Maximum dense growth regarding temperature?

I've realized I have a lot of issues with plants grown when my temps and humidity are off(particularly in the hot summer months, my windows are oddly shaped and I have to use a very inefficient portable A/C). In the summer months my plants are always excessively whispy and in the winter they're too dense. I don't know about everyone else, but I HATE having whispy stemmy buds or small dense ones. I've just done some extra reading on plant respiration and photosynthesis regarding temperature and humidity... and I struck a few ideas.

If respiration increases at higher temperatures in relation to photosynthesis and decreases at lower temperatures, by observing our plants could we not coincide this with it's current development? For instance, after growing a bit we begin to notice when the plant is going through a growth spurt(different for times for all plants, particularly between indicas and sativas). If we were to put the temperatures on the lower end of the ideal range to store energy when the plants are going through a -stretch- spurt but put the temperatures to the higher end of the ideal range when going through a -bulk- spurt, would this not make a significant improvement by lowering their stretch and increasing bulk? I've read that a 10' Celsius change in organisms generally doubles respiration. So I'm assuming that a 2' or 3' Celsius change would still have a decent impact on plants within their ideal growing temperatures. Of course this is provided it does not get too cold to slow photosynthesis or too hot to hinder the enzymes involved in respiration... but still effectively changing when the plants use their energy.

Also, before I try to look it up... does anyone know whether humidity significantly affects respiration or just photosynthesis? I would assume it only effects photosynthesis. I was thinking that if the humidity were raised during cold/photosynthesis periods(when the plants would be stretching) and the humidity lowered during warm/respiration periods(when the plants would be bulking) it may help restrict the use of nutrients during photosynthesis and facilitate the use of nutrients during respiration. Normally, I've noticed a lower humidity decreases PH due to the increased evaporation of water through the plant and soil. By lowering the humidity and providing more water during respiration/bulk periods the plant will be stripping the soil of water faster than during photosynthesis periods. By restricting plant growth and nutrient usage during photosynthesis stages the plant will require less nitrogen and be less nitrogen deficient during flowering. I believe this will make a high PK nutrient more applicable during flowering while still maintaining the desired PH... effectively changing when the plants use the reserved nutrients contained inside of their leaves and soil while helping hold in line PH swings which would otherwise be affected by humidity and require changes in nutrient application.

Forgive me if any of these are ignorant ideas, I haven't had any specific schooling regarding botany and they're mostly based on logic/assumptions. I'd appreciate any input, particularly constructive criticism because I'd like to know where my thinking may be flawed.

I believe I remember a few things..

I know for a fact warmer soil with slightly cooler ambient air helps the plant healthwise, I don't know if it affects density though.

I believe it was Slightly cooler days. Or maybe the other way around.

But I know its only a small difference for max effect.

Hope that helps or gives you something to look up.

I think that helps the plant by encouraging usage of nutrients and growth in the root zone as opposed to foliage (following the concept of higher heat increasing respiration) which would make sense. For actual consumption of energy, I would think a lower root zone temperature and lower air temperature would slow respiration(growth). I think this is in part why it's suggested that nights be cooler, so less energy/sugars and nutrients are used at night than during the day. It helps the plant stockpile nutrients and sugars for usage during day time, or so I would assume. Thank you for posting C.Indica.

Wait does slower growth necessarily induce dense branching?
Can dense growth happen on cannabis in a growth spurt?
I had a plant that got to be like 20 branches tall, but it was like 8" tall.
He was beautiful.
But I killed him when I found his balls.

Stupid manbush.

C.Indica said:

I believe I remember a few things..

I know for a fact warmer soil with slightly cooler ambient air helps the plant healthwise, I don't know if it affects density though.

I believe it was Slightly cooler days. Or maybe the other way around.

But I know its only a small difference for max effect.

Hope that helps or gives you something to look up.

Click to expand...

The most productive temperatures for your plants are 85F day and 75F night. Of coures this will slightly vary from strain to strain. During veg have a much smaller difference in day to night temperature as this will promote growth of the plant's foliage. Ex: 80 day and 76 night. I do not grow in soil but in hydro keep water/medium temps in the 60s. I would think that soil would be pretty similar to hydro regarding soil and ambient air temps so I hae to disagree with a cooler air temp than soil temp. But like i said I don't grow soil may be wrong on that part. Slower growth does not create dense branching, if anything dense branching may slow down the growth of the plant a little, but this is all based on the genetics of the plant. Good Luck.

