Short answer
Grow plants that finish by late September: autoflowers, or photoperiod lines bred to flower early. Start indoors, plant out after the last frost (usually early June at 50° and above), and keep rain off the buds in autumn. Grower practice. Home growing outdoors is legal only in some places, so check local rules first.
Why
Long northern days delay flowering. Cannabis flowers when nights pass a cultivar's threshold, often near 14 h of day [1]. At 50° latitude days stay above 14 h until about 23 August, against 9 August at 40° (calculated, sunrise to sunset). Add the 10–14 day transition and an 8–10 week flower, and a typical cultivar finishes in late October.
Genetics can move the date. Breeding work has mapped Autoflower1 and an early-flowering locus, Early1 [2]. Autos ignore day length; early photoperiod lines simply start and finish sooner.
A head start indoors buys weeks. Seedlings started indoors and hardened off go out at a useful size the day frost risk passes [3].
Autumn wet is the real threat. Gray mould thrives in cool, wet weather on dense buds [4], and northern Septembers deliver both.
The numbers
| Approach | Plant out (N / S) | Harvest (N / S) |
|---|---|---|
| Autoflower, sown direct | Early June / early December | Mid-August–early September / mid-February–early March |
| Early photoperiod line | Early June / early December | Mid–late September / mid–late March |
| Typical photoperiod | Early June / early December | Late October or later / late April or later |
| Light deprivation (~12 h dark from early July) | Early June / early December | Late August–September / late February–March |
What to do
- Choose autos or proven early-finishing photoperiods; avoid late tropical lines.
- Start photoperiods indoors 3–6 weeks before last frost on long days.
- Harden off for 7–14 days.
- Plant out after the last frost, in the sunniest, best-drained spot.
- Use containers, a greenhouse or a hoop cover to warm roots and keep rain off.
- Consider light deprivation for photoperiods.
- Watch forecasts from September and harvest ripe plants ahead of long rain or a hard freeze.
Common mistakes
- Growing a 12-week cultivar where frost arrives in early October.
- Planting out too early and losing plants to a late frost.
- Closing a greenhouse up tight and breeding mould.
- Skipping the local-rules check.
Related
- Full article: The cannabis life cycle, stage by stage
- Answers: Autoflowers outdoors · When outdoor plants flower · When to harvest outdoors
- Terms: light deprivation · greenhouse growing
