@Prawn Connery @Grow Lights Australia @Rurumo @DoubleAtotheRON
How high do you run RH with LEDs?
I am at 3rd week of flowering and 75-80% RH. I guess below 60% RH till the end?
Hi mate, I'm not sure how I missed this. I hope I'm not too late. I'll try to address two issues: VPD (vapour pressure deficit), and fungal infection.
VPD measures the relationship between temperature and humidity to determine healthy plant transpiration (evaporation at the leaf surface).
Warm air can absorb more moisture than cool air. So if your grow room is warm, you can get away with higher humidity than when it is cool. This is because the relative humidity will determine how fast evaporation can occur – which is important for plants, as evaporation at the leaf surface creates negative pressure inside the xylem (part of the stem) which draws nutrient-rich water into the plant from the ground via the roots. Transpiration is one of at least three ways plants obtain nutrient (reverse osmosis and active transport, or ATP, being the others – there is also a fourth way through direct absorption through the leaf via foliar spray).
When transpiration slows, so does water and nutrient uptake. Here is a VPD chart. Notice that the green (optimum VPD) areas have a bigger buffer zone in relation to humidity when ambient air tempertaures are lower, but that higher humidity is required when air temperatures increase. This is to prevent
overtranspiration at higher tempertaures. Which is to say that when temperatures get hot, there will be more evaporation (transpiration) unless there is higher humidity to slow that transpiration down.
Remember, in the tropics it is hot and humid and harder for humans to cool down because sweat can't evapoarate as quickly due to the air already being full of water (humidity). It is the same with plants and transpiration.
At 75-80% humidity, your ideal ambient temperature ranges from 22-32C. BTW, your grow room will always have a higher humidity than the rest of the house because there is a lot of water (wet pots and transpiring plants etc) inside the grow.
On to mould. Let's have a think about this: cannabis sativa evolved in equatorial regions where relative humidity is nearly always very high. So sativa plants have a few defence mechanisms against fungal infection – these include a much less dense structure (skinnier leaves, longer internodes, whispy flowers etc) compared to indicas, as well as natural cell-wall defences.
Indicas, BTW, evolved in Sub and Central Asian highland areas where humidity is lower, hence why they are more prone to mould (due to their denser structure).
Resistance to mould very much depends on strain!
For mould to take hold it needs a spore, a host and moisture.
You can't do anything about the host, because you are trying to grow plants, which are an ideal host for mould spores (because they are also full of water).
SMould spores are everywhere, so you can never really get rid of them.
Moisture is an interesting one because a dehumidifier is the first thing many people think of to control humidity, but as mentioned above, humidity can be controlled to an extent by increasing temperatures. As air temperature goes up, it is able to absorb more water. But contrary to what you think might happen – that humidity also increases as the air absorbs more water – this is not actually the case.
The reason it is called "relative" humidity is becaue humidity is relative to the amout of water a parcel of air can hold, and that amount goes up as temperature increases. That means, if the air temperature increases but there is no source of water to evaporate into it, relative humidity goes down! Conversely, as a parcel of air cools, its ability to absorb and hold water decreases – which is why you get dew on a cold morning, or condensation on a car window (hot, moist air inside the car, cools when it touches the cold window and water condenses on the glass).
The key to fighting mould is AIRFLOW. When you increase airflow, you do three things:
1. Prevent mould spores from taking hold of the host plant (the spores get blown off)
2. Reduce moisture through evaporation in and around the plant (excess moisture is wicked away from the plant by moving air)
3. Remove humidity from the grow area via air exchange (increase air ciculation around the plants, but also increase airflow in and out of the grow area).
So this is all a long way of saying that if you are worried about mould – and before you go out and buy that dehumidifier – make sure your grow room temperatures are optimum (28-30C if you can) and ensure you have lots of airflow not only around the plants (especially through the lower canopy) but through the grow area itself (extractor fans etc).
If you ensure good airflow in and around your grow, you will reduce – if not eliminate – all your mould problems.
