Low Humidity + Intense Light = Yellow New Growth?

[2bags]

High all,

I recently moved two mature seed plants from their vegging area under a 200W blue CFL to the flowering area under a 400W HPS.

The flowering area is a bit cooler but much less humid as the air exchange is much higher. The air coming in is cold and dry. I dont have a humidity meter unfortunately.

The HPS is more than 2 feet above the plants, yet they instantly went into shock once in the flowering room.

They wilted for the first couple of days but are back to normal now, however since then some of the youngest new shoots are yellowing along with some of the unshaded younger fan leaves. The leaves feel dried out and almost look mg deficient but I know it isn't that due to the fact it's only the shoots closest to the light affected. Lower fans and shaded shoots remain dark lush green.

Could this be a result of the low humidity and intense HPS?

I can't raise the lights any more, but I've seen similar problems when lights have been too close, drying out the leaves. I can't see how over 2 feet is too close for a mature plant that has been vegging almost three months.

I'm thinking it was to do with air flow and humidy but what do I know? Do any experts have an opinion on this? If so I'd be much obliged to hear it!

Thanks for your input!

1

 

[DabOnDabs]

VPD - Vapor Pressure Deficit

As implied by the word "deficit" we're talking about the difference between two things. In this case, it's the difference between the theoretical pressure exerted by water vapor held in saturated air (100% RH at a given temperature) and the pressure exerted by the water vapor that is actually held in the air being measured at the same given temperature.


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.

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.

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.


SOOOO what do you want your humidity to be?...Try to keep it in the orange, or the white at worst.

I use 70% for veg-> end of stretch
50%->last 2 days,
as low as possible last two days before chop to promote resin oozage before chop

Source:
http://www.just4growers.com/stream/...-deficit-the-hidden-force-on-your-plants.aspx




I have massive air exchange in my room, but I'm able to get my humidity up using foggers + climate controller to turn off my fans while the foggers are on. Then the fans turn on, change out all the air in the room, cool it down and the process starts over..every 10-15 minutes

 

[DabOnDabs]

As far as your question, your plants could certainly also be suffering from lumen shock, although a 200w increase isnt too much, the different color (HPS vs. Blue) could do it. Along w/ the low humidity, you're asking for a sketchy shift from veg --> flower. (I should know I JUST got done doing the same shit)

 

[2bags]

Thanks for the info DabOnDabs

So do you think lumen shock and/or low RH could cause yellowing and drying out of young shoots?

1

 

[DabOnDabs]

Thanks for the info DabOnDabs

So do you think lumen shock and/or low RH could cause yellowing and drying out of young shoots?

1

Post a picture, but from what you're describing..yes, i do. It could also be something else, but If you we're rocking, in a better dialed environment, and then you took your plants out and put them in this new environment and they started to nose dive..well yea, Im gonna blame that.

Get a fogger, or a ultrasonic humidifier...Fans blowing over buckets of warm water, towels, spray the walls down if you have something on the walls that wont mold. I would try to get the humidity up.