Whats the DIF F20

Added: 2026-05-14

[00:00:01]okay so in today's lesson we're talking about temperature and how we can use it to shape plants last week we talked about uh how light and temperature have an interplay right so light makes sugar and temperature controls the rate that the sugars are burnt uh in this lesson we're looking at using temperature to shift where the sugars are burnt and how that impacts how the plant grows and obviously the background picture here shows tomato seedlings that are compact and short and uh ones that are long and sort of leggy that's basically what this is about uh and and i'll link it back to poinsettias briefly at the end because they're a classic crop and there are some links in this week's folder which i'd recommend you have a look at that talk about using diff in poinsettia crops and in betting plants but they're used there it's used everywhere so let's let's get into it um so in a greenhouse we can control things with temperature in terms of we can control the actual temperature right set a temperature say one at 20 degrees celsius it's 20 degrees celsius but we can also control the duration of that temperature so how long do we want it at a specific temperature and just like the last lesson we can also link light to temperature and so these are things that as growers we have to take advantage of to control our crops uh so something i introduced last week was the daily average temperature right just like i did the dli the daily light integral so you've got your daily average temperature that's that factors in both day and night so if you have a daytime temperature of 20 degrees c for 12 hours and a night time of 10 your average for the day is 15.

[00:01:57][Music] and today i want to talk a little bit about diff right that's the difference between your day and night temperature so and then of course linking light levels like last week but i'm going to leave that out for now that's temperature versus light intensity we'll address that a little later because obviously as light levels drop there's things that we shouldn't necessarily do um so just to remind you that temperature and light have a sort of an interplay and of course sugars are made during the day when the sun is shining but the key thing here and i'll ask this on the midterm uh make sure you know it is that sugars are consumed 24 hours a day right so nighttime consumption of sugars plays a big role in how plants grow so think of it as like during the day the plant goes to the grocery store loads up with groceries and at night the plant is busy cooking up dinner and feeding itself let's never use that analogy before it just popped in my head but maybe it helps you so nighttime is when a lot of growth happens um oftentimes we see a lot of real stem growth and stretch happens in the morning before the sun comes up um this is another look at calculating daily average temperature so here we've got uh a heating system that's trying to maintain 22 degrees celsius during the day and then at night it's dropping uh down to an average of about 18 and then it's back up again so sort of a typical graph this is probably more realistic of a greenhouse by the way the last graph was from the cannabis bunker which because it's not a greenhouse it's fairly steady um although the heating system's not as precise you'll notice it's sort of fluctuated plus or minus one degree which is not that bad anyways in this greenhouse this is actually a screenshot from jeffrey's greenhouse uh jeffrey's green houses the one on lake street um where the yellow line is our actual temperature measurements right and the blue line is our temperature targets as set to the computer and so you can see the greenhouse is sort of reacting to these set points uh it can't follow them exactly for example say we're going from this temperature down to this one um it drifts a little bit off the line um the sun is starting to rise here so there's some energy being brought into the greenhouse so it's a little trickier to to cool it but uh this is just typical chart showing our calculations here this is our min man mean your average so in this case it's 15 degrees celsius average daily average temperature and you can see here's 15.

