Effects of Light Intensity on Plant Growth

Greetings, AFN members, OSP here. I’m a new AFN member moving in from another forum. After sneaking around a number of threads here looking for secret information, I’ve decided to start this thread. I want to evaluate the effects of light intensity on plant growth. More specifically, how lighting can be used to encourage or minimize plant stretch.
In the short time I’ve been a member here I’ve met a number of very knowledgeable growers, and I’m inviting each of you to participate or chime in as you see fit. Since I am new to AFN I’m sure I’ve missed a lot of interested parties. If you are reading this feel free to tag in anyone I’ve missed:
@fettled6 @912GreenSkell @bushmasterar15 @Waira @MedGrower @Son of Hobbes @Nosias @Screwauger @HemiSync @ChroToker @Need4Weed

I've grown photoperiods since forever under High Intensity Discharge (HID) lighting, and stretch has always been a thing to plan for. Several years ago I decided to experiment with an autoflower freebie, and since then I've completed (12) single-plant autoflower grows. To the point that I'm not doing photoperiods anymore. Maybe again one day. I have a Colombian Gold freebie that has brought back memories, but not today.
In the process of migrating from photoperiods to autoflowers, I also began a transition from HID to LED lighting. Over this period I began to see a pattern. With every one of my HID grows, all my autoflowers exhibited healthy stretch. It started quicker than photoperiods, but the overall effect was similar. Low Stress Training (LST) was effective, bud quality was very good and yield was very good to excellent. But with my LED grows, absolutely NONE of my autoflowers stretched. I had complete grows that never exceeded 10” in height.But with LED I can’t help but think much more is possible. Over 44+ years of growing I’ve seen well trained plants out-yield plants with no training every time. So I want the ability to encourage stretch in my plants.
For the sake of other AFN members, I believe the inverse of this stretch phenomena could be very beneficial to growers with a limited amount of grow space headroom. I want to prove or disprove, to some relative degree of certainty, that light intensity can be used to influence stretch when growing cannabis. But first I need to gather data on other growers’ experiences with plant stretch, and its association to light intensity during the plant’s lifecycle.

This is NOT an opinion piece of “is LED better than HID?” LED is certainly different, and in many respects (heat &efficiency) we all know it IS better. And this is NOT an attempt to compare LED lighting brands. There are numerous characteristics of LED lighting that determine light quality, and our most excellent site admin, @Son of Hobbes has started a thread on what parameters can be used to define those. His efforts may be reviewed here: https://www.autoflower.org/threads/what-should-be-on-a-grow-light-review.64857/ I’ve added a few well chosen thoughts on that topic - PAR watts, efficiency, most favored color spectrum, etc.. But I’m not smart enough to make that final differentiation so I’m happy to watch as SOH finishes that. The only facts I intend to present here are those I’ve experienced with my auto grows.

There are a number of other factors besides lighting that affect plant growth. In an effort to keep this thread focused on light intensity with no outside variables, I asked one of AFN’s most experienced members, MedGrower, for his input. His comments, and my follow up confirming those variables were addressed, can be found in his grow thread located at https://www.autoflower.org/threads/...-dwc-hs1-telos-0008.64718/page-2#post-1758834
In consideration to MedGrower, if you have any comments / questions, send them to me or post here. I don’t want to trash his grow thread with my conspiracy theory so I’m asking each of you to please share that consideration with me.

For the sake of this study I’d like to focus on the two main lighting factors that control intensity - light height throughout the grow cycle, and “power” of the light over the grow canopy; i.e. watts per sq.ft.. Light height is an easy factor to define as long as we document the variation during a plant’s growth cycle. Plant canopy area is an easy measure. Power consumption is an easy measure. I know, watts per sq.ft. is not the most meaningful measure of an LED’s efficiency. But until or if we ever reach consensus of an industry standard measure for all lighting types, I am using watts consumed at the wall, per square foot. No matter how much we insist this is not the best method (I agree), wattage consumed per square foot is the only readily available factor across all lighting types. And no matter how much we agree to disagree, it should provide a reasonable enough classification of small / medium / large lighting to support this study.

