How much is too much light?

Is it best to try to aim for only the maximum intensity the sun (at the equator at noon) can produce to avoid light stress? How much more light can you give them then maximum sunlight that they can actually effectively photosynthesize?

For a small grow tent (2x4x5) is a 600w HID too much over a 400W?

Maximum Sunlight at 55°N on Summer Solstice at 12.14pm is a whopping 2500 umols

You'll kill your indoor plants at that level of light intensity in a tent.
800 - 1000 umols at the canopy is all you'll need indoors for flowering.

guitarguy10 said:

Is it best to try to aim for only the maximum intensity the sun (at the equator at noon) can produce to avoid light stress? How much more light can you give them then maximum sunlight that they can actually effectively photosynthesize?

For a small grow tent (2x4x5) is a 600w HID too much over a 400W?

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Maximum intensity sunlight happens only at one time of the day, and varies with the seasons and depending on weather. It's not something you should be aiming to expose your plants to for 12 hours a day or more.

1000 PPFD is oft-quoted as a good guide, but studies have shown cannabis can handle up to 1500 PPFD continuously. This is strain-dependent, of course, and sativas generally handle higher levels of light better than indicas.

But as @Moflow has already pointed out, 800-1000 works, and I go as high as 1100-1200 in my grows.

Start low, say 600- and work your way up (or down!) from there. Monitor what the plant is telling you and adjust.

ill go as high as 1500-1600 but will only adjust to that level for the last couple weeks of flower, and you gotta watch it cause levels that high can degrade your resin. This is with c02

Blast them!......ha

PSUAGRO. said:

Blast them!......ha

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As much as possible lol.

Although I do find that I get weird stunting/stretching if I try to blast them too early.

I might just buy a dimmable ballast then.

I have electricity and other utility bills to pay, I can't just shove a 1000w ballast in there and 'blast them' .... unless you want to pay my electricity bill?

I found this chart that supposedly shows the % of maximum sunlight that various wattage bulbs offers at various heights. That's where I got the idea that you can pretty easily give them too much light (esp. in a small tent where it's hard to raise the lights very high):

Here's a 600 Watt par map

guitarguy10 said:

I might just buy a dimmable ballast then.

I have electricity and other utility bills to pay, I can't just shove a 1000w ballast in there and 'blast them' .... unless you want to pay my electricity bill?

I found this chart that supposedly shows the % of maximum sunlight that various wattage bulbs offers at various heights. That's where I got the idea that you can pretty easily give them too much light (esp. in a small tent where it's hard to raise the lights very high):

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The first thing you have to ask yourself is: what is "natural sunlight"?

It's a very misleading claim, as sunlight varies so much around the world at different latitudes, altitudes, seasons, and weather events.

Is natural sunlight 12 noon on a sunny Australian summer's day, or 5pm on an overcast Dutch Autumn afternoon?

Prawn Connery said:

The first thing you have to ask yourself is: what is "natural sunlight"?

It's a very misleading claim, as sunlight varies so much around the world at different latitudes, altitudes, seasons, and weather events.

Is natural sunlight 12 noon on a sunny Australian summer's day, or 5pm on an overcast Dutch Autumn afternoon?

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That's why I said 'at the equator at noon' .

guitarguy10 said:

That's why I said 'at the equator at noon' .

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Haha! Stoned moment. You did mention that. But at what time of year and what altitude?

OK, so i just looked it up and that's 2200 umol/m2 (PPFD) during each equinox at sea level. Which is about twice what you should be aiming for.

Here's something to read if you're interested: https://www.researchgate.net/publication/225339540_Modelling_photosynthetic_photon_flux_density_and_maximum_potential_gross_photosynthesis

The plants used in this study were determined to have an optimum photosynthetic rate at 700 umol/m2 - this will change by species and strain.