Relative humidity (the relative part is key to understand as that takes temp into account as well) plays a huge role in a plants respiration. The lower the humidity, the more water the leaves transpire causing the plant to uptake more water and nutrient to replace what was lost. Under high heat/low RH conditions this can lead to burning your plants if you aren't careful.

Soil temperatures slightly higher than air temperatures only stimulate underground growth. In a plant's case, this would be root formation.
This could in my eyes be a problem if in flowerpots, as it would eat through dirt and space like a fat kid. But if your growing outdoors in mother natures dirt, it would probably be the best. And it probably happens naturally.

I don't know if this means growth aboveground is slowed, or if it is sped up exponentially due to new roots.

This is just stuff for us potheads to chew on and spit back out, I'm sober right now haha.

the real issue with humidity is this, the Stomata open and close depending on humidity levels. 60% is ideal from a growing point. anything lower and the Stomata start to close up, to the stomata being the photo sin motor open is better to gobble up all that co2. Here is a longer version enjoy and grow big.....

[FONT=&quot]Plantworks: Part 1 – Humidity and Vapor Pressure Deficit[/FONT]
[FONT=&quot]By [/FONT][FONT=&quot]Urban Garden Magazine[/FONT][FONT=&quot]⋅[/FONT][FONT=&quot] July 12, 2010 [/FONT][FONT=&quot]⋅[/FONT][FONT=&quot][/FONT]

[FONT=&quot]“Think like a plant.”[/FONT]
[FONT=&quot]Have you ever been given this odd-sounding advice? Even when we are encouraged to try and understand how plants work, our inherent tendency to personify the natural world is inescapable. Growers often like to draw parallels between humans and plants, after all, there’s no doubt that plants are marvellous, highly specialized and well-adapted organisms. You might even go as far to say they are “intelligent.” But let’s be honest here. Plants are totally different from us, especially in the way they react and respond to their environment. However, if we can get our heads around the world from a plant’s perspective, we become what is commonly referred to as “green-fingered.” We become … better growers.[/FONT]
[FONT=&quot]Have you ever wondered how plants “feel” humidity? An understanding of what humidity is, what it means to plants, and how you can manage it in your indoor garden will help you and your plants stay happy all year round.
The humidity of the air is basically the amount of water in the air. Water can only truly stay in the air when it is the invisible gas – water vapour. Small droplets of water in air, such as fog or mist, are not water vapor; they are simply larger particles of water temporarily suspended in the air that are ready to be turned into water vapour by evaporation.[/FONT]
[FONT=&quot]Temperature plays an important role when it comes to humidity. The warmer the air, the more water vapour it can hold. This means the maximum amount of water that air can hold is directly related to the temperature of the air. As the amount of water air can hold constantly changes with temperature it is difficult to pin an absolute or fixed amount of water that can be held by air. So what’s the best way to quantify humidity if the goal posts are changing all the time? The answer is something called Relative Humidity (RH) – this is a measure in terms of percentage, of the water vapor in the air compared to the total amount of water vapor that the air could potentially hold at a given temperature.[/FONT]

[FONT=&quot]Why is RH so important?[/FONT]
[FONT=&quot]As growers we measure the RH of our gardens using digital or analogue hygrometers. These readings are very important because RH has a direct effect on the plant’s ability to transpire and therefore grow. Generally, plants do not like to lose lots of water through transpiration. Plants have some degree of control of their rate of transpiration through management of their stomata but the general rule is the drier the air, the more plants will transpire.
Now let’s move on to the idea of “pressure” – this is an important concept to grasp when it comes to understanding a plant’s response to humidity. All gasses in the air exert a pressure. The more water vapor in the air the greater the vapor pressure. This means that in high RH conditions there is a greater vapor pressure being exerted on plants than in low RH conditions. High vapor pressure can be thought of as a force in the air pushing on the plants from all directions. This pressure is exerted onto the leaves by the high concentration of water vapor in the air making it harder for the plant to ‘push back’ by losing water into the air by transpiration. This is why with high RH plants transpire less. Conversely, in environments with low RH, only a small amount of pressure is exerted on the plants’ leaves, making it easy for them to lose water into the air.[/FONT]

[FONT=&quot]What is Vapor Pressure Deficit (VPD)?[/FONT]