My light ratings are 62.5W/sq.ft. for HID and 65W.sq.ft. for LED. As to other influences on plant stretch, my grow techniques with HID and LED are as close to identical as one can get for environmental control and nutrient regimen. So enough intro, let’s kick this in the ass as I describe my experiences with autoflowers to date.

My starting system for HID is a 250W cooltube and batwing reflector with a metal halide bulb for veg and a high pressure sodium for bloom. The tent is a 4 sq. ft. system (2' X 2' X 5’3") by Secret Jardin. I configured it with (2) 4” 170CFM centrifugal fans, one for light cooling and the other for odor control through a 12” X 4” Phresh carbon filter). My nutrient regimen is General Hydroponics’ expert recirculating formula with some strength and slight ingredient tweaks. This regimen has not changed between HID and LED grows.
I grow hydroponically, and my system is configured for a single plant in a DIY 4 gallon Ebb ‘n Gro net pot with hydroton clay pebbles. The system floods automatically every two hours for 15 minutes. I call this DIY because I do not rely upon the complex valves and fittings normally associated with commercially available ebb and grow systems. Those systems rely upon multiple pumps, flow control valves and timers opening and closing with each flood cycle to deliver and recover nutrients to/from the plant. A strength of these commercial systems is they enable the reservoir to sit at the same height as the grow containers. Negatives are they are complex and expensive. My system relies upon a passive DIY manifold sitting on top of the reservoir, and the only moving part is an inexpensive and very reliable 170 gph hydro pump. Total ebb n’grow component cost with four net pot buckets was under $100. I bought four net pot buckets to simplify plant positioning changes throughout my grows, and to run parallel grows in separate tents. My DIY manifold does raise system height by the height of the reservoir, but the entire reservoir, tent and plant container system comes in under 7’ tall. It’s a quality system with all the right parts, and it fits in a closet if it has to (mine doesn’t).

I’m going to drift off topic for a moment as I’ve been known to do. I’ve used all types of hydro systems for many years and this is my favorite. It re-oxygenates the root zone after each flood every two hours. Supply to the net pot is positive pressure flow, while draining is passive / gravity, so aggressive root growth is pushed back into the net pot and will never cause a clog in the system’s hydro line. And even if a clog were to somehow mysteriously occur, overflow is not possible due to the design of the manifold. The system is fully automated - I recently took an eight day trip with no worries that the system would stay fully functional until I returned, and it did. A separate reservoir (I use 12 to 14 gallons) from the grow container makes nutrient changes and maintenance simple, and the added capacity gives me plenty of “headroom” in the nutrient mix as the plant drinks.
Here’s a pic of the original HID tent. The manifold is the white bucket to the left outside the tent. The ebb ‘n Gro bucket is slightly out of position on the shelf behind the tent:

 

[Olde School Player]

Back to task. My first experience with autoflowers was a World of Seeds freebie, Northern Lights – Big Bud Auto. I’d grown both parental photoperiods so I felt that experience would help me judge the final product. She stretched normally, yielded 6.5oz, bag appeal was great and the smoke was exceptional. I fell in love and was so high I forgot to keep pictures.

Grow # 2 was my only, first and last ever, attempt at a multi-plant grow using three Auto Seeds’ ChemDawg Autos. This convinced me that auto’s do not like to be crowded, and a 2X2 tent was just not big enough to allow the plants to flourish. I’m certain bigger tents with appropriate lighting would see more favorable results, but that was not where I wanted to go. Those three plants yielded 5.25oz cured total, bag appeal was poor and the smoke quality was nowhere near Humboldt’s photoperiod (one of my all time favorites). But they DID stretch. I was so embarrassed by this grow I refused to take pictures


Grow # 3 was a repeat of World of Seeds freebie, Northern Lights – Big Bud Auto with similar results to the first.

Grow#4 was GreenHouse Seeds’ Kalashnikova auto, which is a re-work hybrid of the Cannabis Cup classic White Russian. The plant developed an open calyx formation as she matured but the buds were solid – no wispiness. This plant nailed it – 11.2 oz of stanky-assed fuck-you-up smoke. To date this is my second-best of all single plant yields. I have two more beans on standby waiting for the urge to strike.

