Difference in Flowering Between MH and HPS

[Jimster]

This season I got a little lazy after vegging and installed my 1000w HPS bulb in one lamp and the other got a 10K 1000w MH. I wanted to see if I could see a difference between buds grown under both lights, and the light's coverage area overlapped in the middle in a rectangular room. I had a couple different strains, but had 2 identical pairs of plants...a Grandaddy Purple and Gorilla Glue pair. I put one of each at the end of the room where they would mostly get one or the other bulb's light.
At 6 weeks of flowering, I'm having issues with branches falling over on both plants and the buds are getting pretty big. The HPS dominant plants were taller in general and the plants are acting like they are finishing up with yellowing leaves. The plants grown under the MH are somewhat shorter but the buds look denser. The plants that were in between the two lights were the biggest of all and I think I need to go back to alternating the bulbs on a weekly basis to give the best of both. What I noticed most was the plants under the HPS, regardless of the strain, seemed to finish faster, or at least the plants appear to be finishing with the yellowing leaves. The MH bunch don't have any yellowing leaves yet and seem like they are still going strong.
This is the first time I did a somewhat controlled side by side comparison for flowering under different lights. The HPS grows bigger plants and seem to finish faster (it's still a work in progress), but I want to see if the MH 10K produces a stronger, more potent bud, as research has alluded to. Until the final product is dried and compared, I'll reserve judgement, since it seems like such a different effect on the plants. Anyone else have any experience with this?

 

[MIMedGrower]

I experimented with hps (hortilux 600) mh (hortilux blue 400,600) and cmh (phillips 315 3100k) in all configurations and overlaps.

Best quality, yield and bud structure was under 2 hps and 1 cmh.

Agree the more blue containing bulbs keep plants shortest and greenest longest but sacrificed about 30% yield compared to hps alone in each case.

For me hortilux blue buds are too leafy and too fluffy. The cmh improves on that but not really enough power in a single 315 to compete with a 600 hps. So again similar loss of yield.

I ran a bunch of clones a few times different ways and got similar teaults each time.

Second favorite configuration and how i would set up if i wasnt likely to buy led next is one 600w horty super hps and one phillips 315 cmh over each 4x6 foot area. Maybe 3.5’ x 5’.

 

[Jimster]

Agree the more blue containing bulbs keep plants shortest and greenest longest but sacrificed about 30% yield compared to hps alone in each case.

While the 10K MH plants are a little shorter, the buds might be more compact... I won't know for sure until harvest. My last grow was the first time I used a HPS and my buds DID come out leafier/less dense than usual... not much, and the buds were bigger, but less dense. I noticed that my finishing time last year and this year is much shorter when using HPS... I'm at 6 weeks flowering and have the yellowing leaves... the MH plants are still growing with no signs of yellowing or finishing. I'm wondering if the extended growing period under the MH might increase the yield by flowering longer. We will find out in a few weeks, but I often flower for 10 weeks or more, but under the HPS, I'll be lucky to get to 8 weeks, I think.

 

[Jimster]

I've been considering what I have seen over the years and I think that I know what the issue might be. HPS is used mostly for flowering. In nature, during the mid to late flowering periods, you get a lot more reds from the sun than blues, simply because of atmospheric diffraction. My next grow I think I will start the flowering with straight MH then transition to HPS about 4-5 weeks later. It seems to me that the extra red in the HPS seems to speed up the flowering stage but might cause it to end prematurely. As I mentioned earlier, I often flowered for 10 weeks but with the HPS, the period is considerably shorter.
Any thought from more experienced growers?

 

[MIMedGrower]

While the 10K MH plants are a little shorter, the buds might be more compact... I won't know for sure until harvest. My last grow was the first time I used a HPS and my buds DID come out leafier/less dense than usual... not much, and the buds were bigger, but less dense. I noticed that my finishing time last year and this year is much shorter when using HPS... I'm at 6 weeks flowering and have the yellowing leaves... the MH plants are still growing with no signs of yellowing or finishing. I'm wondering if the extended growing period under the MH might increase the yield by flowering longer. We will find out in a few weeks, but I often flower for 10 weeks or more, but under the HPS, I'll be lucky to get to 8 weeks, I think.

Hps has much more light output than mh. Maybe you arent giving enough nuteients for them.

All light science testing and my experience as well shows shorter flowering with full spectrum (like a cmh) and shorter leafier plants because of more blue light. And stretchier plants but bigger and denser less leafy with more red like hps.

The 10k bulb is not a normal grow spectrum however. It is a specialty finishing bulb.

 

[MIMedGrower]

I've been considering what I have seen over the years and I think that I know what the issue might be. HPS is used mostly for flowering. In nature, during the mid to late flowering periods, you get a lot more reds from the sun than blues, simply because of atmospheric diffraction. My next grow I think I will start the flowering with straight MH then transition to HPS about 4-5 weeks later. It seems to me that the extra red in the HPS seems to speed up the flowering stage but might cause it to end prematurely. As I mentioned earlier, I often flowered for 10 weeks but with the HPS, the period is considerably shorter.
Any thought from more experienced growers?