[00:04:59]it's kind of right down the middle um so what is the significance of daily average temperature okay so as we increase the daily average temperature so as we increase nighttime and day temperature we increase the rate of node formation which means more leaves more branches faster maturing and faster flowering and a modest increase in internode length keep that one in mind because uh just running it warmer might seem a good idea but there are sometimes a price to pay in this case it would be a stretched plant now depending on the crop this can be beneficial or it can be harmful a lower daily average temperature so that means cooler days and cooler nights will decrease the rate of node formation so fewer leaves and they'll take longer to mature in flower which is logical this is kind of what we'd expect um and from a growth perspective what we're seeing is also we get more root growth so when we talk about shoot to root ratios it reduces it which means we're getting more roots so that's great for earlier stages in crop production uh maybe not so much when you're trying to finish a flowering plant um this is a an interesting chart i don't know if matt got into this with poinsettias but almost all commercial ornamental crops have growth charts that they send out with uh cuttings or you download them in grower sheets and you'll see an upper limit for a height at different weeks and a lower limit and you want to keep your crop in between all the time right so that would mean applying growth regulators or playing with temperature and this crop has it dipped a little bit outside the height range they brought it back it went a little low they brought it back and now they've overshot the last little bits a little bit too tall but not horrible so this is where a grower would put maybe flags out in a crop and go back and measure the same pots over and over again throughout the crop cycle and then here you can see where they decided to apply growth regulators [Music] you can see the correction so here the crop is growing up they probably apply the growth regulator and it it slows down and it starts to recover they apply a growth regulator and it starts to slow down and it starts to pick up whoops that's a terrible line and then it picks up again to here and they apply a third growth regulator and then at that point it kind of just levels off and then you're shipping it out now if they had not applied any growth regulators at all it may well have gone like this and off the chart and you would have a really tall lanky poinsettia that no one wants to buy and if you remember back to my poinsettia lesson the native poinsettias down in mexico and whatnot are really tall skinny like branch things and they're they're not what you're familiar with in a potted poinsettia so that's a really extreme outcome but so that's a growth chart and that's where your daily average temperature uh plays a role so the you have the options of growth regulators and you can also reduce your daily average temperature to to shorten the crop now if you reduce your daily average temperature it's going to take longer to finish so that's a sometimes they don't have that luxury or sometimes your plants are moving too quickly and then you can avoid maybe cancel this growth regulator shot cool it crop down and it'll stay within the height requirement um let's talk about diff diff what is diff diff stands for difference so it's the difference between uh day and night temperature uh and then we'll also talk about uh evening and morning temperatures but for the most part let's just keep it simple and talk about the difference between your day and your night temperature how you calculate diff and it's important you keep this consistent you take the day temperature and you subtract the night temperature because whether this is plus or minus makes a difference okay so keep the sequence the same when you're calculating it so a positive diff means a warmer day a negative diff means a warmer night versus the opposite so in a positive diff your day is warm and your night is cold in a negative diff your night is warm and your day is cold if you're thinking about this you might realize that negative diff would be difficult to accomplish at certain times of the year like in the summer so obviously there is some limitations about when you can achieve negative diffs um what is a negative diff or what does a difference between day and night temperature do so it affects your how sugars accumulate in the plant basically how much and where the sugars go steering sugars uh is a is a really important tool in controlling plant growth you're also your diff affects your net photosynthesis so if you somehow slip off your your daily average temperature you may end up with sort of reduced photosynthesis which means you have less sugars that's where the for example in that poinsettia slide it takes longer to finish because you grew a cool um so if you remember from plant science one plant hormones are distributed in the phloem and of course temperature impacts how sap in the plant flows and typically it flows to the active parts so whatever's still warm so if something is warmer it's going to get more of those plant hormones so if the top of the plant is warm the hormones are going to go to the top so things are going to stay there and have their effect whereas if the top of the plant is cold and the roots are warm things like cytokinin are not going to flow up as much and you get less induction of branching for example depending on the species so again this is the the diff is talking about your difference between your day and your night temperature okay what does diff do