 if 200 is only available you know right next to the window right so what i'm saying is the way that i'm addressing light even though it's more complicated I admit it is complicated but it's because it addresses the the complexities of a natural light setting indoors. And the way that I've just described it's also agnostic of window direction. It's agnostic of what's outside because I'm evaluating what's actually happening, right? I don't care about characteristics out there. I don't care about classifying windows. I care about what's actually happening at the plant.

[23:17] Music.

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[25:10] Jane Perrone More from Darryl shortly, but now it's time for question of the week which comes from Sally and it came in the form of an email entitled 'thorny question'. And Sally's question relates to the thorns on miniature agaves - so there are two three inch terracotta pots with agaves in Sally's Ikea greenhouse and Sally has noted that these thorns are lethal and she's put lethal between asterisks to emphasise the fact that she really means it. Sally's wondering if there's any disadvantage to the plants if the tips were snipped off and writes 'aesthetically I love the look but ouch they are testy plants and in a small space'. One little point here before we get started on how to deal with these spiky bits: officially speaking, these are not thorns they are spines what is the difference it's not any great importance but it's just worth noting that spines are modified leaves whereas thorns are modified stems. So that's why a rose has thorns and an agave has spines. Thought you might find that interesting, Sally. Anyway, on with the question, what do you do about the spines of agave? They are absolutely lethal and I can totally understand why you want to find a solution, Sally, because I too have been attacked by these things and they're, yeah, it's not nice. So I guess the question is, is cutting off those spines going to be detrimental to the plant in any way and balancing that against the risk of you getting an eye poked out or indeed as I saw when I was looking around Google Scholar there was a paper about a young guy who was experiencing problems with mobility, and it turned out that he had an agave spine in his spine so yes this is not worth messing around with.

[27:16] The short answer is that, yes, you can chop those spines off using something like secateurs they are incredibly tough so you probably will need something stronger than a pair of nail scissors, you can chop them off it's absolutely fine just when you're looking at the spine make sure that you're not cutting into the soft fleshy tissue of the leaf and that you're just chopping off the spine. And if you're nervous you can just do it in two segments so that you don't take too much away way but definitely yes you can cut the spines off your agave no problem it is done very very frequently in outdoor landscaping schemes in parts of the world where agaves grow all year round outside for safety reasons apart from anything else and your plant will be absolutely fine for it. If you really don't want to remove the spikes, what you can do is add a piece of cork. I saw this done when I went to the Isle of Wight which is a small island off the south coast of England and they had lots of outdoor agaves there because the climate's very mild and in more public places the tips the spikes would be topped with a cork. So you could certainly try this Sally - if you don't want to remove those spines you could just stick a cork on the end or a piece of cork as they're small ones ones a section of cork that you cut up would work particularly well. I appreciate that aesthetically maybe it's not doing it for you, but if you wanted to keep the plant intact as it were that would be an option. And if you're interested in more agave content do check out On The Ledge podcast episode 59 where Dr Colin Walker who's an expert in agaves talks about these and aloes and some other succulents in a really nice interview. So I'll put a link to that in the show notes if you want to expand your agave knowledge.

[29:16] If you've got a question for On The Ledge, I'd love to hear from you. Do drop me a line to ontheledgepodcast@gmail.com. Now, though, it's time to get back to my chat with Darryl.

Jane Perrone Let's imagine we're taking our light meter and actually using it. The first day I got this, it was probably, it was the depths of winter, I think, when I received this. And I got very depressed because I turned it on and I went around with my plants where I was like, Like, okay, it's hardly even getting past zero here. It's really, you know, it was really like, it was probably like four o'clock in the afternoon and it was a depressing scenario because I was thinking, gosh, my poor plants. This is miserable. I immediately went to flick on a couple of grow lights that I hadn't neglected to put on because I realized that I was, my plants were probably like, why?

[30:09] Is there any particular way? Obviously there's a big silver dome on here, which I presume is the sort of the sensor. Is there a particular way I need to hold? Do I need to hold it horizontal, vertical, pointing it towards the light? How do I actually use the light meter to make sure that I'm getting accurate readings and getting a sense of what it can do?

Darryl Cheng So in fact, the silver mirror dome is just a mirror.

Jane Perrone Oh, okay.

Darryl Cheng And the reason it's there, it's so that when you are measuring something, sorry so the actual sensor is the white dome and what happens is when you like actually i can do it right now uh like if i'm if i'm measuring something that i see the number 73 74 then i can look at this dome and say why am i seeing 74 like what is in my surrounding environment that's given me 71 well look at this big bright spot here it's because i have a light shining on me.

