Observations after 8 weeks under Metal Halide versus LED

[Rama777]

I put 8 kish/Chem plants under Metal Halide and 7 under LED about 8 weeks ago.

Ppfd was somewhat lower under metal halide to keep temps in both tents the same with lights on. Leaf surface temps were very similar, lights on. VPD was usually quite excessive under metal halide and pretty consistently optimal under LED (according to what seems to be the consensus amongst VPD proponents). Plants under metal halide were excessively cool during lights off for the first 4-5 weeks. Fluctuations were much less in the LED tent.

Essentially, conditions were more conventionally optimal under LED and ppfd levels were higher under LED as well. Pot size, soil were the same in each tent.

Results? Metal Halide seems obviously superior, at least in veg. Better color, 25-50% more growth. Oddly, I’ve had more mag deficiency under MH than LED but with less ppfd. I’ve discovered that for certain reasons I think I’ve been undershooting my cal mag (RO water here..) so some deficiency in both tents is expected. I estimate that it was more so under metal halide since they were just growing faster and using Nitrogen more quickly as a result?

Is that just the way she goes? Or can anything be tweaked under LED to match the growth rate of metal halide?

Aside from obviously the different light, and the colder nights under Metal Halide (which you’d think would be a hindrance..), the biggest difference was that the humidity was much lower under MH..

 

[BearWater]

Thats a cool side by side! Are you going to complete the whole run side by side?

 

[Rama777]

Thats a cool side by side! Are you going to complete the whole run side by side?

Not this time sadly! They got sexed and all the Kish/Chem got moved to the MH/HPS tent, plus some Cindy99 x White Truffle

Staying in the LED tent is Triangle Kush and Cindy99 x GSC

I’ll do a full cycle trial with some clones eventually. I think comparisons of flower quality with F1’s under the two lights would just end up being mostly speculative.

 

[BearWater]

F1’s under the two lights would just end up being mostly speculative.

that makes sense. It’s a good start on comparison though! It’ll be cool to see what differences on flower when you get something you want to run.

 

[LoveGrowingIt]

Aside from obviously the different light, and the colder nights under Metal Halide (which you’d think would be a hindrance..), the biggest difference was that the humidity was much lower under MH..

Some believe lower humidity causes faster growth. The reasoning behind that is the plants transpire more in low humidity and thus carry more nutrients to the upper part of the plant, especially the leaves. Thus, with more nutrients, the plant grows faster.

 

[Rama777]

Some believe lower humidity causes faster growth. The reasoning behind that is the plants transpire more in low humidity and thus carry more nutrients to the upper part of the plant, especially the leaves. Thus, with more nutrients, the plant grows faster.

That’s true. Interestingly, studies show that plants subjected to low VPD stress have higher sugar concentration.

I’ve wondered if the same can be said for terpenes? If you could keep humidity at around 70% in flower but the air flow pumping to keep disease away…? Higher THC and/or Terps?

But regardless of all that jazz.. Why do VPD charts show lower VPD as being beneficial in veg? What is allegedly gained by higher humidity to make up for the speedy growth of lower humidity?

 

[LoveGrowingIt]

That’s true. Interestingly, studies show that plants subjected to low VPD stress have higher sugar concentration.

If talking stress, there could be a hormesis-related effect.

I’ve wondered if the same can be said for terpenes? If you could keep humidity at around 70% in flower but the air flow pumping to keep disease away…? Higher THC and/or Terps?

70% humidity isn't low humidity.

 

[Rama777]

If talking stress, there could be a hormesis-related effect.


70% humidity isn't low humidity.

The higher sugars in plants is from low VPD, not low humidity. All the terms can get crossed in your head sometimes..