all right so if we increase the day versus the night so if we make it a positive diff we get stretched we get taller plants and we get larger leaves and we have more photosynthesis if we increase the negative for the night time temperature right so we get a warmer night and a cool day right warm night then the leaves are smaller and the plant is shorter and part of that is you have more respiration because remember respiration happens all the time and at night it's the only thing happening and so if your nighttime temperature is warm you're going to have more respiration happening so more sugars are consumed and the plant can't build bigger as big cells the following it's falling into the next day so the plant actually lays down smaller cells and in a sense is smaller more compact this is what happened to me when i was obviously a baby um my wife's laughing at me anyway so this chart uh this shows obviously just a weird way to represent plant height anywhere from a 18 degree positive diff down to a negative 18 negative diff um that is really extreme i think you're most likely going to be in down around these areas in reality but there's a big difference in height here so we're going from 12 inches to 14 inches to 2 inches height over the plant so there's definitely some control given to the grower this is a picture of a petunia betting plant the one on the right was grown with a negative diff so negative diff and the one on the left with the positive so you can see a pretty large outcome and obviously people don't want to buy petunias that look like this hasn't even flowered yet uh when they get to the garden center so this is the the compact stuff is what people want and this is a great way to keep bedding plants compact without spraying growth hormones uh and then like the cover slide when we started these are tomatoes grown under positive diff versus negative diff pretty profound difference there so diff also affects reproductive versus vegetative growth i've mentioned this a couple times and you'll hear it more in the winter as we talk about uh strategies with vegetable productions but reproductive growth means more flowers or more fruit and vegetative means more leaves and obviously if you just had fruit alone and no leaves you would have hardly any sugar production which would mean you're not going to get fruit so obviously you need to maintain a good balance of leaf growth versus flower growth so that that balance is critical so as you increase the difference between day and night plus or minus this is either direction so as the temperature change variation increases the plants produce more flowers and more fruits as you reduce the difference between day and night right so there's less impact less difference between your day temperature and night temperature uh your plants are more vegetative and this this often happens um when people are trying to grow flowering plants in a house for example they may not have a big difference between their day and night temperature in their house and they may not get many flowers now the biggest reason why they're not getting flowers is because there's low light in the house but if they have a south facing window and they might say well i don't know i'm not getting a lot of flowers you get lots of light well it part of it might be that you don't have enough of a difference between your day and night temperature now if it's right by a window that seems unlikely because cool air is going to come off the window but you never know um so that's uh one other outcome from diffs so the bigger the spread the more generative or reproductive the smaller the spread the more vegetative the more leaves you grow i tried to create a chart here to help your members so this is diff and we're recording the direction of the diff so plus and minus right so warmer day warmer night a cooler night we get a taller plant bigger leaves warm night cool day we've got a shorter plant smaller leaves and then as we increase or decrease the diff regardless of direction right so over here if we have more diff it's generative and if we have less diff it's vegetative now using both daily average temperature and diff what can we achieve right so right off the bat i want to be clear this doesn't work really well on mature plants or slow growing plants so for example you work in a nursery and you're growing boxwood bushes or maple trees diff's not doing much for you but if you're growing fairly rapidly growing plants like bedding plants or our high wire vegetables for example tomatoes and cucumbers especially um peppers a little bit uh the diff has a bigger effect but definitely at the youngest stage of plant develop is where you see the biggest like remember the slide showing the tomato seedlings those are seedlings fairly young and had a huge impact that impact is a bit diminished as the plant gets bigger so oftentimes diff is done at plug production facilities where we're growing young plants for transplanting um and then daily average temperature is our throttle for achieving a certain ship date so we can control the speed of the crops growth kind of relating to last week and it depends on what our objective is right are we trying to uh get a a perfectly shaped plant or do you want to manage a balance between fruit and leaves or do we want to have smaller leaves and bigger flowers or do we want to have a shorter or taller stem length for example cut flower growers try to achieve certain lengths of stems for quality standards so sometimes they have to stretch those stems and they can use diff to do that so minimal diff keeps plants vegetative accumulates the sugars they stay in the leaves they don't