Grow # 5 was a TH Seeds Auto Original BubbleGum that yielded 7.3oz cured (pic below). I’m calling this one out because I did retain pictures, and I repeated this strain under LED, whose results started the formation of my theory of plant stretch. The plant did develop an open calyx form as she matured, but they were solid – no wispiness. And the smoke was impressive enough that I bought more seeds of this strain.

Grow#6 was a Humboldt OG Kush Auto. It failed at 2.6oz., average bag appeal and only OK smoke. Humboldt has not been known for auto’s, and I’m not sure if it was a runt plant, poor genetics, or just a standard phenotype for OG Kush, which is a notoriously poor producer. Someone else can try this one before me.


Grow # 7 was a G-13 Labs Pineapple Express Auto, which I aborted when it showed as a full male. I cut the bastard with prejudice.


Grow # 8 turned out to be one of my all time favorites; Blimburn Seeds Mamba Negra Auto . . This is one of the most impressive strains I've ever grown. Frost was off the chart (did over 12% hash by cured sugar trim weight using an ISO quick-wash) and smoke was some of the best ever. She stretched beautifully, filled the tent under HID, and in 70 days yielded 8.46oz after a 30 day cure. I can’t say enough good about this nasty girl:

To summarize, all seven of my first autoflower HID grows stretched like crazy.

By the time grow # 9 rolled around, I decided to test the waters with LED. I wasn't quite willing to bet the farm on a technology I was unfamiliar with at the time, so I bought a MARS II 700 LED. This model draws 263W at the wall in veg mode, and 325W at the wall in full bloom mode. In retrospect, I have been completely satisfied with the light. I’m not claiming it’s the best on the market, but for a budget conscious buyer (or in my case just unsure of the technology) it has proven to be very effective.

With the MARS-II I added a second tent, 2'3" X 2'3" X 63"; 5 sq. ft., also from Mars Hydro. I’ve been 100% satisfied with this, it’s a quality piece of gear. And I liked the increased footprint size, thinking being this minimizes differences in watts/sq.ft. between the old 250W HID and the new 325W LED:

 

[Olde School Player]

To help keep comparisons meaningful, I chose a second round of TH Seeds Auto Original BubbleGum which had delighted me from the previous HID grow. This first LED grow scared me, but ultimately got me started on this plant stretch theory. The plant never stretched, and this was the first time I’d seen this phenomenon. I was really afraid it would be a shit yield, but it ended up being a personal best single plant yield of 11.76oz. after a 30 day cure. It also made rock hard buds (no open calyxes this time). Wonderful smoke, every bit as good as the first grow under HID. She almost filled the tent, but NO STRETCH. 10" tall when harvested. At the time I wondered if this difference from the first HID grow was a phenotype thing. But I couldn’t help but wonder if something else was at play. This is my girl:

After BubbleGum, I did another G-13 Pineapple Express. I’m glad I gave G-13 Labs another chance as this grow came in at @ 8.07oz cured, and excellent smoke for the auto version of an urban legend strain. But still no stretch.

By the time my grow # 11 came around my theory was solidifying. Yield was looking pretty good overall, but I just knew that if these sexy girls would grow some long legs I would be happier, and yields would improve. So I decided to experiment with a Dutch Passion Blackberry Kush, and I ran it under my original 250W HID. As I suspected, she went back to the stretch-like-crazy trait, but developed open calyxes halfway through bloom. Pretty see-though wispy. Very good smoke but a disappointing 3.7oz. yield. But she was a beautiful girl - she's in my avatar, and here:

After that it was back to LED with a Dutch Passion Cinderella-Jack. Loved this strain, yield came in @7 oz., but still no stretch:

Followed by my current Dutch Passion Blue Auto Mazar, who just came down today (guessing 4- 5 oz). Nice dense buds, smells like full-on skunk in the tent but has a nice sweet pine scent when touched. But guess what? Still no stretch. This has been the most compact plant I have ever grown – that’s a 12” ruler in the pic. I could have fitted four of her in that little 5 sq. ft. tent. But homey don’t do ‘dat.

So to wrap up my experiences, most of my auto grows were OK to great yielders. The OG Kush & Blackberry Kush disappointed under HID and the Blue Auto Mazar will probably disappoint under LED. But to the point of this thread, ALL of the HID and NONE of my LED grows stretched at all. Zero. Zilch. Nada.

