Lighting Greenhouse veggies

Added: 2026-05-13

[00:00:01]okay so in this lesson we're going to talk about some concepts about lighting uh vegetable crops um we're talking primarily about tomatoes and a little bit about peppers today so we're by now we're very familiar with what photosynthesis is and what it does it takes sunlight and converts carbon dioxide and water and into sugar right and as a sort of a base fact for you to remember you need about 80 to 110 watts per square meter of uh light intensity for photosynthesis to happen so if you go below that level of intensity you end up with not much happening um now one you might say well then all you need is under 10 watts except with the vegetable crops they're very tall and there's a lot of leaves which cuts into the amount of penetration into the canopy so typically we we're looking at around 500 watts per square meters needed for optimal growth um that way we know that there's light penetrating down to the lower parts of the canopy to make those leaves useful all right uh less than five percent of the sunlight that actually hits your crop uh you know is actually used for photosynthesis the rest of it is reflected or turned into heat and of course we know that plants look green because a lot of that is reflected off the plant we'll talk more about this in another lesson about spectrums and things like that so don't worry that's coming um there we also know that you can have too much light remember we talked about photo oxidation so anything more than say 1500 micro moles per square meter is going to risk burning your crop damaging the leaves or even the fruit in the case of peppers um and then of course we know that light also heats up things right and so solar radiation also contributes to evapotranspiration that means it's warming the plant and it's removing water from the plant causing transpiration to happen which is a good thing um and and of course it kind of works nicely that as the sun gets higher and more powerful the plant will respond with more transpiration so this is sort of a mutually evolved system um this this graph here just going back to we were talking about shows you what what wavelengths are transmitted into the canopy and green light does filter down lower into the canopy as does in far red light remember far red light stretches goes down to the bottom of the crap crop causing stretch and green light interestingly uh makes it through or bounces off leaves to get to the lower leaves and what is interesting is that green is completely not useless because it does trigger some photosynthesis so green actually green light actually improves the efficiency of lower leaves making them contributors for sugar production so this green is an important part of a spectrum not hugely important but it is useful so there are three types of artificial lighting that uh we need to remember about right there's what we call replacement lighting replacement lighting is the kind of lighting you would see in an indoor chamber that has no glass that means that all the spectrum should be replaced at appropriate intensities for the crops to grow properly then there's what we're kind of talking about today which is supplemental lighting and supplemental lighting means that okay so sunlight is there but we're augmenting that with our own lights as best we can for the most efficient use of electricity that's supplemental lighting or sometimes called top lighting then there's photoperiodic lighting which we're not going to talk about today we will talk about photoperiod but not photoperiod in our other lesson for crops too we talked about mum production and how we use a regular bulbs in the middle of the night not a huge amount of light but enough to trick the plants into thinking that the you know there's a different photo period happening so that's a different type of lighting so as far as supplemental lighting goes there's uh there's the choices you have to make as opposed to sort of when you're going to turn the lights on and for how long um when you're looking at the calendar of your year we know that the day length changes you know in the winter we have short days here in canada and in the summer we have long days which means it's an entire continuum between the shortest day and the longest day so when you're looking at uh less than four and a half hours per day there's definitely room for supplemental lighting and the other thing that not only just the length of the day but the height of the sun in the sky has a big impact uh on the intensity we talked about this in the fall with the climate in the scent in the winter the sun is so low in the sky a lot of the light is you know pushed through more atmosphere and bounces off you're almost five percent of the sunlight uh useful sunlight in compared to the summer so that's a pretty big loss in the winter time so it's obviously in the times of year where we use supplemental lighting is when the sun is weakest right that's when it's most beneficial so this graph is just showing us the hours of uh daylight for a given latitude and we're sitting at 43 degrees north the red line is 50 degrees north so we're just under that let's say we're here that puts it seven hours of daylight in january this is our january month so there's there's uh not a lot a light in january we just went through the winter we know that you know december and january not much happens i mean we have grow lights thank goodness but the rest of the crops not much happened um terms of peak light you know that that also changes because remember i talked about the eye the angle of the sun in the atmosphere so the shorter days the sun's lower in the sky so our peak power output is also lower so let's take a look at that so uh 40 degrees is between red and green let's say it's around here and for january we're looking at yeah 100 watts per square meter not a lot of power remember i said you know with tall tomato crops you're looking at 500 watts by the time that light trickles down to the lower leaves so we don't have enough to drive real growth which is why a lot of farms you know don't plant their tomatoes and peppers until december then the plants are small they don't care and as the plants get big the by the time they come to the size