managing the late flower cycle
What is Happening During Late Flower?
For the weeks leading up to late flowering, your plants were focused on adding size and producing various secondary metabolites and plant alcohols. As week seven begins for nine-week varieties, the plant will shift its focus away from creating flower clusters and begin the senescence process.
With this will come some slower feeding and drinking. It will also come with cool color changes in the foliage, flowers, and trichomes. It's no secret that in today’s market, flower color and potency are differentiators that can increase demand for your product.
Let’s look at how growers can alter their environment, fertigation program, and light to optimize the final stage of plant growth and achieve the highest quality flower for you or potential buyers.
Point Break
Environmental Conditions
Regardless of light fixture selection, late flower is when you want your vapor pressure deficit (VPD) higher than 1.0 kPa to reduce the risk of pathogens taking hold and causing crop loss.
I like to push how much color I can get out of my plants during this period, so cool and dry conditions are the goal.
I’ve taken rooms as low as 68 degrees Fahrenheit during this time, but I'll lay out some guidelines due to the practicality of what most setups can do. Here’s a breakdown of how this should look week over week.
Week 7: Ambient | 75F/ 54%, Leaf Surface Temp | 73F, giving you a 1.2 VPD.
Week 8: Ambient | 74F/ 52%, Leaf Surface Temp | 72F, giving you a 1.2 VPD.
Week 9: Ambient | 74F/ 50%, Leaf Surface Temp | 72F, giving you between a 1.3 VPD.
I typically recommend that the temperature drop 8-10 degrees from your daytime setpoint to keep a healthy balance between night and daytime.
However, late flower can benefit from a few more degrees of difference. Increasing your daytime and nighttime temperature differences by 10-12 degrees will encourage each flower to express its unique color fades that much more.
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.
Fertigation and Substrate Concentration
Your fertigation plan is also crucial to consider during the ripening period.
One point of contention between cannabis growers and consumers is when plants are fed more than they need too late into flower, causing undesirable flower morphology and may cause pain to your throat via a “harsh smoke.”
Though things like high moisture content can also cause a harsh smoking experience, overfeeding is undoubtedly a cause for this as well.
Depending on your goals, a few approaches can be taken during ripening. I always recommend to others that the feed concentration remain unchanged from early flowering until the last few weeks.
Feed concentration can taper up to 50% of the original concentration as you progress through weeks eight and nine. It's crucial to NEVER feed lower than 50% of your initial feed concentration, especially in hydroponics, to avoid osmotic shock and weak cell structure within your flower tissue.
Soil growers will never really have a true “flush” in the sense that it's understood today, but that’s honestly okay since it’s ideal for the plant always to have essential nutrients.
I also recommend a generative irrigation strategy during this time, as I’ve had the best success with it.
This means large drybacks, large shot sizes between 4%-8%, and a smaller total irrigation window from your first re-saturation event to last. If you use substrate sensor technology, you may notice a decrease in substrate concentration as you taper your feed downward.
Scooby Snacks
Grown by me, bred by Fletcher at Archive in Oregon
Light Demands
As you enter late flower, your plants have just finished taking peak light intensity values for the cycle. Depending on the goals of your facility or grow space, you’ll want to take one of two approaches to light intensity:
If growing for the highest quality smokable flower, you’ll want to begin slowly tapering your light intensity in week seven. This will mimic a naturally decreasing light intensity and day length experienced outside, promoting the best conditions for flower ripening. An ideal ramping down would be to reduce the light intensity at the canopy by 75-100 micromoles each week.
If you are growing for yield, whether raw flower yield or extraction, you can continue pushing light intensity values through the end of the plant’s life on harvest day. In tandem with keeping your feed concentration the same, the plant will never receive the signals to ripen correctly. This is good for those growing for weight, as the plant will continue to produce new flower tissue.
Remember that the CO2 you supplement should increase and decrease in line with your light intensity values. For example, 1200 micromoles dropping to 1125 micromoles should also be paired with a 75ppm decrease in CO2 from 1200 to 1125. These values need to stay in balance with one another to promote the best plant health and crop production. This rule applies throughout the plant life cycle as you increase and decrease light intensity.
As it pertains to the light spectrum, there are also a few approaches that can be taken. Those with a fixed spectrum light can’t apply the following techniques. For those of you with spectral tuning capabilities, whether presets or complete control over each wavelength:
If growing high-quality smokable flower, it’s advisable to utilize a spectrum with a higher concentration of blue photons toward the end. This has increased the resin production of many varieties, similar to how Type 2 cultivars respond very well to UV exposure in-grow.
If growing for extraction, keeping your light spectrum with a higher concentration of red photons through the end of the cycle is advisable to promote yield increases further.
Ocean Beach ripening