Now here I am, starting my 14th autoflower grow. I'm waiting for an old HID favorite to germinate, Blimburn Seeds Mamba Negra Auto. So far the first two seeds have failed, my 3rd and last is in the rooter, and I’m waiting anxiously for her to pop or flop. I'm hoping for another run under LED's at a much greater height to see if I can generate some stretch that's somewhere comparable to my old HID grows.
And for those of you who have followed along through this long-assed post, there’s two sides to this equation. Anyone struggling with limited headroom should be able to use what I’ve already learned to their advantage. Lowering the light, increasing light strength, and/or decreasing the grow canopy area should make 10 - 12" max height plants a reality. With minimal LST, every one of my auto LED grows have stayed below 12" and I know many growers will LOVE that!!! I feel I already have enough information to assist growers who wish to minimize stretch., so feel free to reach out with questions.

The other side of the equation you’ve probably figured out. I'm betting a tall starting LED light height, followed by closer finishing height, is going to yield the results I'm looking for. I’ve seen grow threads on here where plants HAVE stretched. I WANT that so I can train my plants for higher yield, and I WANT that light intensity to encourage dense bud formation like I’ve seen with LED. If I can get both of these I’ll have what I’ve always chased – a better way to do single plant grows. Let's face it, a 4 oz grow is not too shabby, but with 8 to 12 oz grows behind me it’s not in my nature to settle.

 

[Olde School Player]

So here’s where you can help. I’m asking for each of you who have had successful grows under LED lighting to provide some basic statistics on your grow. I’m not trying to be rude by restricting this to successful grows. But I do need to acknowledge MedGrower’s point that there are other factors affecting plant growth, and eliminate those from the equation. So if something went bad on your grow - heat, nutes, bugs, pH drift not controlled, your room mate smoked it all when you weren’t looking - whatever - please defer posting until you complete a successful grow. Here’s what I’m looking for:

What is the breeder & strain and final cured yield of the plant you are reporting

Watts per sq. ft. documented from wattage-at-the-wall of your light(s) and with size and square footage of your grow area (size to identify odd shaped areas). If you use multiple lights, list each with a total wattage consumed.

Finished height or plant length. Difference being a measure of training effectiveness to final height. For example, if you took a 30” tall plant and brought her down to 12” with LST, then report total plant length at 30”. Remember, I want to measure stretch, more so than final plant height. The simplest method to measure stretch of a trained plant is with a string following the contour of the main trunk from base to tip.)
If you did no training & allowed the plant to follow its natural form, report No Training and normal plant height.

And finally, the magic sauce –
What light heights did you maintain throughout veg, transition and bloom? Ranges fine, for example:
Veg – start @ 30”, down to 16”,
Transition – start @ 16”, down to 12” (I frequently skip this, feel free to as well)
Bloom –maintained at 12” throughout bloom.

I don’t care if you raised the light to finish final bloom.
And maybe I should care about at what age the plant stopped growing vertically? Of all the info I gathered during my auto grows, I never thought to document that. If you did, report it.

As your reports come in, I will try to summarize these on the next post in table form, and maintain a running average of light height to plant height. If I find that’s not working out or for unknown reasons just not making sense, I’ll find some way to summarize & report your results. I’ll attempt to maintain that table by editing my thread post immediately following this.

Thanks for bearing with me, I know it’s a long start for this thread. If you don’t know yet, there are two things about me – I won’t do anything half-assed, and I don’t know how to write short posts

 

[Olde School Player]

Post # 5 of this thread is reserved for future editing

 

[Savage Garden]

Excellent stuff and i am subbed.

 

[puggle6]

Man, OSP you sure know how to bring it! lol

Just briefly I'll add some observations.

T5, (I have a 110w 'starlight' model, a bit more efficient than reg T5)

T5 seem very good for germ trough to day 10-14, seedling stretch seems minimal (we don't really want leggy seedlings)
By this stage some decent leaf area is available for the plant to properly photosynthesise and readily use a higher intensity light.
I have 6500k and 2100k tubes but obviously only use the 6500k for seedlings.

MH-HID 600/1000w, I regularly use this up to pre flower and stretch is predictable and controlable. Nothing really new here I guess.
Globe selection has usually been comprimse between brand/price etc, haven't tried pimped out high end offerings.

