Outdoor seasons by latitude: when flowering starts

Move a photoperiod cultivar from northern Germany to Malta and the night it needs to trigger flowering arrives on a different date, and arrives more gently. Near 55°N, night length lengthens by roughly 4.5 minutes a day around the equinox; near Malta's roughly 36°N, the same swing is about half that. Nothing about the plant changed. The latitude did.

This page is about that latitude effect specifically: how far you are from the equator sets both the calendar date your local night crosses a cultivar's trigger, and how fast the trigger arrives once it starts moving. For the mechanism itself, the way an uninterrupted dark period switches the plant from vegetative to flowering, see how photoperiod triggers flowering; this page assumes you already know the plant is reading darkness, not daylight, and builds on that.

The switch: unbroken darkness past a threshold

Outdoor photoperiod cultivars flower once the naturally lengthening night crosses a cultivar-specific critical duration, commonly generalised as about 10 hours of unbroken darkness ^[1][2]. Below that, the plant stays vegetative; at or above it for enough consecutive nights, it commits to flowering. That generalisation is a reasonable planning default, not a fixed biological constant: real cultivars vary widely either side of it, and the next section gives the range.

How latitude sets the date and the pace

Earth's axial tilt is why night length changes with the seasons at all, and why that change is not the same size everywhere. At the equator, night stays close to 12 hours all year. Move towards either pole and two things happen together: the night gets shorter in local summer and longer in local winter, and the rate at which it changes around the spring and autumn equinoxes gets steeper. A cultivar's critical night length is a fixed line on the calendar; latitude decides how quickly the season's night length runs up to meet it.

Two numbers are worth carrying with you. The full-year swing (longest night minus shortest night) grows fast with latitude: about 2.4 hours at 20°N, 4.9 hours at Malta's 36°N, 6.9 hours at 45°N and 10.2 hours at 55°N ^[3]. And the speed of change right at the equinox grows even faster in relative terms: roughly 1.1 minutes a day at 20°N against 4.5 minutes a day at 55°N, about four times as fast ^[3]. Every latitude reaches exactly 12 hours of night within a day or so of the same date, 22-23 September; what differs is how long before that date each place already crossed a lower cultivar-specific threshold, and how briskly it was moving when it did.

Malta, and why a gentle swing matters here

Take a cultivar with a widely-cited 10-hour critical night length and place it at three latitudes. The night reaches that threshold in late July near Malta's 36°N, in mid-August at 45°N, and only in the last week of August at 55°N ^[3]:

Malta's gentle swing cuts both ways. The mild trigger date gives a long, unhurried run into autumn on a Mediterranean latitude with almost no frost pressure. But that same gentleness means Malta's midsummer night never drops much below about 9.6 hours, so a cultivar bred with a shorter critical threshold gets little or no unambiguous vegetative window outdoors, even in June. Nearer the equator this gets more extreme still: below about 20°N, the modelled midsummer night never drops under roughly 10.8 hours at all, which is part of why cultivars labelled sativa-dominant, historically grown at low latitudes with a long, mild season, differ so much in habit from cultivars labelled indica-dominant, historically grown further from the equator or at altitude with a shorter one. Treat the label as a loose starting point for expected season length, not a guarantee for any specific cultivar.

The season still has to be long enough to finish

Latitude sets the trigger date. It does not, by itself, guarantee the season stays workable afterwards, and that is a separate constraint worth checking before you commit a cultivar to an outdoor plan. Most photoperiod cultivars take roughly 8 to 11 weeks to finish once triggered, though this varies widely by cultivar (grower practice; see the life cycle explainer for typical stage lengths). A late-July Malta trigger has a wide, mild runway behind it. A late-August trigger at 55°N has a shorter one, with autumn cold and heavy rain arriving sooner, which is why growers at higher latitudes lean towards cultivars bred to finish fast, or towards forcing an earlier trigger artificially (see below), rather than towards long-flowering lines bred for a longer season.

When the calendar says trigger and the plant disagrees

A date worked out from latitude alone assumes the plant only sees the sky. In practice, stray light after dusk can hold a plant vegetative well past its calculated trigger date, or interrupt flowering that has already started. Recent controlled work found the most sensitive cannabis cultivars responding to light pollution as dim as about 0.01 µmol·m⁻²·s⁻¹ (10 nmol·m⁻²·s⁻¹), delaying flower initiation and slowing inflorescence development; red light disrupted the response more than white light of the same intensity, consistent with a phytochrome-mediated effect ^[4]. For scale, full moonlight measures around 0.002 µmol·m⁻²·s⁻¹, below that threshold for most cultivars but close enough that the most sensitive ones may still respond on the brightest nights ^[4]. A streetlight, a neighbour's security light or a nearby grow-light leak sitting in the canopy's line of sight after dusk is a far larger and much more common problem than the moon.

If an outdoor or greenhouse plant is weeks past its calculated trigger date and still hasn't shown pistils, walk the site after dark and look for any light source the canopy can see, not just ones aimed at it. A single distant streetlight is often enough.

Moving your own trigger: light deprivation, briefly

Because the trigger is really about the length of unbroken darkness the plant experiences, not the calendar, you can impose it early with blackout material instead of waiting for the season to catch up. This technique, light deprivation, covers plants with a lightproof material for enough of each 24-hour cycle to hold darkness above the critical threshold on demand. It works for exactly the reason this page has been describing: the plant cannot tell the difference between a naturally long night in late August and an artificially long one in May. The build, materials and daily schedule belong in the outdoor grow-spaces guide; this page only explains why the trick works.

Find your own numbers, then log what actually happens

The first season, write down the date you actually see the first pistils and compare it with what this page predicted. Cloud cover, tree lines and your own microclimate will move the real date a little every year; a season or two of your own logged dates will beat any latitude chart for your specific site.