Managing DLI and photoperiod correctly

DLI (Daily Light Integral) is the photosynthetically usable light amount summed over 24 hours, measured in mol·m⁻²·day⁻¹. It is calculated from PPFD multiplied by the lighting duration in seconds, divided by one million.

Photoperiod, on the other hand, refers to the division of light and dark phases within a 24-hour cycle and controls flowering triggering in photoperiod plants.

Since DLI is a pure product of intensity and duration, different PPFD/hour combinations yield the same DLI — 600 µmol·m⁻²·s⁻¹ over 18 hours mathematically produces the same daily total as 900 µmol·m⁻²·s⁻¹ over 12 hours.

Flowering induction in photoperiod cannabis is detected via a phytochrome-controlled system that measures the length of the UNINTERRUPTED dark period, not the light duration itself — hence the term 'critical dark period'.

Seedling/early vegetative stage: a lower DLI (~15–25 mol·m⁻²·day⁻¹) is sufficient to avoid overwhelming the young photosynthetic apparatus.

Main vegetative stage (18/6): DLI of 25–35 mol·m⁻²·day⁻¹ at moderate PPFD over a long light duration.

Flowering stage (12/12): a DLI of 35–45 mol·m⁻²·day⁻¹ requires a significantly higher PPFD than in the vegetative stage with only 12 hours of light, to reach the same or higher daily total.

For stable flowering induction, photoperiod cannabis needs at least about 12 hours of uninterrupted darkness per cycle — even a brief light intrusion during this phase can be interpreted as a 'second day'.

Repeated light intrusions can lead to reversion to vegetative growth or stress-induced hermaphroditism, as the plant interrupts the signaling chain that maintains flowering.

Delayed or absent onset of flowering despite a correct 12/12 cycle: first check for light leaks in the grow room during the dark period (door gaps, LED standby lights, external light sources).

Isolated male flowers on otherwise female plants (stress-induced hermaphroditism): retrospectively check the light-leak history and timer reliability of the past weeks.

Increasing PPFD in isolation, without calculating the actual DLI by accounting for the lighting duration.

Using timers without redundancy or battery backup — a single power outage can shorten the dark period and disrupt flowering induction.

Treating autoflowering genetics with the same strict photoperiod rules as photoperiod plants, even though they respond primarily to DLI rather than dark-period length.

Some growers deliberately use a brief interruption of the dark period with far red light to trigger certain physiological responses — however, this intervention requires precise control and is error-prone without experience.

Autoflowering lines can also be grown under continuous light (20/4 to 24/0) due to their photoperiod independence, though the optimal DLI upper limit varies by strain.