HPS-HiD 600/1000w, my usual bloom lamp and again very predictable and productive stretch into full bloom, again, it's been around forever!
Mid range globe selection, Phillips son-T generally and have been very happy with the price/performance.

CMH-HID 860w retrofit on a 1000w MH ballast. This is newish to me, I haven't managed a full grow with this lamp as I was interrupted and had to pull early.
Wow, the stretch was out of control... I need to understand this lamp a bit more, a lot of heat for the 1mx1m tent (looking to get a 1.2mx 1.2m for a bit more training room and temp control!)
The CMH guys report great results from the native 315w and 630w and soon I'll have some data on the 860w
This is the globe.
https://growershouse.com/philips-cdm-860w-allstart-lamp-4000k

There are adjustable ballasts that will run this globe as well as MH/HPS from 400w to 1000w but they are expensive and I'm not going to go there as Phillips might discontinue the range and I'm trying to move to efficiency so I'll run these globes out on the anchient 1000w MH ballast and then go to a native CMH or COB solution. That said, what I experienced with the 860w it would be nice to tune it down to have stretch control!

COB and LED, no experience to speak of apart from 2x 60w 'Blurples' which I can only report as being only a little superior to the T5


I am a very technical person however I use a balance of rationalization and intuition to guide me rather than pour over charts and numbers to the third decimal place. I have to do enough of that to get my paycheck and growing is a joy and release for me. The relationship I have with my growing system and choices are very much a result of what gets whispered between my plants and I lol!

A few things I wonder about lighting..

Kinetic energy, heat I guess (and radiation?) Not sure how it is explained for lighting but I wonder.. with turbochargers kinetic energy forms a part of the efficiency curve. Is the lighting equivalent the reason why some much less powerfully lights can achieve the same or better results. Might have to have a good lighting refresh to find out if/how/what that connection relates (and be up on weeding out marketing bullshit)!

Color index and other environment factors.. we all automatically jump to light time cycles to infulence stubborn plants that don't want to flower or hold in pre flower, or simply won't fill out.

Is it possible there is a formula for (not just) cri/pH/temp/humidity etc along with light cycles that can more heavily infuence a plants lifecycle? Yea we know a lot but I think there are still some secrets to be discovered!

Either way I'll be moving to CoB in the 1mx1m tent and using up the fresh 860w globes I have in a new 1.2mx1.2

Hopefully the 860w and a tent of CoBs back to back!

More to come.. hmmm

Keep on rocking the free world OSP!


f6

 

[MedGrower]

This is a very interesting topic of conversation and I am looking forward on providing my data on it when I have time. It may be a very hard thing to pin down as there are so many factors that vary between what genetics are at play and at what conditions. However lets give it a good go and hope we could all learn from this

 

[G.Leave]

A brilliant post, and giving us lots to think about!! Thanks for posting and repping you now for your efforts!! Thanks for taking the time!!

 

[Olde School Player]

@fettled6 @MedGrower @912GreenSkell thanks for the quick replies Gents, lets me know my thread made sense & maybe I'm on to something here.
You all said it well, there are so many extenuating factors in this, especially genetics as MedGrower states. Over my twelve auto grows, multiple strains, all HID grows did longer buds and LED did more compact rocks.
I think my experiences with HID and LED over a few years have led me to believe that color spectrum is pretty well under control. Yes, some tailoring of LED spectrums might offer (minor) improvements. Equating to the old MH-veg and HPS-bloom, better spectrums for their specific task.
And certainly all the other things we need to do to insure a successful grow have an impact. However, given the attention I have always given to those (pay attention to the small stuff and the big stuff will take care of itself) I believe standard grow factors are comparable across the two grow platforms. So I keep coming back to light intensity having the biggest effect on a plant's growth.
I see improvements in bud density with LED. I see better stretch with HID. If I can't fix this with light height, then plan B is to dust off my HID for veg and transition, then finish with LED.

I do wonder - a number of growers have expressed the opinion that HID intensity for better for penetration into the plant canopy while LED is better for bud density. I wonder if LED is just fine for penetration, but its increased intensity has kept stretch at bay so penetration became less of an issue? Hmmmm.
Thanks again, Team!!!