My plants (that are claimed to be 8 weeks by breeder) go 10 to 12 weeks green only under hps all the time. But i dont use a bloom ratio. They get ballanced nutes all the way through.

The high pk in bloom nutes can cause faster onset but premature finish.

Nutrients also play a big part in results. I have tested nutes with repeated clone runs as well.

Even different forms of nitrogen can change how the plant grows. I got much stretchier plants with pure blend pro and organic soil than with dyna grow and pro mix hp. Ammoniacal nitrogen grows stretchier and nitrate more compact.

 

[Beachwalker]

This season I got a little lazy after vegging and installed my 1000w HPS bulb in one lamp and the other got a 10K 1000w MH. I wanted to see if I could see a difference between buds grown under both lights, and the light's coverage area overlapped in the middle in a rectangular room. I had a couple different strains, but had 2 identical pairs of plants...a Grandaddy Purple and Gorilla Glue pair. I put one of each at the end of the room where they would mostly get one or the other bulb's light.
At 6 weeks of flowering, I'm having issues with branches falling over on both plants and the buds are getting pretty big. The HPS dominant plants were taller in general and the plants are acting like they are finishing up with yellowing leaves. The plants grown under the MH are somewhat shorter but the buds look denser. The plants that were in between the two lights were the biggest of all and I think I need to go back to alternating the bulbs on a weekly basis to give the best of both. What I noticed most was the plants under the HPS, regardless of the strain, seemed to finish faster, or at least the plants appear to be finishing with the yellowing leaves. The MH bunch don't have any yellowing leaves yet and seem like they are still going strong.
This is the first time I did a somewhat controlled side by side comparison for flowering under different lights. The HPS grows bigger plants and seem to finish faster (it's still a work in progress), but I want to see if the MH 10K produces a stronger, more potent bud, as research has alluded to. Until the final product is dried and compared, I'll reserve judgement, since it seems like such a different effect on the plants. Anyone else have any experience with this?

Great info! Did this back in the 90s for awhile, noticed pretty much what you said re: tighter, denser buds but lesser yield by maybe 20% or so and shorter plants under MH; don't recall if there was a difference in length of bloom time

 

[Jimster]

Great info! Did this back in the 90s for awhile, noticed pretty much what you said re: tighter, denser buds but lesser yield by maybe 20% or so and shorter plants under MH; don't recall if there was a difference in length of bloom time

I used the 10K finishing bulb specifically for the extra UV and planned on swapping the two positions about every week, but got lazy and it got hard to move around, so they stayed in the same location since the start of flowering. The nutrients shouldn't be an issue as I have grown the same way for decades, and I used the same nutes for both the HPS and the MH plants. I had 2 clones pairs and each one either was put in the HPS or the MH side, to try to keep things the same as possible. None of the MH plants look nearly done, although they are getting pretty fat. The HPS look nearly finished. The only real difference was the light, and I didn't think it would make such a drastic change. When I used to use just a std MH bulb, I'd flower for a full 3 months. I have one plant in particular that is midway between the two and it looks impressive. I think next time I'm going to use the MH for the 1st 6 weeks of flowering, then switch to the HPS. Never to old to learn!

 

[growsince79]

MH and HPS are a great combo. I start with a 100w mh. From veg until 3 weeks of bloom they get 2 100w mh and 150 hps in the middle. About week 3 I switch to 2 150w Hps with 100w mh in the middle. For me, this combination makes short, dense buds fast.

 

[Beachwalker]

I used the 10K finishing bulb specifically for the extra UV and planned on swapping the two positions about every week, but got lazy and it got hard to move around, so they stayed in the same location since the start of flowering. The nutrients shouldn't be an issue as I have grown the same way for decades, and I used the same nutes for both the HPS and the MH plants. I had 2 clones pairs and each one either was put in the HPS or the MH side, to try to keep things the same as possible. None of the MH plants look nearly done, although they are getting pretty fat. The HPS look nearly finished. The only real difference was the light, and I didn't think it would make such a drastic change. When I used to use just a std MH bulb, I'd flower for a full 3 months. I have one plant in particular that is midway between the two and it looks impressive. I think next time I'm going to use the MH for the 1st 6 weeks of flowering, then switch to the HPS. Never to old to learn!

Good evening Jim, hope you're well! I think it's completely reasonable to believe that the more yellow spectrum finished the plants sooner. What I remember as the biggest difference was the hardness and density of the buds

 

[visajoe1]

I read an article before about this topic (I'll search for it) and the conclusion was to run MH through stretch and then finish the last 2 weeks with MH. Running HPS in weeks 3-7/8/9/10/etc depending on the strain.

The reasoning if I remember correctly was HPS promotes flowering which is concentrated in the weeks above. MH helps control stretch in early flower and helps promote resin production at end of flower.

When you look at the spectrums of MH vs HPS, this makes sense. MH generally has higher Kelvin rating and more UV, which we already know will cause plants to be shorter and UV increases resin production.

I followed this theory during my run, and added UVB late flower also. We'll see what the end result is.