So anyone have input on starting flower with mh during the stretch or just do hps? Would mh promote more veg growth during the stretch? I may have asked this already a while ago.
You'll find lots of differing opinions on this one. Personally, when I switch to 12/12, the first week I'll use MH. This does many things, among them: promote additional vegetative growth, and helps keep internode length shorter, imo. You can use MH the entire grow and you'll see little difference. Here's why...
To answer your question, scientifically speaking, there are a few things you can do to help keep the stretch down. The first, and most important thing is to make sure you have sufficient light for your area. Eliminating that factor, it is not the MH or HPS spectrum, per se, that causes a plant to stretch. It is a natural response as a result of flowering hormones that it is time to compete for passing pollen (cannabis, like corn, is anemophilous). To be successful, the plant grows taller and branches so the flowers have a better chance of reproducing. So, I would assert, that neither MH or HPS would significantly impact the stretch. More important would be light penetration (whether MH, HPS, LED, CMH, flouro, or plasma), plant density, and distance from the canopy.
Hope this helps and happy growing!