I have written about speeding up germination and about speeding up vegetative growth And speeding up flowering and I want to add to how to speed up flowering thanks to a scientific experiment (LINK) done by my friend. Basically, the cooler you have the night temperatures the faster the flowering response will be.
My friend just had a second child, and his grow closet is now his youngest kids closet. He moved his grow lights into his garage. The garage is attached to the house, but is not heated and it is only about 10 degrees above outside temperatures, and it is on the south side of the home so it gets very hot on sunny days. He can no longer grow over the winter months, or in the middle of summer. He started the seeds inside his house under a fluorescent light then after a few weeks he put the plants in the garage under an HPS. One thing he did was he switched his light and dark cycle, so that the lights are on (and making heat) at night when it is cooler outside/in the garage. He is growing in a grow tent to prevent light from affecting flowering. This also helps insulate the plants from the more extreme temperatures in the garage.
He keeps good notes on flowering time and he has data showing that since his plants are growing in the garage flowering occurs 4 days earlier with colder night temperatures. When the lights are off he frequently has temperatures in the 40’s and even in the upper 30’s one time. This would kill basil and some other sensitive herbs, but will cause most plants to have a faster flowering response. Especially plants that flower in the fall/ with 12 hours of darkness. These plants will ‘think’ fall is coming so they better get flowering and reproduce.
I don’t recommend growing in an unheated garage to speed up flowering, but you could consider keeping the vent fan running even during the dark cycle when you begin flowering to lower the night temperatures as much as possible. Even if you can’t get the temperatures into the 40’s the greater the difference between the high of the day and the night temperature the faster you will see a flower response.
Dr. E.R. Myers