[31:08] And in most cases, if you're doing it by window, you'll see like the reflection of the window. And earlier when I talked about just simply moving further or closer to the window, you'll see the profile or the shape of that reflection change and realize that it has a profound impact on this number, right? So that means when I evaluate light, I'm not evaluating it for the window as a whole. I'm evaluating for every square foot next to that window and in front and behind that window. Every square foot of the area where you can put a plant is going to be different. So you're not just going into the center of the room and holding the light meter and going, oh, it's a hundred foot candles here. You've got to go in a bit more depth, right? And presumably the more measurements you can get over time and in different spots is going to add to the accuracy.

[31:59] Yes, but it's more like add to your sense of this area typically gets such and such levels, right? And on my website, which I think I need to maybe make it a bit more clear, it's that you don't need to care if the difference is between 150 and 160. That's not going to make a difference. What makes a difference is the average range. Range so in the ranges that i've kind of specified it's you know under 100 foot candles if it's always on under 100 and and maybe coming close then you're kind of at the bare scraping the bottom of the barrel here between 100 and 200 is the next sort of level and then 200 to 400 now the reason why that range got bigger is because the bigger your window is the more the the variance is going to be, right? And then the last range is 400 to 800. And if you're in the 800 foot candles of purely indirect light. It basically means you probably have floor-to-ceiling windows across spanning the whole wall. A lot of modern apartments or condos have this because they just give them floor-to-ceiling windows. That's why the maximum range of indirect light tends to be 400 to 800 and also excellent for plants.

Jane Perrone I'm holding mine here on my desk. This is why, if you look at the background here, there are no plants visible apart from a terrarium which I've turned the grow light off off in there because it strobes uh when it's on video but um yeah there's there ain't a great deal of plants many plants in here because I'm looking at this and it's telling me 20 about between about 25 to 30 foot candles i have got a grow light on the desk over here and if i'm moving it close to that then we're getting up to uh well yeah as soon as we're under the grow light we're up to a nice sort of 2000 so we're going but when I'm holding I'm not that's when I'm like 20 centimeters away from it so um that just shows you the power of the grow light right and that's just an Ikea bulb in there uh so that is really interesting about the mirror, I hadn't realized that so I'm glad that you told me that that's a really useful thing and I should also point out that you've got the temperature and the humidity on there as well which is useful to know as well.

Darryl Cheng I mean yeah in fact there's no other light meter that has all all three measurements that's crazy why is that not standard that seems well because I think you know when they make devices they just make them like because the device like a light meter is so specific to I mean usually for workplace design it has to be brightly lit enough if they're doing factory work or something on the table then it needs to be a certain brightness for them to see comfortably right and that's the typical the first usage of of light meters.

[35:01] Then, of course, when it comes to plants, Whenever you start with the most, let's call it commercially viable use of growing plants, we're talking about agriculture. So agricultural levels of DLI are well beyond what you need for houseplants. So they need more specialized light meters for that type of growing. Whereas with houseplants, as I mentioned, you're dealing with much lower light levels in terms of what's practical. So you don't quite need like light meters in the hundreds several hundreds or thousands of dollars even right um actually i did i did want to address one thing that i didn't talk about which was you asked like exactly how do you orient the light meter in order to make the measurement and really so if you think about this as just telling you how bright is it right now like how how many, like how, how, yeah, how, how is the, the, the light sensor catching photons, then you just have to imagine that if, if you, if you orient this differently, that's just going to be the leaves oriented that way in order to get the highest number, right. Which is why, of course, your plants wrote like, you know, um, lean into the, into the windows, because if the highest level of light is when you turn towards it, then the leaves are going to orient themselves to be orthogonal as possible right so so in fact there's no you don't have to always hold the light meter a certain way just go around where the leaves are and that and that will tell you uh the range that you're working with for that plant.

Jane Perrone Designing a light meter must have been a blast - I'm sure there were some challenges though - what did you actually learn from designing your very own light meter?

[36:51] Darryl Cheng So here's something interesting, which I learned. And that is that, you know, when, when you're designing something for a specific purpose, you really have to know, I guess that the science like very deeply behind it. Otherwise manufacturers can only do things as specified, like by the specifications, but then when you test it and it's not behaving in the way that you know based on the science then you have to have i guess the the knowledge to understand how to correct this now that's all very conceptual but let me tell you exactly what happened with this light meter which is that uh light sensors like i'm talking about the electronic chips they just catch photons and any photons because remember i said visible photons is what what we care about for plants, 400 to 700 nanometers. Now, here's a little trick you can do to sort of test for yourself is when you take a remote, like your TV remote control and you press the buttons and you look at the front where that little, the thing that actually sends the light out.

