After 2-3 weeks of the 12-12 light schedule, most cannabis plants will show the first signs of their gender (they either are a female plant which starts growing buds, YAY! or they are a male plant which start growing balls/pollen sacs, NO!).
If you're growing a cannabis plant grown from a random seed ("bagseed"), unless you somehow have an auto-flowering seed, you will need to understand about cannabis life stages and how they are affected by light periods.
In the wild, as the days get shorter and nights grow longer, a marijuana plant "realizes" that winter is coming and will start budding/flowering. It "knows" it's approaching the end of its life cycle so it frantically starts making buds in time before winter.
Cannbis plants have two life stages:
Marijuana plants have an internal process that allows them to detect how long they receive darkness each night. This is because they are a "photo-period" plant, specifically a "short-day" plant which means these plants start making flowers/buds when days start getting short.
Cannabis at the vegetative state is hardier than a baby plant; hence, it can withstand higher VPD concentrations. Generally, aim to slowly raise the VPD throughout this stage until it tops at 1.0 kPa. Note that CO2 absorption is vital here, so try not to increase your VPD dramatically.
Lastly, the flowering stage can withstand higher VPD but requires lower humidity. Slowly raise the VPD from 1.0 kPa until it hits around 1.4 kPa.
The key to calculating the right VPD is to check the growth stage of the plant. The plants are relatively fragile at the clone or baby stage, so they can’t handle too much stress. They also need higher rates of nutrients to compensate for their underdeveloped roots. So, as a rule of thumb, aim for a VPD range of around 0.5 – 0.7 kPa.
Finding the Right VPD for Each Growth Stage
The stomata are the first area of the plant where you’ll notice a change. Higher VPD will cause the stomata to constrict to prevent excessive water loss. As a consequence of smaller stomata, the amount of CO2 the plant absorbs reduces as well, which will directly affect photosynthesis. Thus, adjusting VPD will enable you to fine-tune this process.
Light intensity is one of the main factors that can indirectly affect VPD as it can increase or reduce the plant’s temperature. Higher temperatures lead to more VPD content.
Higher VPD content will also speed up the transpiration of water from the leaf to the air. In turn, this increases the ability of the roots to absorb nutrients from the soil. However, there is also a downside to excessive VPD and is the reason why you need a cannabis VPD chart. Weed can be highly sensitive to VPD, as more of it can stress the plant, negatively affecting its quality, growth, and yield.
How Does VPD Affect Cannabis Growth?
It’s recommended to get bulbs with a higher intensity from the start. That’s because it’s easier to reduce intensity than it is to increase it. What’s more, higher levels lead to denser plants with higher yields.
Making sure the VPD isn’t too low where the plants are unable to evaporate enough water to encourage the transport of nutrients is very important for plant health. In cases where VPD is extremely low, water condensation can provide a situation for fungal growth and other diseases, which is exactly what we are trying to prevent.