At WeedSeedsExpress, we’re dedicated to bringing you the latest scientific findings to help you grow stronger, healthier cannabis. And today, we’ve got yet another fascinating study to share, this one digs into a question every home grower asks sooner or later: How much light do my plants really need during the vegetative stage?
Researchers at the University of Guelph set out to test cannabis under different light intensities, and what they found might surprise you. If you’ve ever wondered whether cranking your LEDs to full blast is worth the electricity bill, or if your modest light setup is enough, you’ll want to pay attention to this one.
Why This Study Matters
For many of us, the veg stage can feel like waiting at the starting line. Plants are stretching, stacking nodes, and building the framework that will support those frosty flowers down the road. But the truth is, the quality of your veg phase directly sets the stage for yield.
Too little light? You get tall, spindly plants with weak stems. Too much? You risk heat stress, wasted energy, and possibly light bleaching later. Most advice floating around online suggests modest light levels in veg, often in the 250–650 µmol·m⁻²·s⁻¹ range. But this study shows cannabis can handle far more, and even thrive under it.
The Study at a Glance
- Plant material: Clonal cuttings of the strain Gelato.
- Medium: Rockwool blocks, with standard nutrient solution and controlled drip irrigation.
- Environment: Commercial greenhouse setup with temperature held at ~77 °F by day, ~71.6 °F at night, and humidity around 37–40%.
- Lighting: LEDs providing five different canopy-level intensities, 200, 450, 700, 950, and 1200 µmol·m⁻²·s⁻¹, with a 16-hour photoperiod.
- Duration: 21 days in vegetative growth.
In other words, this was a tightly controlled, well-run experiment, the kind of trial most home growers can’t pull off in their garage or tent. That’s why studies like this are gold: they give us real numbers to work with.
What They Found
The plants responded differently depending on the amount of light they received. Here are the big takeaways, broken down simply:
1. Growth Rate: Midpoint Sweet Spot
Plants hit their best balance between height and structure around 600 µmol·m⁻²·s⁻¹. At this level, they weren’t stretching too much, but they weren’t stunted either.
2. Biomass: Bigger with More Light
Aboveground dry weight (the actual plant material) kept increasing as light intensity went up, hitting its stride around 900 µmol·m⁻²·s⁻¹. Beyond that, returns started to level off, so blasting your plants with more than 1,000 probably isn’t worth the electric bill.
3. Morphology: Thick and Dense
High light levels gave the plants shorter internodes, thicker stems, and more nodes overall. Think stocky, strong plants with dense foliage, exactly what you want heading into flower.
4. Leaves: Smaller but Smarter
At higher intensities, leaves were smaller in size but thicker and richer in chlorophyll. That means the plants were packing more light-harvesting machinery into each leaf, getting better at using the photons they received.
5. Water Content: Drier but Tougher
Plants under high light held a little less water in their tissues. This may sound like a bad thing, but it actually makes stems sturdier and less prone to collapse later.
What This Means for Your Grow
Best Overall Light Intensity
If you’ve got the power and ventilation, aim for around 900 µmol·m⁻²·s⁻¹ at canopy level during veg. This produced the most robust plants with great biomass and structure.
Energy-Saving Option
If you’re more worried about your power bill than squeezing out every gram, 600 µmol·m⁻²·s⁻¹ still gives you solid plants with slightly more open architecture (more airflow, less chance of mold).
Transition Strategy
Consider ramping up light intensity in the last week or two of veg. This “hardens” your plants and gets them ready for the high-intensity demands of flower. The study found that plants adapt better to strong flowering lights if they’ve already been exposed to high PPFD in veg.
Practical Tips for Home Growers
Because I know you might be halfway through a joint while reading this, here’s the simplified grower’s checklist:
- 200 µmol·m⁻²·s⁻¹: Bare minimum, plants will survive, but expect stretchy, weak stems.
- 450 µmol·m⁻²·s⁻¹: Decent for early veg or small tents.
- 600 µmol·m⁻²·s⁻¹: Sweet spot if you want healthy plants without a monster power bill.
- 900 µmol·m⁻²·s⁻¹: Go-to level for strong, compact, high-yield-ready plants.
- 1200 µmol·m⁻²·s⁻¹: Overkill for most home setups, your plants can handle it, but your wallet might not.
Tip: A cheap quantum sensor or PAR meter is worth every penny if you want to dial this in. Eyeballing light is like guessing how strong your beer is, you might be close, but you won’t know until it hits you.
Why This Study Breaks Industry Myths
For years, many cultivation guides recommended keeping veg lights on the lower side, saving the high intensities for flower. This study turns that thinking on its head. Cannabis is far more light-hungry in veg than most people realized, and it doesn’t just survive under higher intensities, it thrives.
That means a lot of home growers are leaving growth on the table simply by under-lighting their plants. The takeaway? Don’t baby your plants more than necessary, give them the photons they can use.
Wrapping It Up
The University of Guelph’s research shows that cannabis in veg can handle light levels up to 900 µmol·m⁻²·s⁻¹ with no stress, giving you dense, strong, well-prepared plants for flower. If you’re running a lean setup, 600 µmol·m⁻²·s⁻¹ still gets the job done while saving electricity.
So next time you’re adjusting your LED dimmer, remember: your plants are tougher than you think. Light them right in veg, and you’ll be thanking yourself come harvest.
Here’s what you can do today:
- Check your canopy-level light intensity, don’t guess.
- If you’re below 600 µmol·m⁻²·s⁻¹, consider upgrading or lowering your fixtures.
- Ramp up towards 900 µmol·m⁻²·s⁻¹ as veg matures.
- Watch your plants: compact nodes and thick stems are the green thumbs-up you’re on track.
And as always, stay tuned here at WeedSeedsExpress. We’ll keep bringing you the latest research, stripped of jargon and ready to apply in your grow room.