[38:01] You won't see any light because it's infrared light. But if you take your phone camera and presumably doesn't have infrared light filter and you point it towards the camera and you look at it. You'll see like a little kind of purplish light flashing, right? So there is infrared light that's invisible to our eyes, but it's still photons. And when I first designed this meter, they just put the sensor in there raw with no filter in front. And so what was happening was I was noticing much, much higher numbers than what other light meters were telling me. And then I took a remote control and pointed it at the sensor, sensor and it shot up like crazy and then i realized oh it's not filtering out the proper wavelengths and so then i had to go and source a uv and infrared filter so that it it cuts off anything that's higher than 700 lower than 400 so that only the photons between you know 400 you know 4 to 700 are captured by this sensor and so that's an example of something that if i didn't know, like, that's what the science is going on behind with the circuitry, then I might have produced an inaccurate meter.

[39:15]Jane Perrone Now, one of the things that also, I mean, I have not taken this up, but I do know that when you, people buy this meter, I think I'm sure I remember getting an email about your workshop. You do offer people a workshop for learning about their light meter, which is obviously like, as we've seen, I didn't know about the mirror. So clearly I needed to go on your workshop. Why did you decide to offer that because because presumably for the reasons we've been talking about like people's understanding of light is in general much more basic say than their understanding of temperature so you need this work you need an extra insight?

Darryl Cheng

I think i've mentioned before like the analogy of like this is just a basic tool like a measuring tape but in order to understand light for your houseplants it's like saying you you're trying trying to build the house, right? So just having a light meter, yes, it will tell you the brightness, but understanding how to interpret those measurements and even how to do the measuring itself is an area that I think no one really talks about. Especially with natural light. Now, now let's move just quickly to the context of growlights. Growlights are in fact a lot simpler. So if we think about the analogy of speed versus total distance traveled, right?

[40:33] Okay. I'm going to bring calculus here, but on a graph of speed versus time, and if you had a constant speed for a set amount of time, then you stopped it going that speed, you'd have a rectangle. And the area under that rectangle is just speed times the time. So because a grow light stays at the same brightness all the time and you control the time, it's very easy to calculate the daily light integral for a grow light because you simply turn it on, you take your light meter and measure at the leaf and say, you know, like you said, it was 2000 foot candles, convert that to a PPFD, which by the way, is very easy to do with white grow lights. Um, then multiply by the time you keep it on and then you, you have your DLI. Whereas because for natural light, the number keeps changing. You cannot just multiply it and get the same number.

Jane Perrone Yeah. I mean, I guess that's the joy of growlights. As you say, you can program a light, a smart plug and get them set up for a set number of hours and you can concretely say how much exactly how much light your plant is getting.

Darryl Cheng Exactly. And one thing I will caution about growlights is that there are tons of people who advertise or even promote using growlights. Not one of them, maybe a couple of them, talk about measuring it. So to me, that's the same as telling someone to bake something, but not knowing what temperature it's set at. Because as we saw with the light meter, every inch that you change the distance from a grow light changes the number on the reading so much. So that's why sometimes I get questions of people who are saying like, oh, my plant's doing so poorly, even though I use grow lights. And then I say, show me a picture. And it's like a growlight five feet away, like a weak grow light five feet away from the plant, thinking that that's supposed to make a difference. But of course, be objective and measure it from five feet away. It's like 20 foot candles. And then I can tell you, well, your plant's hardly getting any light.

Jane Perrone Yeah, exactly. The distance is vital in this calculation. Now, not everyone obviously is going to be - I can't remember how much your grow light (I mean light meter here) retails for - is it 90 bucks or something? I can't remember, tell us.

Darryl Cheng Yes, $99 US.

Jane Perrone It's a really good investment I would say, but obviously not everyone's going to have the budget to to have one of these - perhaps it goes on the Christmas list - but if people don't have a light meter, it's just not something on their schedule uh financial budget wise, what can we do using our eyes or any other tools we might have just to get a better idea of light, in our particular homes?

Darryl Cheng I think it it's not as much get a better idea but rather, just put your plants as close to the window as possible and realize that your plants will grow differently from someone else just purely based on simply the size of your window and how many hours of sun hit the plant.

[43:43] And I know it's like not the answer you want to hear, but what I'm saying is I can't, I can't give, I can't tell you, oh, you'll like bright and drag light is right in front of a so-called east facing window. Because if two people claim that they have east facing windows, that has nothing to do with like what I would actually see on a light meter, right? Right. It's almost as if to say any guidance that you give on light without a measurement has to actually line up with what you would get if you did measure. The best example of this is that outdoor gardeners, their parameters of full sun, part sun, part shade and full shade are all based on a common metric of hours of sun exposure, which does line up correctly with DLI. But bright indirect light versus medium versus low has no correlation to dli and it also has no similarity between two people who two people two rational people who say i have bright indirect light there is no guarantee that they actually have the same dli versus two people who say i planted my outdoor plant in quote unquote full sun if they're both understanding that it's six or more hours, then their DLI should be similar.

