The flower cycle, week by week
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Beginner · Flower

The flower cycle, week by week

This paper maps the eight to ten weeks from light flip to harvest. You will be able to read the plant stage by stage, set the right climate and feed for each phase, and use trichomes rather than the calendar to decide when to cut.

Beginner9 diagramsEvidence-linked · 8 sources~12 min read
In this guide
01 Purpose and scope02 Definitions03 Evidence and limitations04 What the 12/12 flip does to the plant05 Stretch and bud set (weeks 1–4)06 Bulking and ripening (weeks 5–10)07 Reading trichomes to find the cut date08 Troubleshooting09 Expected results and limitations10 References
01 · Start here

Purpose and scope

Cannabis grows in two big phases: vegetative (building leaves, stems and roots) and flowering (building the buds you actually harvest). Flowering is triggered by changing the lights to 12 hours on and 12 hours off, and it runs for roughly 8 to 10 weeks for most plants. This guide walks that stretch one week at a time.

You do not need to have grown before to use it. Flowering is the final phase, lasting about 8 to 10 weeks for typical hybrids. Pure indica types can finish in 7 to 8 weeks, and long sativa types can take 11 to 13.[6]

Only photoperiod plants need the light change to flower. Autoflower plants flower on their own with age, so this guide focuses on photoperiod plants. The plant tells you what week it is in, so you read the plant (pistils, trichomes, leaves) rather than the calendar alone. Getting the climate, light and feed right each week is the difference between dense, potent buds and loose, mouldy, or harsh ones.

The two-phase life cycle1Seed / clonestart→2Vegetative18/6 light,build the frame→3Flipswitch to 12/12→4Flowering8-10 weeks, budsform→5Harvestcut on trichomeread
Figure 1. Flowering is the back half of the grow. Everything before the flip builds the plant. Everything after it builds the buds.[6]
Who this is for

Anyone about to flip their first photoperiod grow who wants a week-by-week map. Pairs with the crop-steering and harvest and cure papers.

02 · Vocabulary

Definitions

This page uses a handful of grower words repeatedly. Learn these six and the rest of the guide reads easily. None of them are complicated once defined.

Flip / 12/12Changing your light timer to 12 hours on, 12 hours off. This tricks a photoperiod plant into thinking winter is coming, which starts flowering.
PistilsThe fine white hairs that emerge from bud sites. They start white and darken to orange or brown as the plant ripens.
TrichomesThe tiny mushroom-shaped resin glands that cover the buds and nearby leaves. Their colour change tracks chemical maturity like a traffic light: clear means the compounds are still building, milky (cloudy) means peak accumulation, and amber means degradation has started. Under a 60× loupe or pocket microscope they are easy to read. They hold the bulk of the THC, CBD and terpenes and are the true harvest signal — pistil colour is a rough first hint, but trichomes are the real clock.
Calyx, bud and colaA calyx is one teardrop floral pod. Many calyxes stacking together form a bud, and a large bud cluster on a main stem is a cola.
EC and VPDEC (electrical conductivity) measures how concentrated your nutrient solution is — the strength of the water, not the volume. Picture a cup of salt water: more dissolved minerals means a higher EC number. A drip EC between roughly 1.8 and 2.8 mS·cm¹ covers most of flowering; below that range plants starve, above it they burn. VPD (vapour pressure deficit) is the air’s pull on the moisture inside the plant’s leaves. Think of a wrung-out towel versus a soaking-wet one: dry, warm air is the wrung-out towel — it pulls hard (high VPD), so the plant opens its stomata wide and drinks fast; cool, humid air is the wet towel — it barely pulls (low VPD). VPD combines temperature and humidity into one number in kPa so you can tune how hard the plant has to work to stay hydrated. Cannabis flowers best between roughly 1.0 and 1.4 kPa depending on stage.
Defoliation and lollipoppingRemoving leaves to open up airflow and light (defoliation), and stripping the lower third of the plant so energy goes to the top buds (lollipopping).
From calyx to cola1Calyxone floral pod→2Budstacked calyxes→3Colabud cluster on astem→4Pistilswhite to orangehairs→5Trichomesclear to milkyto amber
Figure 2. The same flower, named at different scales. Pistils and trichomes are the two signals you watch to time the harvest.
Parts of a flower (bud)what you are actually looking atCola (flower cluster)Calyx (the bud core)Pistils (the hairs)Trichomes (resin)Pistils change colour with maturity; trichomes (not pistils) are the reliable ripeness signal.
Diagram. The parts of a flower: cola, calyx, pistils and the resin trichomes that signal ripeness.
VPD: the climate sweet spotVapour pressure deficit blends temp and humidity into one number; steer to the band for the stage.too humidclones / vegflowerdrystress0.2 kPa1 kPa1.8 kPa
Diagram. Hold VPD in the flowering band as the canopy fills in.
Lollipopping: keep the top, strip the bottomlight and air reach the buds that payKEEPREMOVElarf & shadeRemove the lower third (small ‘larf’ buds and shade leaves) so energy, light and airflow go to the productive canopy.
Diagram. Lollipopping and defoliation in flower open the canopy to light and air.
Evidence assessment

