How to force flowering

Should the drop between 18 hours of light and 12 hours of light be gradual or immediate?
Surrey, England

Marijuana’s growth stage is determined by the number of hours of uninterrupted darkness it receives rather than its chronological age. When it receives 18 hours of light or more daily it grows vegetatively, producing more branches and leaves. When subjected to an uninterrupted dark period of 12 hours daily, its growth switches from vegetative to reproductive and it begins to flower. There is no reason to make this switch gradually. The plants respond rather mechanistically to the light regimen.

Outdoor cultivated marijuana flowering is also determined by the number of hours of uninterrupted darkness it receives. During the summer, when the day length is longer nearer the poles than the equator, a particular marijuana variety flowers much earlier in Florida than in Vancouver. It begins to flower when a critical dark period is reached. For most varieties this seems to be between 8 and 12 hours of uninterrupted darkness daily.

You can find out a plant’s critical light level by observing plants outdoors to see when they start flowering. Check the length of the darkness period when you observe the first flowers. Indoors, you can start at 9 hours of darkness and increase the dark period by 15 minutes each week. Figure the critical period as 15 minutes shorter than the dark period length when you see the first flowers. Suppose a plant’s critical dark period is 10 hours. This means that it will flower under 14 hours of light per day. This is 2 hours longer than the 12 hour light period which is generally recommended. The two hours extra light each day is a 16 percent increase in light to the plant. This is critical because light = growth.

A shorter dark period may lengthen the flowering period a bit. To get the buds to ripen quickly, the dark period can be lengthened to 12 or even 14 hours.

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