Jane Perrone Yeah. It's like the people who suddenly, you know, go on, progress on from houseplants and get themselves a greenhouse. And suddenly all their plants are burnt because they are realizing that outside, you know, a greenhouse that's got, that's not got shade cloths up in some parts of the world is going to just fry everything because suddenly the light's a heck of a lot brighter.

Darryl Cheng As you say, that's a big difference. And on top of that, if their approach to watering is also what I would call the poor approach, which is a specific schedule based on the plant type, then that situation is going to ruin their plants. Whereas I talk about watering as being based purely on observing the soil dryness. So if your Monstera, let's say, you water it whenever the soil reaches about halfway dry. Well, if you put it in a greenhouse, it's going to reach halfway dry in one or two days. Then you will keep up with watering. Maybe a longer term problem would be leaf bleaching. But what I'm saying is, at least with the watering part, because your so-called watering algorithm adapts to the situation, then you won't get messed up by when the situation changes. Versus if you just had a rote schedule, then if you change some other parameter and your own internal algorithm doesn't change with it, then that's where the problem, as you said, of plants burning up in greenhouses.

Jane Perrone That's another whole episode, watering. But it's been really fascinating to talk to you about this. I should say that there's going to be a transcript up at janeperrone.com, which I advise people to go and read because obviously there's a heck of a lot of information that you've imparted there that's really, really valuable. And I'll put all the links to all of the things you've talked about, that table and so on in those notes as well. It's been a delight to talk to you about this subject. And let's hope that if even one person sees light in a different light today, then you've done your job, Darryl. But thank you so much for joining me today.

Darryl Cheng Yes, thanks, Jane. It's been a pleasure

[47:11] Music.

[47:19] Thank you so much to Darryl Cheng and check out the show notes at janeperrone.com for all the information about Darryl's light meter, which is excellent, by the way. I can safely say this was a really good investment purchase. I think it's $99 American and I know that I'm going to be using this absolutely loads over the coming weeks, months and years. Also you'll find information about his two books. The New Plant Parent and The New Plant Collector which is out later this month and loads more brilliant plant science information on his website, houseplantjournal.com. So do go and check that out as well as the transcript which you'll find at janeperrone.com. That's all for this week's show. I will be back in two weeks with another episode for you, but until then, I leave you with the words of victor hugo who said "to love beauty is to see light". Have a great week - bye.

[48:26] Music.

[48:34] Jane Perrone The music you heard in this episode was Roll Jordan Roll by The Joy Drops, The Road We Used to Travel When We Were Kids by Komiku and Dead From The Beginning, Alive Till The End by Doctor Turtle. All Tracks are licenced under Creative Commons. Visit the show notes for details.

[48:54] Music.

Host Jane Perrone talks to Darryl Cheng aka House Plant Journal about the science of light for houseplants and how a light meter can up your houseplant game, and the Q&A covers how to tame an agave.

This week’s guest

Darryl Cheng is the author of The New Plant Parent and his new book The New Plant Collector is out later this month.
Check out his website House Plant Journal for more information - you can buy Darryl’s light meter here.
Darryl’s Instagram is also definitely worth a follow.
You can find the table on bright direct light he mentions in this interview here.
Listen to my previous interview with Darryl here.

Patreon subscribers at the Ledge End and Superfan levels can watch a video podcast of An Extra Leaf where Darryl and I talk about his insights into the light spectrum - especially in relation to growlights.

QUESTION OF THE WEEK

Sally wants to know how to stop getting injured by an agave with lethal spines. Can she cut them off? The short answer is yes - this is common practice when landscaping in outdoor areas where it’s warm enough for agaves to grow outside. Use a strong pair of scissors or secateurs as the spines are extremely strong, and just be careful to only cut the spine without venturing into the softer leaf tissue behind.

If you don’t want to have to cut the spines, you can also add a cork or a piece of cork to the end of the spine to render them safe.

Side note: do you know the difference between a spine and a thorn? A thorn is a modified stem - think roses - while a spine is a modified leaf.

Want to ask me a question? Email ontheledgepodcast@gmail.com. The more information you can include, the better - pictures of your plant, details of your location and how long you have had the plant are always useful to help solve your issue!

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HOW TO SUPPORT ON THE LEDGE

Contributions from On The Ledge listeners help to pay for all the things that have made the show possible over the last few years: equipment, travel expenses, editing, admin support and transcription.

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