Photoperiod Trigger Calendar

When will short-day cultivars trigger flowering at your latitude? Annual day-length curve with critical thresholds.

Your latitude

Latitude (°) Positive north, negative south

Cultivar / threshold Cannabis (12 h dark) Chrysanthemum / poinsettia (13 h dark) Strawberry June-bearing (13 h dark) Soybean (~13 h dark) Hops (cone formation, ~16 h light cessation) Custom threshold

Critical dark hours Min hours dark for trigger

Quick locations — Select — Boston, MA (42.36°N) St Louis, MO (38.95°N) Nashville, TN (35.15°N) Los Angeles, CA (34.05°N) Phoenix, AZ (33.45°N) Houston, TX (29.76°N) Minneapolis, MN (44.97°N) Seattle, WA (47.61°N) London (51.51°N) Stockholm (59.33°N) Sydney AU (-33.87°S)

Trigger date (start) — Day length drops below threshold

Release date (end) — Day length rises above threshold

Trigger window —days Per year at your latitude

Pre-trigger reference — 2 weeks before trigger (start prep)

Annual day-length curve

Day length across the year at your latitude. Shaded area = photoperiod-trigger window for selected cultivar.

Outdoor light deprivation ("light dep")

Outdoor cannabis growers at temperate latitudes face a problem: by the time natural day length drops below ~13 hours of light (around mid-September in the northern hemisphere), the season is already cooling, and frost may arrive before harvest.

Light deprivation solves this by manually shortening the day with blackout cloth. Pulling tarps over the canopy at, say, 7pm during August forces a 12/12 photoperiod and triggers flowering 6-8 weeks earlier than nature would. By harvest, weather is still warm and plants finish before frost risk.

The Trigger Calendar tells you when natural photoperiod transitions through critical thresholds at your latitude — useful for planning when to start light-dep, when to stop, when natural light alone will trigger your crop.

Greenhouse photoperiod control

At higher latitudes (45°N+), summer days exceed 15-16 hours. Short-day flowering crops (chrysanthemums for fall sale, poinsettias for Christmas, strawberry June-bearers for spring) need blackout cloth to extend the dark period.

For long-day flowering or vegetative-promotion crops, supplemental lighting can extend day length artificially in winter. At 60°N in December, day length is barely 6 hours; supplementing to 14-16 hours requires substantial lighting.

Indoor relevance

Indoor growers control photoperiod directly via timer, so this calculator is most relevant for:

• Outdoor / greenhouse photoperiod-sensitive flowering planning
• Light-dep schedule for outdoor cannabis
• Planning travel / vacation — knowing when natural photoperiod will or won't trigger flowering for your outdoor crops
• Latitude-aware purchasing decisions — autoflower vs photoperiod cannabis seeds, day-neutral vs short-day strawberries

Free under CC BY 4.0. Cite as "OAT Photoperiod Trigger Calendar (openagriculturetechnology.com)".