Flower week 1: the transition after the flip to 12/12

Nothing visible happens in flower week 1, and that is the correct result. Leaf colour, growth rate and canopy shape all look exactly like they did on the last day of veg, maybe with an internode fractionally longer if you're measuring with a ruler. What has changed is inside the plant, and it started the moment the first full dark period landed. Most week-1 trouble is a grower who expected to see something, and started adjusting a system that was not broken.

Stretch (internode elongation) begins

5–10days after the flip

one controlled trial, one cultivar [3]

Minimum dark nights to start committing

3+consecutive nights

same trial; commitment strengthens with more [3]

Pre-flowers visible to the eye

2–3weeks

grower consensus, not the lab timing above

Feed EC this week

1.6–2.0mS/cm

hold late-veg level or step up slightly

Day 0 is the first full dark night, not the day you touch the timer

Day 0 is the first night the plant gets an uninterrupted, roughly 12-hour dark period, not the calendar day you reprogrammed the timer. Flip the schedule on a Tuesday afternoon and the first full dark cycle runs Tuesday 20:00 to Wednesday 08:00: Tuesday is day 0, not the Monday you edited the settings, and not Wednesday morning when you first look in and see nothing has changed. Get this fixed point right, because every week label on this page, and on the ones after it, counts from day 0. If you have not made the switch yet, see preparing to switch to 12/12 first, including how long to hold vegetative growth before you flip: one survey of 26 published grows found vegetative periods anywhere from 13 to 180 days, with 30 days the median and 21 days the single most common length, so there is no one correct week count to aim for ^[1].

The 12-hour dark period is a convention, not a hard biological rule for every cultivar. In one indoor trial of ten drug-type cultivars grown side by side under six photoperiods, four cultivars showed no flowering delay at all out to 14 hours of light, and two showed a 1.3- to 2.3-day delay at 13 hours of light compared with 12 ^[2]. If your own plant seems a day or two slow to respond on a clean, unbroken 12/12 schedule, cultivar variation is a more likely explanation than a mistake in your setup, so check the schedule is genuinely unbroken (see below) before you assume something is wrong.

What's actually changing while the plant still looks like late veg

The visible plant is running well behind the decision it has already made. In one controlled trial that tracked a single cannabis cultivar in a research growing room, gibberellin (GA4) and auxin levels at the shoot tip fell gradually over the following three weeks, measured in samples taken on days 4, 6, 9 and 19 after the switch to short days. On an unbroken 12/12 schedule, the first signs of inflorescence initiation were visible under magnification by day 7. A separate arm of the same trial gave plants only a handful of dark nights before switching them back to long days: more than half had still begun forming an inflorescence by day 10 after three consecutive dark nights, and every plant had after seven ^[3].

That is a lab result from one cultivar, grown in a controlled research room, not a promise about your tent. With an ordinary timer and normal room light spill, most growers see nothing that actually confirms sex, the small calyx or the smooth pollen sac, until somewhere around two to three weeks in. What week 1 itself usually gives you: internode spacing that is very slightly longer if you measure it, and not much else worth acting on.

Inside the plant · one controlled trial, one cultivar [3]

1. 1. Day 0: the first uninterrupted dark night of the 12/12 schedule.
2. 2. Days 0–10: auxin (IAA) and gibberellin (GA4) fall gradually at the shoot tip.
3. 3. Plants given 1, 3, 5 or 7 dark nights, then put back on long days: 10, 60, 80 and 100 % had begun forming an inflorescence by day 10.
4. 4. Days 5–10: internodes and the main stem elongate fast. The stretch begins.
5. 5. Day 7: flowers visible under a 40× loupe (at least three pairs of stigmas) on unbroken 12/12.
6. 6. It holds only while the nights stay dark: plants moved back to long days reverted to leafy growth, even after 21 short days.

What you can see · grower consensus

1. A. Week 1: no visible change. Same colour, same height, same leaf shape.
2. B. Week 2: the stretch becomes visible and internode gaps open up.
3. C. Weeks 2–3: pre-flowers, a small calyx or a smooth pollen sac, usually visible to the eye.

That gap between committed and visible is the whole reason week 1 causes panic: the plant is doing its job on a schedule you cannot see. Resist retraining, defoliating hard or feeding heavier to "wake it up"; there is nothing asleep to wake.

The first stretch starts before you can see it

Vertical growth rate itself, not just internode length, typically starts climbing within the first week. In the trial above, overall plant height kept accelerating for about the first two weeks under short days before levelling off, while the fastest internode elongation ran from day 5 to day 10 ^[3]. Week 1's rate is the quiet start of that curve; you will usually only notice it in hindsight, comparing a height measurement taken today against one from seven days ago.