where they're starting to produce fruit you're already into february and the sun is stronger and you see more growth so what are some supplementary [Music] rules here so it's more efficient to provide a lower amount of radiation so that's not radiation in terms of you know i'm going to die uh irradiation is sunlight over a longer time so less light over a longer time is better for plants than a lot of light over a short time that's a really important lesson to learn so lower light intensity over a longer period of time will give you what's called a daily light or a light integral right and so you can have the same like a brighter intense light over a shorter time that has technically the same light interval but the plant has a better has better use of the lower light level over a longer time now it doesn't mean that you should just go buy yourself a week with a light bulb and give the crop a week because we have to come confine ourselves to the photo period so there's a compromise right but generally speaking you don't need to have a massively powerful light uh and then just turn it on for a short time that's not helpful um it's more efficient uh sorry it's better for example to light a crop with at 50 watts per square meter for 18 hours than it would be to light it with a hundred for nine hours so that's kind of a more realistic for comparison although the those wattages are kind of low for my example but it was just sort of to help you follow along but essentially plants use light uh in such a way that if you if too much light is arriving at the leaf the plant can't keep up we've talked about this before right and if there's a lot of light the the electrons actually damage right that's photo oxidation but even if you're not at that damaging level what happens is that light that's arriving is is uh asking the plant to do something that it can't keep up with it's like having all sorts of mail arriving at your warehouse and you don't have time to sort it and then the next morning you're expected to deliver it right so the leaf falls behind so there is there is an incremental benefit right so and the idea is that we we generally speaking we want to extend the duration of the day not heighten the intensity of the day right so we're talking about adding supplemental light to the day so beyond the daylight so usually lighting in the evening or morning is most effective uh so not during the day so much but more as the sun starts to go down we turn the grow lights on keep the plants active don't let them slow down to give them keep that growth moving into the end of the day and then turn them off and let the plants rest so as far as uh sl stands for supplemental lighting uh there's typically the computer systems use sort of a a threshold to decide when to turn lights on and that may be at 100 joules per second per square meter uh or 100 watts per square meter per second ppf um you would set that in your controls you can ask matt about that in his class and then you know if it falls below that then the grow lights turn on and then extend the day for that time so looking at different crops cucumbers and roses uh they they don't mind 24 hours of light they can actually manage the sugars and shunt them to where they need to go while the light is on so i don't know many farms that light cucumbers for 24 hours i think that ultimately there's a cost benefit thing that you have to look at but the point is 24 hours doesn't harm them uh tomatoes on the other hand should only be lit for about 16 hours 18's already kind of pushing it beyond that actually we'll talk later there is there's negative effects on the crop so typically and that's i'm saying that as an example for looking at 24-hour versus shorter times so continuous light otherwise known as 24-hour lighting often has quite a few negative impacts on plants and tomatoes they start to get necrosis and die leaves fall off so typically the grow lights then would be turned off when the sunlight uh reaches around 320 micro moles per square meter per second ppf if you were thinking about watts versus micro moles that's sort of the equivalent so in your preva you would set uh the the minimum setting for supplemental lighting so in the morning at a certain point you would say okay the lights can come on now and we'll run them until the sunlight reaches 320 micro moles ppf and then it would turn the grow lights off and then in the afternoon when it goes below that it would turn them back on again within the confines of your overall photo period right to make sure that the crop is is not getting too much light um so typically looking at april through september the crop receives the ppf that it needs without supplemental lighting so we don't need it through those months which is a huge cost savings right okay so there's some studies have been done to look at supplemental lighting bringing you know your overall dli up to that 320 sort of micro mole target and that means that in the morning when the sun is coming up the light levels are lower the grow lights come on and then as you hit your target then the grow lights turn off and then towards the end of the day you go below your micro mole and then you you add it so we're talking about adding 100 uh to 150 micro moles of supplemental and the increases were in the order of 20 increase in yield to 36 increase in yield with the grow lights so there's definitely a return on your investment by going up the 150 micro moles uh again depending on your location but for where most of these farms are located that seems to make sense so there is a justification for it in uh in crops now in tomatoes when we increase the light the the overall ppf with supplemental lighting we're increasing the sugar content which is directly related to flavor right so this is where you know some of the criticisms come from you know all the flavor of hydroponics a lot of it's genetics but some of it's how they're grown right and does the farm have supplemental lighting or not are they allowing the plants to make that extra sugar and on top of that the plants are also when they have sugar they're able to produce other metabolites such as ascorbic acid um so vitamin c and whatnot and those acids also contribute to the flavor so there's a it's a complex uh