How do you force plants to stretch vertically?

One of the plants fell out of the cup into soil and stretched on its own about 2-3x as tall before sprouting.. fighting for light possibly but it all went unnoticed.
Does partial darkness or weaker light make them stretch? Set amt of total darkness?
How do I force a plant or a number of plants to grow on my terms? I pretty much know how to do it horizontally through training techniques.
I think it might be the roots.. softer dirt / less water more oxygen.

Ya, crap light will make them stretch early sprout (not enough light), having the light on one side will make them stretch (left or right), growing under a flower spectrum instead of veg lights will make them stretch. Also packing them really tightly together makes them fight for light and grow taller and less bushy. There. Theres a couple ways to make em your bitch. I like the hps way and packing them close together the best for tall.

Raising the light will make a plant stretch.

Why do you want stretch? I heard on a YouTube video by some weed-science guy, that 13+ hours of light in the flowering phase will result in more stretching and root growth. More light in your 12/12 schedule (like 15on/9off) creates "Fall" conditions as opposed to "Winter", so the plant continues in "Semi-Veg" and grows taller, leafier, and rootier haha. That light schedule also creates a larger yield because there is more light-energy absorbed (as opposed to 12/12) - however, there is less dark/grow-time, so the harvest will take longer. Peace! GL!

I've been stretching mine by switching from 18/6 to 14/10 for 8-10 days before switching to 12/12, keeping the lights (LED's) higher than normal, like 24", significant extra stretch vs switching straight to 12/12. A side benefit of this is once they're switched to 12/12, buds show up within a couple of days.

I've got everything necessary to try all that stuff. will begin. thanks.

Increasing the DIF causes stretch beyond regular stem elongation. DIF is difference between day and night temp.

Example src: http://www.sciencedirect.com/science/article/pii/S030442380200002X

Usually the opposite is desired though...