sunlight vs electric lights

this has been on my mind and was wondering if anyone can help me? why do we have to keep lights on for 18 hrs a day to keep our plants in veg ,when outside the longest day we have is only 15 hrs and they veg all summer

Marijuana plants continue to photosynthesize as long as there is light and their other needs are met. They do not need a rest period. Turning the light off for 25% of the day decreases growth rate by 25%. This is actually costly because it requires the plants to spend a longer time in the vegetative stage.

- Ask Ed

sunlight

sunlight

but why are the plants not flowering until fall

I think that really depends on the strain. I had some indica strain outside once that started to flower in august.

You don't "have" to keep plants under anything more than the "critical" nightlength. If for example you have a plant that would trigger into bloom at 13.75 hours then anything above that limit will keep it in veg. The longest day here in SoCal according to the Old Farmers Almanac is 14 hours and 47 minutes in mid June for several days. Keep in mind that is for Sunrise and Sunset not the dawn or twilight periods! This can add to the day length too.
Most growers used 18/6, 20/4 or 24 to give more growth as flurs have such low lumens to get more growth. So they have larger plants going into bloom, this, imo, has been carried to the extreme. Some folks are veggin' "forever" (months actually!)to get a certain yeild per plant. This is a deceptive figure as the "total time" grown is not factored in! Veg time uses more juice and time, so its better to know what max size you can get for a yeild in the shortest period rather than veg til its Xmas, lol!!
Example; grower A vegs for two months to get a size of 30+ inches beore going into bloom, he gets finished in 70 days after the veg time, total time 60 days of 18/6 plus 70 days of 12/12 = 130 days but 60 days were at 18/6 so total juice = 1080 veg + 840 bloom= 1920 total juice! On the other hand grower B, vegged for 20 days(24/7) and bloom for 70 = 90 days( 40 days less!) veg juice = 480 and bloom is the same at 840 total of 1320 or 60 hours less! This grower could be on his 2nd crop 40 days sooner!! The grow that took longer would have more total yeild than the shorter crop but not the % of the 2nd one added in!! This grower also used less nutes and co2!!! So lesson here is be a more efficient grower!! Factor this for a year and you get more for less, not bad eh?