Question... How do you know when the plant is mature enough to start flowering?

I am using three 36" floresent grow tubes, keeping the plants just inches from the light.

grow space is a closet, about 6' by 3' and right now there are only 2 plants that are close to flowering?

well you can start flowering and like KNOWM said you can tie them down and limit the height that way and actualyl get more buds possibly

I think I am going to give them a couple more days and then start.

One easy way to know when your plants are ready to flower is the pre-flowers. But if hieght is an issue then go by hieght.

IMO a lot depends on if it was started from seed or clones. seeds I let veg longer than I do clones.. with my set up as soon as my clones get a root they go right back in the bloom box..