interplay that is to be quite honest is still being researched and understood especially now in light of leds because we have even more interesting outcomes that are needed to be studied looking specifically at tomatoes so as far as photo periods so the length of your day right when the grow lights come on and when they go off if we extend it to 14 hours right so remember in january we had what seven hours we go if we go up to 14 hours with 150 micro moles of ppf we have the largest yield and quality increase uh so remember tomatoes don't like 24-hour lighting if you go to 17 we end up with some problems uh you can get a more vigorous plant but there is issues with uh sugar distribution and also should the light come on in the middle of night we don't know why but if there is interruption you get quite a bit of necrosis forming on the tomato plants so i think it's kind of cool that there's stuff we don't know yet right that that's kind of cool so yeah 20 14 hours is the sweet spot for tomatoes um if you extend tomatoes to 24 hours with 150 of ppf you end up with less fruit so you have a lower total fruit weight they're smaller so you get fewer fruit and they're smaller and you end up with lots of necrosis and crops actually start to go they don't shrink but they start to die uh which is just horrible uh you know that's that's not gonna improve your yields at all looking at peppers there are different beasts uh they are able to handle up to 20 hours of photo period and the same 150 ppf and they see typically you know dry weight goes up in the chute which means of course that there's more tissues being laid down they're sturdy um although if you think back to the fall with our chat with st davids they were saying that thinner stems are better so that that's an interesting outcome so i see some conflicting stuff but i'll go with st davis because they they grow pretty good crops so um they also noticed that the leafs were bigger and that is maybe good maybe not hard to say the fruit camp was highest that's good so we have the highest yield with 20 hours and we have the best quality so 20 hours is the sweet spot but what where things get a little tricky and if you remember the st david's tour or if you haven't watched it you should go back and look at it um one of the issues is cost you know grow lights don't run for free so that's where the cost for all that supplemental lighting was not seen in the increased yields in terms of revenue generated versus revenue spent for the electricity to run the lights so that's why a lot of peppers to this day are still not lit there's a few greenhouses that have grow lights on peppers but not a lot and if they do they only have maybe a small percentage of their overall farm and they do that so they get to market earlier because early product gets a higher price but demand is also sort of not always there um 24 hours of peppers uh interestingly so 24 hours had the same amount of yields as 15 to 16 hour photo period so basically there's no point in paying for the electricity for all those nine hours or more that you know eight to nine hours that you're adding it doesn't work uh you actually get decreased fruit numbers and smaller fruit size and the leaves start to wrinkle a little bit they don't die like tomatoes and you know just poop out on you but they the uh they do suffer so in case you're wondering as often i get students asking what if we light the crops for 24 hours generally speaking not a good idea um now what what some of this stuff does is alert us to some interesting concepts right um why doesn't yield keep going as we increase light levels right you might think this is photosynthesis we add light we make sugar we make more sugar we get more growth right well it's not quite that simple because um light plants make sugar for the day and then they turn it into starch and then they feed on that as they build and fuel their growth for the next day and then they start the process again but you know you may have an increase in 45 of starch in your leaf after a 16 hour photo period but at the end of the day you go past you know 18 24 hours you actually have a lower rate of photosynthesis uh 26 to 29 less photo photo uh photosynthesis or less sugar production than if you had a 12 hour photo period so more is not better once again we see that you know 16 hours seems to be about top so what is it about the inability of tomato plants you know to make use of that right well there's a couple of theories about what's going on and i'm going to tell you right now we don't have it fully figured out the first one is source sync imbalance now we've talked about source sync before so we'll talk about that again and the other is sugar metabolism issue so let's start with the source sink um remember the source is a leaf and the sink is the fruit or essentially a sink is any growing part of the plant so that could be apical and lateral meristems or it could be flowers or fruit that's been you know flowers fertilizers growing fertilized growing fruit so you have your leaf and you got your flour and we're trying to move sugar from the leaf to the flour and if you have sugar you know the rate of production of sugar is exceeding what the fruit can use at that moment then the sugars get locked up in the leaves and are less available uh or sometimes just make you know you can form starch and then the starch is given time it forms hard starch crystals uh that are you know if you're eating for example celery it's very crunchy and people often say oh it's crunchy because it has lots of water it's actually got a lot of starch and that's the starch grittiness that's in your teeth and that's giving you that potatoes you eat and eat an uncooked potato sometime and you'll feel the grit in your mouth in your teeth that's starch crystals that are long-term storages uh storage molecules for for sugar so if we're making lots of sugar and the fruit's not getting it on time the plant starts to stitch it into starch which forms crystals and then it's locked out and the fruit doesn't get it so that's the source sync imbalance right so your your issue is with loading and unloading of sugars from the phloem into the fruit it's just the speed of it's not happening um i had mentioned before like the