How do you control the flowering stretch?

Yes. Temp differential works.

One of the best ways to control stretch is to run clones off the same mother. Should be the same, all things equal.

I have noticed that when I used bud blood a week before flip and the week of flip it kept my sativa a little shorter. It was the same batch of seeds Cali-o BUT not the same plants(two separate grows one after the other). I also noticed that the stretch on my other varieties(afgooey and white widow blue balls and kish) all stopped stretching at the same time under the same 315cmh.

I just feed calcium nitrate and a micro-nutrient blend during veg. When I switch to 12/12 I add monopotassium phosphate but don't cut back on what I was feeding during veg.. I use fulvic acid throughout the grow and add some additional magnesium sulphate "epsom salts" around the end of week three through week five. Then I start lowering the strength of the nutrient solution so when I harvest I'm at around 1/4 strength. Any stretch I get is filled up with bud.

I dont experience unusual stretch with any variety here. I dont stop c02, I dont change temps temps, I dont use pk boosters, I dont change up nutrients,I dont add any extra anythings, I dont switch lights(mh/hps all the way through) ...

yet I dont get the stretch. must be more to it than the controls I mentioned?