tend to move down to the roots because there's not a lot of temperature difference to encourage the sap to flow so the leaves tend to get larger thicker and if we did a sugar analysis in the leaf we'd see a lot more starch in the leaves so we get a bigger rapid increase in canopy size but you can keep in mind you can diff for a day or two or a few days and then go back to normal if you want to make a change in your crop increasingly negative diff right so night temp temperature is getting bigger than day uh plants become much more generative they start forming buds they start to build more biomass more branching the inner nodes are more compact so some of these are really desirable traits and the downside is that the flowers may may form faster over a smaller canopy that can be good can be bad right but typically and this is definitely a down there are fewer buds and smaller flowers so nothing in life comes for free right so you might get a more compact crop and save on growth regulators but you might have a few smaller and fewer flowers so that's what going back to that chart this is what this part is showing here that more diff ends up with nice compact but slightly fewer flowers and that may or may not be an issue if you're going bedding plants as long as there's sunflowers you're good they're not going to count how many flowers are on it so you're saving a lot of money on growth regulators to from having to spray all your bedding plants to keep them under control before you ship them combining negative dip with your daily light integral is also important right so your nighttime temperature obviously affects how sugars flow from the previous day's photosynthesis and during when there's days where there's actually very low dli so in the winter you don't have a lot of light unless you have grow lights but let's say there's no grow lights oftentimes the plants don't produce enough sugar to grow because the sugar they do produce is consumed by respiration at night so that's why through you know november december early january crops really don't grow a lot unless they have supplemental lighting because they're barely using the sugar providing enough sugar to maintain their own respiration and typically what little sugars they have they've done distributing early in the evening so it doesn't make much difference if you do negative dips in the middle of winter so why pay to have your greenhouse warm at night if your day daylight integrals are very low the following day or the previous day so be mindful of that so with low light integrals we have to apply less temperature increase at night so we don't want to we want to avoid the that negative diff for the most part uh as you get higher light integrals you can do more but you know in the dead of winter not a really great tool uh i can't remember what's on the next slide but the other problem with the diff is obviously diff only works seasonally to some extent so they say the winter doesn't work so well unless you have grow lights but the other problem is if it's in the middle of summer your nighttime temperatures aren't very cold so you can't even achieve much of a negative a reversal in temperature right you can't so diff applies primarily to shoulder seasons which is why we we talk often about fall for poinsettias or early spring for bedding plants because that's when the climate is perfect to use diff to control plant growth without having to spray a lot of growth regulators but in the dead of winter in the middle of the summer not gonna so here we're looking at light accumulation your dli is this red line as the sun is shining your dli starts to accumulate and we're not achieving a significant amount of um dli and so temperatures are floating a little cooler as you have a brighter day and you have a larger dli uh you can let temperatures float higher it's a lot of scribbling but i think you get the idea so oftentimes your computer system automatically tracks and adjusts night temperature and your dli uh accordingly so that's nice you don't have to constantly sit on it but you should monitor check the graphs to see what's going on make sure something's not wrong um one other thing about diff uh if you change the f to a p we call it dip and this is a probably i would say even more common than an outright diff partly because a bunch of us greenhouse guys are cheap and the idea that instead of heating your entire greenhouse for the whole night really warm what a few few people started to find was that if you just did a drop in temperature in the morning uh you could have a larger diff outcome right and that would help a couple of things it would impact sugar flow but it would also shock the plants remember big diffs make plants more reproductive so you could get them to jolt into flower a little bit but as a diff you know it it it basically allows you to sort of control some of the sugar flow uh and and shorten the plants up a little bit anyways it's a bit of counteractive because before this we were talking about uh you know a pre-dawn dip that would seem like that would be more effectively increasing a positive diff right um but the the the reason why this shows differently or behaves differently than a full on negative diff where your warm temperature let's say you have a nighttime temperature and you come into the day and this would be a negative where it's warmer in the night and cooler in the day the difference here is that in the morning is when a lot of the stem stretch happens a lot of growth uh typically if you think about it cells are dividing during the night and as the sun rises those cells fill with water and they start to stretch that's when roots