Cycling light intensity during flower (interesting article)

[Slippy One]

Apologies if this has been discussed before, I stumbled across this accidental discovery at a Oklahoma cannabis company. It's about cycling light intensity during flower, thought it might be interesting to share/discuss. 

 

This only applies to those with light dimmers, but if you're between grows looking for cost saving mods, or just curious, week 1-3 of flower keep the lights dimmed - weeks3-6 full lights, then back to dim for week 6-9+. 

 

I'd be interested how many already do this, and any advice or tips regarding PAR levels etc. Happy plants, bigger, denser buds, less electric.. win/win?

 

Anyway, here's the article: dated 2021

 

"

How we stumbled upon this discovery about cycling light intensity: Our Story

So we built a new grow room that ended up having a lot of electrical issues. I’ll spare you the details of this catastrophe of contractors, but it’s safe to say, we had problems.

After the room was constructed, we immediately noticed problems. We had constant issues of flickering lights. We eventually discovered the cause, and had new electrical circuits installed. After the new electrical work was completed, we thought we were good to go.

Under the impression that things were good to go, we placed our plants in this room, under the 12/12 light cycle. We were using HPS lights in this room, and we had about 60 total lights.

Almost immediately, we had problems. Lights were still flickering. This time, the circuits were correct. But the actual fixtures themselves were defective. Which meant we had to make a warranty claim and get these lights replaced.

The problem: we already had about 600 plants in the room. And we need to stay on schedule.

We were down to only 15 HPS lights out of 60

Each passing day, we had more and more lights going out — and the amount of warranty claims going up.

We ended up with only about 15 lights on and functioning, in a room built to have about 60 lights. That means we only had one-fourth of the amount of lights blazing and we were now a couple weeks into flower.

We worked diligently with the hydro supply store to get our warranty claims submitted. And then waited on the new lights to arrive.

As we were waiting for the replacement lights to arrive, we nervously watched this room. We had 600 beautiful plants under only 15 lights. We kept worrying that we were just wasting potential, that there was not enough PPFD in the room to get the light density needed to produce real flower.

We were three weeks into flower, and everything looked like it was ready to just take off. The plants had been pruned in veg, and the tops had grown almost perfectly through the trellis netting. So far everything was looking great!

Installing the new lights during Week 4 of Flower

Now in the fourth week of flower, we were beginning to freak out. The plants looked phenomenal, but we just couldn’t imagine that things would continue to get better. Surely, the quality of our bud would take a nose dive with only 15 lights in the room. There is no way that could be enough intensity to produce quality bud.

But then, some of the replacement lights arrived during Week 4. We hurried to assemble and hang the extra lights in the room. And once they were hung, we fired them up.

Later in the week, more lights arrived. And then in Week 5, we received another shipment. We just continued to hang lights as they arrived.

Observations about the plant response to the additional lights

After the replacement lights were hung, we now had close to about 60 lights in the room. The plants never once looked like they were suffering. And now they were thriving.

By Week 6, the room just looked fat. I mean, fat fat. These plants were huge, the colas were chunky. Everything looked healthy and happy.

And what we began to realize was that a theory we had begun to develop was being validated right before our eyes. And from there, we began changing some of our grow practices and subsequently seeing fantastic results.

The Change of Intensity from Veg lighting to Flower lighting can shock cannabis plants

So we have suspected for some time that plants go through stress in the transition from veg to flower. This would make logical sense if you think about it.

In the veg room, our plants are under fluorescent lighting that is around 300 watts (or LED equivalent). Subjecting those same plants to 1,000 watt HPS lighting a day later can be quite shocking.

You may have noticed in the Week 1 video that I remarked how the plants looked kinda stressed after the flip into flower. This was not the first time we had noticed the plants looking droopy and sad. We just thought it was part of the normal stress that occurs when going from 300 watts to 1,000 watts.

And droopy plant compositions is a reasonable response to the stress of high powered HPS lights. But is it necessary to make such a drastic change from veg to flower? That is a question we have pondered, but never done anything about.

But with the results we were seeing in the new flower room — where we started with 15 lights, and then added lights as weeks went on — we were convinced there was something productive about increasing light intensity over the first few weeks of flower.

The Cycling of Light Intensity has shown healthy responses from the plants

Based on this case study, we began implementing these changes in our other grow rooms. Whether the room was equipped with HPS or LED lighting, we began to cycle the intensity of the light. We will explain how exactly we have executed this tactic.

Introducing a Bell Curve of Light Intensity to our plants

If you were to chart our light intensity on a graph, it would look similar to a bell curve. In the typical bell curve, the amount increases to a peak, and then decreases back to starting. The end result is the shape of a bell. We have decided to use our light intensity in a similar pattern.

In veg, our plants are under 300 watt lights, more or less, on an 18/6 schedule. Therefore, we want to gently expose them to greater and greater amounts of light, but slowly so as not to shock them.