[FONT=&quot]VPD can be defined as the difference (or deficit) between the pressure exerted by water vapor that could be held in saturated air (100% RH) and the pressure exerted by the water vapor that is actually held in the air being measured.
The VPD is currently regarded of how plants really ‘feel’ and react to the humidity in the growing environment. From a plant’s perspective the VPD is the difference between the vapor pressure inside the leaf compared to the vapor pressure of the air. If we look at it with an RH hat on; the water in the leaf and the water and air mixture leaving the stomata is (more often than not) completely saturated -100% RH. If the air outside the leaf is less than 100% RH there is potential for water vapor to enter the air because gasses and liquids like to move from areas of high concentration (in this example the leaf) into areas of lower concentration (the air). So, in terms of growing plants, the VPD can be thought of as the shortage of vapor pressure in the air compared to within the leaf itself.[/FONT]
[FONT=&quot]Another way of thinking about VPD is the atmospheric demand for water or the ‘drying power’ of the air. VPD is usually measured in pressure units, most commonly millibars or kilopascals, and is essentially a combination of temperature and relative humidity in a single value. VPD values run in the opposite way to RH vales, so when RH is high VPD is low. The higher the VPD value, the greater the potential the air has for sucking moisture out of the plant.
As mentioned above, VPD provides a more accurate picture of how plants feel their environment in relation to temperature and humidity which gives us growers a better platform for environmental control. The only problem with VPD is it’s difficult to determine accurately because you need to know the leaf temperature. This is quite a complex issue as leaf temperature can vary from leaf to leaf depending on many factors such as if a leaf is in direct light, partial shade or full shade. The most practical approach that most environmental control companies use to assess VPD is to take measurements of air temperature within the crop canopy. For humidity control purposes it’s not necessary to measure the actual leaf VPD to within strict guidelines, what we want is to gain insight into is how the current temperature and humidity surrounding the crop is affecting the plants. A well-positioned sensor measuring the air temperature and humidity close to, or just below, the crop canopy is adequate for providing a good indication of actual leaf conditions.[/FONT]

[FONT=&quot]Managing Humidity[/FONT]
[FONT=&quot]Managing the humidity in your indoor garden is essential to keep plants happy and transpiring at a healthy rate. Transpiration is very important for healthy plant growth because the evaporation of water vapor from the leaf into the air actively cools the leaf tissue. The temperature of a healthy transpiring leaf can be up to 2-6°C lower than a non-transpiring leaf, this may seem like a big temperature difference but to put it into perspective around 90% of a healthy plant’s water uptake is transpired while only around 10% is used for growth. This shows just how important it is to try and control your plants environment to encourage healthy transpiration and therefore healthy growth. So what should you aim to keep your humidity at? Many growers say a RH of 70% is good for vegetative growth and 50% is good for generative (fruiting /flowering) growth. This advice can be followed with some degree of success but it’s not the whole story as it fails to take into account the air temperature.[/FONT]

[FONT=&quot]Humidification systems to increase RH.[/FONT]
[FONT=&quot]Table 1 shows the VPD in millibars at various air temperatures and relative humidity. Most cultivated plants grow well at VPDs between 8 and 10, so this is the green shaded area. Please note that the ideal VPD range varies for different types of plants and the stage of growth. The blue shaded are on the right indicates humidification is needed where the red shaded area on the left indicates dehumidification is needed.[/FONT]

[FONT=&quot]By looking at this example we can see that at 70% RH the temperate should be between 72-79°F (22-26°C) to maintain healthy VPDs. If your growing environment runs on the warm side during summer, like many indoor growers, a RH of 75% should be maintained for temperatures between 79-84°F (26-29°C.)[/FONT]
[FONT=&quot]The problem with running a high relative humidity when growing indoors it that fungal diseases can become an issue and carbon filters become less effective. It is commonly stated that above 60% RH the absorption efficiency drops and above 85% most carbon filters will stop working altogether. For this reason it is good practice to run your RH between 60-70% with the upper temperature limit depending on your crop’s ideal VPD range, in the example it would be 64-79°F (18-26°C.)[/FONT]
[FONT=&quot]The table also shows that if your temperature is above 72°F (22°C), 50% RH becomes critically low and should generally be avoided to minimize plant stress.
Please understand that by presenting this information we do not want you to go to your indoor gardens and run your growing environment to within strict VPD values. What’s important to take from this is that VPD can help you provide a better indication of how much moisture the air wants to pull from your plants than RH can. If you want to work out for yourself the VPD of your plants leaves you can follow the steps below:[/FONT]