Worked example, home tent. A plant measured 55 cm (22 in) from the pot rim to the growing tip on the day of the flip. Measured the same way seven days later it reads 61 cm (24 in), a 6 cm (2.4 in) gain that is barely more than a typical week of late-veg growth and easy to miss without writing the first number down. By the end of week 3, growers commonly report a photoperiod plant finishing the stretch at 1.5 to 2 times its flip-day height, though that range is grower consensus rather than a measured trial figure and varies hard by cultivar; the number that actually decides whether your headroom holds gets its own maths in flower weeks 2–3: stretch, not here. This page's job is only to tell you the curve has already begun, and that a 5–10 cm (2–4 in) gain in week 1 is the start of it, not a problem.

This is the highest-risk week for a light leak, and the easiest one to protect

Week 1 carries more light-leak risk than any week that follows, simply because the routine is new: nobody has checked the door seal yet, the timer has not been watched through a full cycle, and anyone walking into the room after dark still reaches for the switch out of habit. Once the routine settles in over weeks 2 and 3, the same mistakes get rarer without anyone trying.

The plant does not need pitch darkness, but it cares about the wrong wavelength more than most growers expect. In one study, near-infrared light invisible to the eye, delivered during the dark period, delayed cannabis flowering: about 2 days at an intensity comparable to standing roughly a metre from an infrared floodlight, and up to 12 days at a higher dose ^[4]. The mechanism is the same phytochrome system that reads ordinary light, so it is not only bright white light that can reset the clock.

One finding sits alongside those risks and should be reassuring rather than alarming: reverting a plant to long days after only a handful of interrupted nights early on mostly cancels the switch rather than damaging the plant permanently, in the same controlled trial ^[3]. A single bad night in week 1, caught and fixed, is a delay, not a disaster. What is far less forgiving is a leak that repeats night after night without anyone noticing.

Worked example, commercial room. A 24-light flower room flips on a Wednesday; day 0 is logged as that Wednesday's date against the room's batch ID. On the Friday of week 1, a technician doing an unrelated equipment check opens the door at 22:00, well into the dark period, for about ninety seconds under a torch left on the wrong setting. Nobody notices anything different that week, because nothing is visible yet, but the flip is re-logged as delayed by a day and the harvest-window forecast for that batch is adjusted rather than left to drift. The fix that actually prevents a repeat is not blaming the technician; it is moving the equipment check to before lights-off, so nobody has a reason to open that door in the dark at all.

1. Test the timer through one full cycle before you rely on it2 min to check

   Watch it switch off and on once, at the exact times you expect, rather than trusting the programmed schedule alone.

2. Walk the room in full dark before day 0

   Look for door and vent gaps, standby LEDs on nearby equipment, and spill from an adjoining room; see light-sealing a tent or room for how to close them.

   Warning Check from inside with the door shut, not from outside looking in.
3. Agree a no-entry rule for the dark hours

   Anyone who genuinely needs the room during the dark period uses a dim green torch, never the main light switch.

Hold the feed where it is; don't chase a jump

Feed EC this week usually stays at whatever it was for late veg, or steps up slightly toward the low end of a typical early-flower range, commonly put at around 1.6 to 2.0 mS/cm depending on your base water and how strong the veg feed already ran. That is grower and manufacturer convention, not a controlled trial's finding, so treat it as a starting corridor and adjust from what your own runoff EC tells you, not as a number to hit exactly. For the fuller switch from a vegetative to a flowering feed programme, see switching to bloom nutrients; this page only covers what changes in week 1 itself.

The reason to hold rather than jump is uptake, not chemistry: the root system that will eventually support a much bigger flowering canopy has not grown yet, so a big EC increase this week mostly means more salt sitting in the pot than the plant can use, not more nutrition delivered. Save the increase for when the stretch above is visibly under way and demand has actually gone up.

Worked example. A tent ran its final veg feeds at EC 1.4 mS/cm, pH 6.0, with runoff sitting at EC 1.5. On the day of the flip, the grower moves the feed to EC 1.6, keeps pH at 6.0, and checks runoff after the next full feed rather than guessing: it comes back at EC 1.7, pH 6.1. That is close enough to the input EC to say the roots are taking up water and nutrient at roughly the rate they were before, with no build-up and no reason to back off. If runoff EC had come back at 2.4 or higher against a 1.6 input, that would say salts are accumulating faster than the plant is using them, and the fix is a plain-water feed to flush the pot back down, not a further increase.

Climate can shift on the same gentle schedule: nudging the room toward the early-flower VPD corridor this week costs nothing, because transpiration demand is still close to what it was in late veg.

What's not normal this week, and what to check first

A few things are genuinely abnormal in week 1, and each one points back to the schedule or the feed rather than to some new flowering-specific problem.

Everything else, unchanged colour, unchanged height, a feed that reads the same as last week, is week 1 working as intended. The next thing you will actually see is the stretch itself.