The stretch occurs in a photoperiod plant in about one week after the lighting schedule changes from 18/6 to 12/12. This is also known as the flowering stage, and it's when the plant starts to stretch and elongate in growth during this stage in its development. The plant that I'm flowering is Gorilla Glue #4, which is a clone from a mother plant that I started from a female feminized seed.
THE DIFFERENCE BETWEEN SATIVA AND INDICA
There's a noticeable difference in the stretch between a sativa and indica strain. Indica dominant plants will not stretch as much as a sativa does. This is due to the plant's origins, and the hours of sunlight in certain countries with defined seasons. Indica strains generally originate from countries that have defined seasons while sativa's thrive in tropical environments with robust sunlight consisting of 12/12 sunlight for most of the year.
WHY IS THIS IMPORTANT?
The stretch is dictated by the number of hours a plant is exposed to sunlight. More sunlight means a better vegetative growth rate while less sunlight tells the plant to stretch or initiate the flowering stage. When you change the lights from 18/6 to a 12/12 lighting schedule, the reduction in the lighting time will make the plant stretch up toward the light; therefore, causing it to flower.
It's been my experience that sativas like to grow tall, fast, and thin. They also thrive in hot and humid conditions while indicas are thick, bushy and can be manipulated eaiser for more bud sites. The first signs of the stretch also means it's a good time to change your nutrients from grow to bloom and slowly add bud enhancers as the plant develops.