E-mail – HPS and Internode Stretching: Is It Time for an LED or Conversion Bulb?

Dr Myers,
    Is it true that hps bulbs cause more Internode stretching if the spectrum is not balanced with blue? do you believe an LED could be used to provide blue light to the flowering plants effectively enough to decrease Internode stretching?

I have only used hps bulbs so far. The 18 month mark came and I replaced all 3 with new htgsupply hps bulbs just like the ones that came with the light kits, and I feel this "pang" of wondering if i could get better results by either adding a halide conversion bulb or adding LED's. I understand the only way I can find out would be through scientific experimentation myself but my emotions keep me from spending the money on LED's. I am afraid to swap out 1 of the 3 hps bulbs with a conversion bulb because I don't yet have a light mover and the conversion bulb would light only one 3' x 3' bed - and that doesn't seem like an actual change to the spectrum in the entire room would take place. Is it time for me to bite the bullet and start the experiment or do you think I’m way off track here?

Thanks for the E-mail. Yes, HPS will cause internode stretching in most plant species. This is due to the light output being more in the yellow/red range, and a lack of blue light. However, if your plants are flowering, I’d not use a conversion bulb, they have a much lower output of light. (Read my post on flowering and HPS vs. MH)You may want to bend or prune your plants. You want the maximum amount of light to maximize your yield. If you are using this room for vegetative growth, then you may want to use a conversion bulb, or get an MH, which is much better for vegetative as far as internode stretching (there is no problem with MH’s)  but and HPS putts out a lot more light intensity than an MH.

As far as LED’s, you could put in an all BLUE LED to correct the problem. You could get 2-3 panels and put them beside the HPS you already have to cover the grow area. This blue light won’t need to be as intense; you just need to increase the blue to decrease the internode stretching. A Tri-Band LED would also reduce the internode stretching. I have only used a 90 and 120 Watt LED, but I am going to use a 300 W from HTGSupply.com this semester, and I am assuming it will be compatible to a 400 W HPS. I will let you and other readers know.

You may want to think about a light mover, since you have multiple lights. You could increase the grow area, and at the least you would get better growth on the peripheral areas. Without moving the light or plants the plants on the edge of your grow area won’t get the optimal amount of light. You could then put an LED or MH in the room and move it around with the other lights. I think this is the best option, but it is also the most expensive one.

Good Growing,
Dr. E.R. Myers