start to drink and the stem elongates and by cooling the air at this stage when the cells are elongating we can actually reduce the influx of water before the cells harden up and we can actually keep the plant short so it's a little counterintuitive to what a positive negative diff was that's what we call it a pre-dawn dip we don't want to call it a diff because it's kind of got a different outcome uh and the nice thing about it is that it works great in seasons when there's cold air outside right because all you do is open your vents or open your curtain and cold air dumps from the top that this is your greenhouse and you got your curtain cold air is trapped up here and then when you open the curtain or the vents this cold air pours down onto your crop and shocks them with cold temperature and you get this pre-dawn dip other options for plant height control that we've talked about but let's might as well review them higher light levels right so brighter light levels limiting phosphorus we talked about that in plug production uh water stress with holding water that for sure works but you have to be careful don't damage the roots mechanical stress is something i have not talked about [Music] mechanical stress is really interesting if you brush plants with your hands it actually causes them to react to that and stiffen their walls which also keeps the cell walls from stretching and so the plants are mechanically are kept short there's a greenhouse in burnington called aldershot that grows miniature roses potted roses and i don't know if you've seen the irrigation boom we have at the college matt's hopefully talked about that he's not using it right now because it doesn't work well on poinsettias but what uh aldershot does is they they drag a plastic bar under the boom as it rolls over the benches to spray to irrigate them and that plastic bar bangs against the plants as it moves along and you might think oh that's going to damage them it doesn't it just gently drags over the plants and that mechanical motion causes the plants to become stiffer stronger and more compact and they've cut their growth regulator applications down i think almost by 75 percent by simply dragging these plastic pipes they did different experiments one of their houses has clear vinyl straps like you see in a cooler hanging on the boom as it drags along both work the other thing we talked about was uh spacing plants right leaving don't be greedy don't put too many plants on a bench otherwise they just compete for height so typically what we try to say is we want to avoid leaves overlapping as soon as one leaf overlaps on other the one that's been covered is going to try to stretch to get over top the other and so you end up your entire crop just starts racing for the sky and it gets stretched [Music] cooler outside conditions before transplanting or hardening off that's possible maybe more of a nursery application from the lighting perspective we'll talk more about this as we go through the program but red to far red you want that high red ratio to keep the cuttings compact you'll notice the lights over our propagation bay have a lot of red in them and that will help to keep plants compact in that area you can also pinch like on the poinsettias but that costs money because you're paying someone to pinch and pinching can be inconsistent and then of course we've also mentioned growth regulators finishing up i got a an interesting slide here this was a study done down the states where they looked at using different temperatures not so much diff between day and night but difference between the zone so this was what we call root zone heat so they had heat right on the pots and they had cool air temperatures and what they found was as they raised the temperature of the root zone substantially they were able to achieve uh a really interesting outcome so looking at the finished product it's almost identical to the last column which is conventionally produced with growth regulators this side has no growth regulator so that's pretty cool uh i don't know that a lot of people do this because running really warm like 27 degree roots on temperatures costs money so anyways just thought you might find that interesting since we've been talking about poinsettias all right lastly uh finish up with this as opposed to the pre-dawn dip at the beginning of the day there is also a pre-night dip and this is interesting from a tomato production perspective this is common practice especially in tomatoes and i mentioned this in one of the labs at the greenhouse where just before sunset you drop the temperature of the air abruptly and what happens is the fruit which is thick and has lots of water in it which has a high heat capacity the fruit stays warm and the leaves and the stems cool off and what happens is sugars will flow because the of the warmer fruit sugars will flow from the leaves into the fruit and that's how you make tomatoes sweeter from a commercial standpoint so pre-night dip increases sugar flow to the fruits so as the air temperature drops at the end of the day in the greenhouse when you open your vents briefly the sugars that are in the leaves will then flow to the more active fruit so you got your your leaf here and then you got a branch with tomatoes growing the sugars will flow to the fruit not sure why i drew that doesn't even look like anything okay so we've talked about daily average temperature we've talked about diff diff causing if you even have a difference that it's more reproductive if there's no difference between day and night the plan is vegetative and then we talked about uh pre-dawn dips and now pre-night dips all right thank you