1. [FONT=&quot]Measure the leaf temperature and look up the vapor pressure at 100% RH on table 2 below.[/FONT]
2. [FONT=&quot]Measure the air temperature and relative humidity and look up the nearest vapor pressure figure on table 2.[/FONT]
3. [FONT=&quot]Subtract the air vapor pressure from the leaf vapor pressure[/FONT]

[FONT=&quot]Example:
Leaf Temperature = 24°C (100% RH) Leaf VP: 29.8
Air Temperature = 25°C @ 60% RH Air VP: 19.0
VPD= 10.8[/FONT]

[FONT=&quot]Humidity’s Effect on Plants[/FONT]
[FONT=&quot]Plants cope with changing humidity by adjusting the stomata on the leaves. Stomata open wider as VPD decreases (high RH) and they begin to close as VPD increases (low RH). Stomata begin to close in response to low RH to prevent excessive water loss and eventually wilting but this closure also affects the rate of photosynthesis because CO2 is absorbed through the stomata openings. Consistently low RH will often cause very slow growth or even stunting. Humidity therefore indirectly affects the rate of photosynthesis so at higher humidity levels the stomata are open allowing co2 to be absorbed.[/FONT]
[FONT=&quot]Leaf roll on Thai basil- Localized humidity stress causes by the lights being too close.[/FONT]
[FONT=&quot]When humidity gets too low plants will really struggle to grow. In response to high VPD plants will try to stop the excessive water loss from their leaves by trying to avoid light hitting the surface of the leaf. They do this by rolling the leaf inwards from the margins to form tube like structures in an attempt to expose less of the leaf surface to the light, as shown in the photo.[/FONT]

[FONT=&quot]For most plants, growth tends to be improved at high RH but excessive humidity can also encourage some unfavourable growth attributes. Low VPD causes low transpiration which limits the transport of minerals, particularly calcium as it moves in the transpiration stream of the plant – the xylem. If VPD is very low (95-100% RH) and the plants are unable to transpire any water into the air, pressure within the plant starts to build up. When this is coupled with a wet root zone, which creates high root pressure, it combines to create excessive pressure within the plant which can lead to water being forced out of leaves at their edges in a process called guttation. Some plants have modified stomata at their leaf edges called hydathodes which are specially adapted to allow guttation to occur. Guttation can be spotted when the edges of leaves have small water droplets on, most evident in early morning or just after the lights have come on. If you see leaves that appear burnt at the edges or have white crystalline circular deposits at the edges it could be evidence that guttation has occurred.[/FONT]
[FONT=&quot]

[/FONT]
[FONT=&quot]Guttation on tomato plants caused by high RH and wet coco coir.[/FONT]

[FONT=&quot]Powdery Mildew from poor humidity control.[/FONT]
[FONT=&quot]Most growers are well aware that with high humidity comes and increased risk of fungal diseases. Water droplets can form on leaves when water vapor condenses out of the air as temperature drops, providing the perfect breeding ground for diseases like botrytis and powdery mildew. If humidity remains high it further promotes the growth of fungal diseases. The water droplet exuded through guttation also creates the perfect environment for fungal spores to germinate inviting disease to take hold.[/FONT]
[FONT=&quot]Quick reference chart:[/FONT]
[FONT=&quot]Low VPD / High RH[/FONT]
[FONT=&quot]High VPD / Low RH[/FONT]
[FONT=&quot]Mineral deficiencies[/FONT]
[FONT=&quot]Wilting[/FONT]
[FONT=&quot]Guttation[/FONT]
[FONT=&quot]Leaf roll[/FONT]
[FONT=&quot]Disease[/FONT]
[FONT=&quot]Stunted plants[/FONT]
[FONT=&quot]Soft growth[/FONT]
[FONT=&quot]Leathery/crispy leaves[/FONT]
[FONT=&quot]So hopefully now you are not just ‘thinking like a plant’ – you’re ‘feeling it’ too![/FONT]
[FONT=&quot]Next time, part two of Plantworks will be looking at foliar spraying and how plants absorb nutrients into their leaves.[/FONT]
[FONT=&quot]
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So the 40% humidity that a lot of growers aim for in their flowering room is too low and will cause the stomats to close up???