pre-pre-dawn dip in temperature where the fruit stays warmer than the leaves and that allows the fruit to be physiologically active and steer the sugar i had one student who sent me an interesting article questioning that theory and for the most part the theory is still kind of regarded as fact but the research does show that we're not entirely sure right is it because the fruit is still warm that the sugars are able to move to the tomatoes to make them sweeter we still operate on that theory but there is also the competing theory of sugar metabolism so the sugar metabolism hypothesis is a bit confusing because it looks a lot like the temperature one that i was talking about but when some scientists did some studies they noticed if they controlled for temperature that something similar happened and it seems that there is some evidence that the sugar accumulation is actually related to a photo period than more than anything and the the idea is that what they're what they found was that these two molecules sucrose phosphate synthase and sucrose phosphate phosphatase they're two enzymes that you don't need to remember those by the way that they're responsible for processing starch to grow things right so um and what they found was that the plant would switch these enzymes on and off depending on photo periods and so by extending the day or shortening the day they could influence the expression of these two enzymes and they would that would impact the distribution of sugar and so that it could also mean that you know playing around with your photo period also messes up with the enzyme so i think in truth like most things in life it's a bit of both uh that's my opinion on it but i thought you might find it interesting that not everything's figured out right so there you go it's very similar to the temperature sugar is made all day and then uh and then it you know prostate processed at night and if if there's not enough at night time then it's not sufficiently processed and it gets stuck in the leaves but like i said there's a there's a photo period element to that let's look at a at a farm and see what their individual practice is kind of instead of just looking at it in terms of you know science and blah blah blah this is a tomato farm in quebec uh this grower's name is audrey bouyen and she works for a farm called savora and they grow tomatoes and they use supplemental lighting no not we don't call it are officially called supplemental so that's in a greenhouse there's sunlight as well right it's a pretty big farm 16 hectares uh and they have two of those hectares have uh lights so uh i sorry two of them do not have light so most of it's lit and some are not now looking at their strategy they provide light through september remember september you start get to shorter days so they increase they supply light during the day and they increase the day length so they're looking at their overall dli going up so typically they're adding up to three to four hours in the morning to get 16 hours per day so not so much at the end of the day but in the morning and in during the day if it goes over 250 watts per square meter they turn the grow lights off if it's cloudy the lights come on uh you know they try to keep things in the cool temperature um now when they get into december through to february that's our dark month right we're talking quebec that's an even higher latitude than here um they provide supplemental lighting every day for 17 hours uh for their beef steak and 18 for their specialties which is your tov and your cherries uh that's a lot of light that's right near the maximum of what tomatoes can handle and they also have to deal with temperature especially when they get towards the later part i've often said march and april very tricky months to grow because even though it's cool outside it can get very hot in the greenhouse quickly and if tomatoes get too hot you end up with a heat delay which will reduce your yield for up to you know you can compress your production for a week or two and you can't have that we need to have fruit coming off consistently so when the grow lights are on they add their own heat and so the greenhouse is set to be more aggressive at opening its vents so such that it takes about 30 minutes to cool by three to four degrees celsius so that means that the vents will open bigger and sooner like larger gaps and sooner to aggressively cool the greenhouse down that's what they call a heat anticipation setting um so that the vents are moving more aggressively because of the added heat [Music] another really interesting practice that they do kind of related to today's talk but i'm going to add it anyways is they in all their tomatoes they typically remove the leaf that is directly above the topmost truss in the plant and they do that so that the sun gets more access at the the growing fruit and if this leaf is gone the light actually comes right to the fruit and hits it directly and that does two things it warms it which helps steer sugars and it accelerates the growth because just warmer tissues grow faster so they get larger tomatoes from that perspective so that's interesting so they go when they're doing their crop management they look at the top of the growing the growing tip and they remove the leaf that is directly above the current flower truss okay um this is just a picture of their farm they do uh inter-crop lighting with led through with a grant they got with the government and with phillips they have this is a lower crop it's growing up and it will cover this so that plants are on either side intercrop lighting is a cheaper way to get light down lower into the crop remember when we started the lesson we talked about light hitting the top and we needed more light because it has to filter down through all those leaves to get to the bottom of the tomatoes well if you put the light in the middle then you can eliminate the lower leaves directly without worrying about heat because it's an led so this is a potential real positive uh impact of leds in the vegetable industry i think a lot of growers would would be doing this right now if it wasn't because of the cost if the cost is significant um but you know it's neat to see all right that's the end of my little talk about lighting