Decrease growth ? Well that would mean purposly harming your ladies if you wanted to slow the growth in all ways you would haveto mix a very strange nute mixture you could purposely cause a nute lockout. I would never recommend this. Though if its just the size of the plants your concerned with ie: Height. you can cut them in half and they will still grow and produce. HST is one of the best forms of training. you can even cut every branch in half if you want. though I wouldnt recommend it. What I would recommend is like 2 or 3 more Lights. lol thats a big room.