Just guessing is totally fine – it's only about a first impression.
Several things are possible – but the normal case is the threshold. Both plants read the same clock in the sky, they just have differently set alarms. That is exactly what makes the harvest month a varietal trait.
The light course explains how the plant measures the night: via phytochrome, a molecule that flips one way in light and slowly back in darkness. If the night is long enough, the flowering signal arrives.
Indoors you set that clock yourself – you switch to 12/12 and the plant follows.
Outdoors nobody sets the timer. The timer is the sky. And it runs on a calendar you cannot negotiate with.
The decisive number is the critical day length: the day length below which the plant starts to flower. It is not a constant of the species but a trait of the individual variety.
This was measured on 27 hemp varieties in climate chambers, in half-hour steps from 12 to 18 hours. Result: the thresholds lay between 13 hours 45 minutes and over 15 hours 30 – depending on the variety's origin.
Northern varieties flower under longer days. They had to, because their summer is short. A landrace carries its home climate around with it as a built-in harvest date.
An early-flowering variety is not a special technique and not a trick. It is a perfectly normal photoperiodic plant with a perfectly normal flowering duration – its threshold simply sits higher.
A higher threshold means the day drops below it earlier in the year. The plant starts flowering in July instead of August and is done in September instead of October.
That's the whole magic. And that is why "done in September instead of the October rain" is a variety decision, not a growing technique. You buy it in spring or you don't have it.
Both finish early. Both are sold as "easy outdoors". Yet they are two completely different things – the difference lies in what the plant responds to at all.
Early flowering responds to light. Autoflowering responds to age.
The early variety waits for its day length – it just comes earlier. Cover it up and it flowers immediately. Keep the days long and it waits.
The autoflower ignores light entirely and flowers 21 to 35 days after germination, whatever the sky does. Genetically that is not a shifted threshold but a different mechanism – and it hangs on at least two loci that are inherited in opposite ways. There is no single autoflower locus.
In a trial with ten THC varieties, all of them still began flowering at 14 hours of day length, three even at 15. Sounds like you could count on much earlier dates outdoors than expected. You can't. Why?
Because starting to flower and building flowers are two different responses.
At 15 hours the plants showed stigmas – and then nothing more happened. Not a single flower tissue developed further; the data had to be removed from all yield analyses. Even at 14 hours the flowers stayed smaller and less developed.
"It's showing hairs" is no proof that the light duration is good enough to harvest. The plant can hear the starting gun and still not run. Incidentally also the reason 12/12 is the indoor standard: not because 12 hours is the threshold, but because it gets every variety reliably through.
A cross circulates online that supposedly flowers at 18 hours of day length – and the seller stresses it is expressly not an autoflower.
Let's check that with what this lesson gives us.
First: the highest threshold ever measured in a variety that genuinely responds to light is about 15 hours 30. Every candidate examined that also flowered at 18 hours also flowered under continuous light in follow-up tests – that is not a high threshold, it is no threshold at all. Which is exactly the autoflowering that was ruled out.
Second: the genetics supposedly come from Iran. At that location the longest day of the year is about 14 hours 38 minutes. A plant cannot have evolved an 18-hour threshold there – it would be undercut all year round and therefore meaningless.
The description contradicts itself. That is no proof the variety is bad. It is proof that the number is worthless.
Grow eight plants of a landrace and you get eight different flowering dates. That is not a fault in the seed, that is the normal case.
A genome study of 145 Iranian landraces put numbers on it: between the genetic groups, mean flowering dates lay only about seven days apart. Within a single group, the spread reached up to 53 days.
Work it out and only about five per cent of the variation lies between the groups. All the rest sits inside the group itself.
Origin tells you little, the individual plant tells you everything. Anyone growing a landrace should mark the earliest plant – not judge the variety.
Why does nature use day length of all things as its seasonal signal, and not temperature? Because temperature lies. A warm October day says nothing about how much year is left – day length, by contrast, is incorruptible.
The plant is not alone in this. The predatory bug Orius, familiar from the beneficials course, enters reproductive dormancy on short days – even at 25 °C. It doesn't care how warm it is. It counts hours, not degrees.
Plant and predatory bug read the same clock. And one more thing in common: in Orius too the critical day length is not species-specific – the spread between strains of the same species exceeds the difference between species. Exactly as with the landraces.
The plant measures uninterrupted darkness. If the night is regularly broken or brightened, the flowering signal fails to arrive or arrives late – this is documented as a yield problem. Indoors it's called a light leak, outdoors it's a street lamp, a yard light or the neighbour's terrace. How strong the effect is depends on brightness and distance, but "outdoors it doesn't matter" is wrong.
One of the answer options just now read: „outdoors the moon evens that out." It was wrong – but the question behind it is legitimate and follows directly from the previous card.
A reader put it to a magazine in 2012: his plants flowered earlier than usual, and July had brought a new moon, so darker nights. Does the explanation hold?
No – and the reason is already in the question. The moon changes phase every month. Flowering, however, does not wander through the summer with it: it depends on day length, and that repeats on the same date every year. If the moon were the trigger, flowering would jump by weeks from year to year. It does not.
The idea is tenacious all the same. In the archive of the scene magazines „moon planting" appears as early as 1979 – not as an observation but as a selling point for a growing guide. And in 1985 a magazine printed the counter-evidence itself (ten years of data, „no fluctuations that had anything to do with the lunar cycle") only to conclude in the same paragraph: „Anyway, I happen to KNOW that astrology works, so there you go."
That is the anatomy of a myth in two sentences: the data are not disputed, they are declared irrelevant.
The streetlight is untouched by this – there it was a light source that burns the same every night. That is exactly what the moon does not do.
🎯 Hands-on mission
Take the varieties you have in mind for next year and look for a statement about the harvest date. Is there a month, a flowering duration in weeks – or a day length in hours? You will almost never find the third. That gap is exactly what the next lesson is about.
Continue in the Academy →✓
If the harvest month sits inside the seed – what else is in there, and how do you tell whether a claim about it holds?
Something still unclear?
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