Managing cannabis plants in midflower
What is happening mid-flower?
At this point in the cannabis plant life cycle, your plants have bolted or stretched as much as they can and won’t be growing vertically anymore. They’ve also set all the points at which flowers will grow, known as flower sites.
From weeks four to six, your plants will focus on increasing their flowers’ overall size and density in hopes of receiving pollen. However, cultivators who grow to consume and produce at scale aim to prevent that, so the flower created is seedless.
A lot of money can be made and lost for those producing it commercially, so it’s crucial to understand how to alter your environment, fertigation plan, and light to optimize this period.
Two tables of The Soap, Week 5
Environmental Conditions
Regardless of light fixture selection, mid-flower is when you want your vapor pressure deficit (VPD) at 1.0 kPa or as close as you can get to that value when you start week four. As the weeks progress, you'll slowly increase the VPD to 1.2 kPa.
I want to keep daytime or lights-on temps between 75F and 79F during this time. The relative humidity should stay between 56% - 58%, and leaf surface temperature should vary between 73F - 75F.
Here’s a breakdown of how this should look week over week.
Week 4: Ambient | 78F-79F/ 58%, Leaf Surface Temp | 75F, giving you a 1.0 VPD.
Week 5: Ambient | 77F/ 57%, Leaf Surface Temp | 75F, giving you a 1.1 VPD.
Week 6: Ambient | 75F/ 56%, Leaf Surface Temp | 73F, giving you between a 1.1 - 1.2 VPD.
To keep a healthy balance between night and daytime, the temperature can drop 8-10 degrees. Just keep in mind that as this temperature decreases, your VPD will increase by default without altering your humidity to slightly lower when lights are off. Youll want to alter your humidity to keep the same VPD regardless of your lights being off.
But as you can see from the guidelines above, you’ll want to slightly increase your VPD as you approach late flower, a period of flowering we will cover in next week’s newsletter.
Fertigation and Substrate Concentration
Your fertigation plan will also greatly dictate how well your flowers will bulk up during this time.
I always recommend to others that the feed concentration remain unchanged from early flowering until the last few weeks, when it can taper by up to 50% of the original concentration. I’m a huge advocate for using the same input EC from veg to late flowering. The main thing that should change from veg feeding to flowering is not the concentration, but the nutrient composition itself.
In tandem with an ideal VPD that allows for good transpiration, your fertigation volume and frequency should also be catered to during this period.
During bulking, a few different approaches can be taken. I see some growers altering their fertigation schedule to a more vegetative shot size during this period before switching back to a generative approach late flower with great success.
I recommend a generative irrigation strategy during this time, as I’ve always had the best success with it personally. This means larger drybacks than early flower, resulting in a higher substrate EC. It also means heavier shot sizes between 4%-8% and a smaller total irrigation window from your first re-saturation event to last. This will ensure optimal nutrient uptake by your plants, allowing you to achieve the best yield and flower quality possible.
Grease Monkey, Week 5
Light Demands
Research on light science has demonstrated that light is a dynamic growth input. Light is the culmination of its intensity or power, spectrum or the combination of various wavelengths and colors, and duration or photoperiod.
Midflower is when your plants are bulking up and adding most of their flower weight. To achieve the greatest flower yield and quality possible, it is ideal to implement the following strategies:
Find each variety's light saturation point. Your entire light schedule should revolve around this value.
Achieve peak light intensity during mid-flowering. Your flowers have set. Your environment is optimal. Now is the time to full send
Utilize a light fixture with a fair concentration of red photons to promote further yield increases. If you have the luxury of spectral change, experimenting with a higher concentration of red and far red photons during this period will encourage yield increases,
Using a nine-week variety as our example: You’ll want to push the light intensity values upward until you reach your peak intensity in week six to get the greatest yield and quality out of your crop.
If you have varieties that prefer lower light intensities overall, you’ll look to push them to 1000-1050 micromoles. Most varieties under systems that can push higher light intensity values seem to like the 1200-1300 micromoles range. Some varieties have extremely high light saturation points. I’ve grown varieties that can handle upwards of 1600 micromoles. Still, studies have demonstrated that some varieties can have a linear relationship between crop yield and increasing light intensity up to 1800 micromoles.
Though it may sound crazy to those who’ve only grown indoors and under lower light intensity, a clear summer’s day can produce more than 2000 micromoles at the plant canopy. But growers should always consider that each variety has its own unique light saturation point, and beyond that point, you’ll see plant health and flower quality diminish.
The Soap - (Animal Mintz x Kush Mintz)