Therefore, when flipping into flower, we start out lights at 600 watts on a 12/12 schedule. We leave that intensity at 600 watts for about 10 to 14 days.

In Week 3, after completion of two full weeks at 600 watts, we then increase the intensity to around 825 watts. We watch how the plants, now deep into their post-veg stretch, respond to the 800+ watts.

We push the hardest between the middle of Weeks 3 and through the end of Week 6. That is where we pump to 900 to 1,100 watts, depending on the number of lights, size of the room, and how high the lights are above the plants. This time between Week 3 and Week 6 is where the most growth occurs.

Why Cycling the Intensity of Lights is beneficial to cannabis plants

During the first couple weeks of the flowering period, your plants are still in a vegetative state. They are immature, in that they do not have buds. They are flexible and pliable, still capable of being trained, like an immature plant. Except for the fact that the light cycle has changed, the plants really are more like vegetative plants.

Minimal intensity is needed during Weeks 1 and 2

The changing of the light cycle itself, from 18/6 to 12/12, is enough of a change in routine to stress the plants to a degree. There is no need to pile on top of that stress with a huge increase in light intensity. A small increase is all that is necessary.

In addition, the plants are going stretch and growing long during those first couple weeks. The growth pattern is much more akin to vegetative growth than flowering growth. For this, the plants do not need to take a pounding. Again, the changing of the light schedule is enough to send them into the post-veg stretch. Therefore, the increase in intensity need only be minimal.

The greatest amount of intensity should be used during Weeks 3 through 6

It is during Weeks 3, 4, 5, and 6 that the plants go through the most maturity growth. There is where bud development occurs. It is during these weeks that we want the greatest amount of light intensity.

This is where your plants will grow the most, in terms of bud development, bud density, and overall size of the buds, including the lower branches and top colas. If ever there was a “go” time, it’s during Weeks 4, 5, and 6.

Begin decreasing intensity in Weeks 7, 8, and 9

After the completion of Week 6, your plants may not grow much more in size, but they will continue to “peak” in terms of flower quality. Your aromas and colors will also continue to deepen and advance during this time.

At this time, you can start mimicking the seasons and start retreating the light as it would during fall. This also correlates with our advice to reduce temperatures near the end of the flower cycle, as well.

All of these tweaks go toward the objective of the bell curve approach. First, we slowly introduce a greater intensity of light, but not too much to shock the plants. Next, we peak the intensity as the plants are in their prime flowering stage. Last, we rescind the amount of light as the plants complete their flowering.

The Benefits of Cycling Light Intensity for your marijuana plants

In sum, we much prefer this “new” method of cycling light intensity during the flowering stage. We have found the plants just respond better. They look healthy and happy in those initial weeks of flower. And they just blow up in size during the middle of flower.

We have found that by cycling our light intensity — slowly ramping it up, holding it, and then ramping back down — that our plants grow bigger. The bud development is better as well. We were afraid of smaller, less dense buds, but the opposite has been true. Our buds are bigger and rock hard in terms of density. And best of all, overall yield is higher. Much higher.

Some times, you just stumble on to these discoveries. For us, having no choice but to go through the first half of flower with a fraction of the light intensity was nerve racking. But the end result was delightfully surprising.

 

"

 

 

[cypher79]

I'd already been running my flowering light at 75% for the first 2 weeks, before turning it up to 100% for the rest of the flowering period... I'll try turning it back down for the last few weeks and see how it goes.

[Slippy One]

Yeah they pretty much stop growing at the end, dimming makes sense mimicking nature, I’ll definitely give it a try. 

Edited November 7, 2023 by Slippy One

[GSZZ]

I like to reduce DLI towards the end of flower anyway, it draws them to a nicer more uniform finish imo - but I'm not sure about reducing the light intensity during the first few weeks of flower that sounds like it would actually reduce yield. The plant tends to grow to its conditions during those first couple of weeks of flower ime, giving them a lower intensity will grow a smaller framed, less prepared plant for the higher intensity later... it was a bit easier to see it back int day when you'd leave the MH in for too long before switching to the HPS. I tend to double the DLI when I switch to flower

[ratdog]

Just now, GSZZ said:

less prepared plant for the higher intensity later...

 

 

yes, i like strong stems early on for fat tall bitches

[MidgeSmith]

Yeah, I have been turning the temperature down a bit at the end to try to preserve temps and felt like it was mimicking nature.  Turning the light intensity down at the same time would probably be a good thing.

So, does it follow that you'd turn up the UV intensity in weeks 4-7 and drop it off after that, but keep the far reds going at the end of each day throughout flowering?

[joshbunnybun]

These guys veg at 300 watts then flower at 1000 watts, is it any wonder their plants are stressed, surely from the end of seedling stage intensity should be built up so when flipping to 12/12 the lights are already at max?