E-mail- Best 2nd Light and Stem Elongation

Hello,
I got a question about HID lighting. Currently I have a 600w hps and would like to upgrade to another 600w. I am using a Digi ballast, and would like to figure out whether or not to use a HPS/HPS combo for flowering or HPS/MH. I know MH aren’t as lumen efficient as HPS bulbs are but I also have been doing a lot of reading on how plants don’t really care about the lumens as much as the PAR watts. So before I go buy another 600w combo I’d like to figure out whether or not I'd be in my best interest to have a more mixed spectrum or just a enhanced spectrum HPS like I have now. Lastly id like to say I’m going for the maximum yield.

Hello and thank you for the question.
As usual the answer is it depends… If you are seeing any problems with stem elongation, (you are tying plants down so they don’t grow as tall, or propping them up because they are tall and lanky) I think you need an MH, or an hps/mh combo. You won’t have the light output as with an HPS but you will increase yields over your current set up for sure. If you add another hps you will have more stem elongation than you do now, the yellow/red combo so loved for flowering also increases stem elongation especially when plants are in a vegetative state. The more blue light output of an MH causes plants to have shorter stems and would actually decrease stem elongation compared to what you have now I’d imagine.

If you do not see stem elongation with your current set up I would add another hps. I say this because the yellow/red light of an HPS increases the flowering response more than the widespread light output of an MH. If you are looking for flower/fruit yield I would also go for the two HPS, again as long as you are not having a problem with stem elongation.

I think you have the right idea about PAR. An MH may have a slightly higher PAR, but the hps has more PAR in the light that induces flowering. Most other light colors (wavelengths) are given off in sufficient quantities by an HPS so a second HPS will not be a limiting factor.  Yet another reason I’d recommend an HPS, an MH bulb can lose up to 50% of its output in a year. It will take an HPS 2-3 years before it has lost 50% of its initial output. That is also something to consider, you will be buying replacement bulbs for an MH yearly if you are looking at yield.

I hope that helps, feel free to drop me a line and let me know what decision you made and how it worked out.

Good growing
Dr. E.R. Myers