Evidence and limitations

How sure is this paper?

We've gone to great lengths to keep these guides honest. One of the main ways we do that is self-review: we actively look for claims that are subjective, only lightly backed by literature, or based on grower practice rather than a controlled study — and we call those out instead of dressing them up as settled science.

Often there simply is no paper for the decision you're making. In those cases we're drawing on what other growers report and what has worked in our own rooms. That can still be useful — but it is not a lab proof. Do what works for your plants, your room, and your meters. If a table disagrees with your crop, believe the crop and log the difference.

Solid
Well supported by plant science, standards, or broad multi-source consensus
  • Photoperiod cannabis needs long nights; light leaks can disrupt flowering
  • Trichomes and pistils are maturity cues; calendar alone is weak
  • Dense late canopies + high humidity raise Botrytis risk
Operational
What many growers and rooms actually run — start here, then tune
  • 8–10 week hybrid flowering maps and stage climate bands
  • Stretch expectations that vary widely by cultivar and spectrum
Grain of salt
Subjective, thin literature, single studies, or “this works for us” practice
  • Amber trichomes = CBN = couch-lock as a clean pharmacological switch (multi-factor; human CBN evidence limited)
  • Long plain-water 'flush' improving smoke quality (controlled tests often show little benefit)
  • Phone-LED / tiny indicator light as a proven herm trigger (night integrity matters; genetics + multi-stress dominate herms)

See something glaringly wrong? Tell us and we'll fix it. Please open a GitHub issue with the paper name and what looks off (include a source if you have one): Report an accuracy issue. Local law, labels, and licences always override any recipe here. Inline notes labelled grain of salt flag the highest-risk over-trust points in the text.

03 · How it works

What the 12/12 flip does to the plant

Photoperiod cannabis measures the length of the dark period to decide whether it is spring (grow) or autumn (reproduce). When you give it 12 hours of uninterrupted darkness, it reads that as the days shortening and switches into flower production.[2]

The darkness must be truly dark. Uninterrupted darkness is required to trigger and maintain flowering, and even small light leaks during the 12-hour night can stress the plant back toward veg or cause it to make seeds (hermaphroditism).[4] In vitro work shows photoperiod plants will revert when night length is broken.[8]

Twelve hours is the safe default, though research finds slightly longer nights can also work for some cultivars.[1] Most strains show their first pre-flower pistils within 7 to 14 days of the flip. Flip the plant when it is healthy and roughly half to two-thirds of your final desired height, because it will stretch a lot after the flip.

Light vs dark: veg schedule against flower scheduleThe flip is the switch from the top schedule to the bottom one. The 12h dark block must be sealed.18h light (veg)6h dark12h light (flower)12h dark, light-tight0h12h24h
Figure 3. Vegetative runs long days. Flowering runs an equal split, and the dark half is the trigger. Break that dark period and the plant can revert or seed.[4]
The dark must be dark

Stray light from chargers, timer LEDs, or door gaps during lights-off can cause re-vegging or hermaphroditism. Tape over indicator lights and seal the room before you flip.

Photoperiod: the light schedule flips the planthours of light vs dark over 24hVegetative 18/618h light6h darkFlower 12/1212h light12h dark12 hours of uninterrupted dark triggers and holds flowering. A light leak in the dark can revert or stress the plant.
Diagram. The flip to 12/12: twelve hours of unbroken dark triggers and holds flowering.
04 · Weeks 1-4

Stretch and bud set (weeks 1–4)

For the first 2 to 3 weeks after the flip the plant stretches, often nearly doubling in height as it builds the frame to hang buds on, while white pistils appear at the nodes.[6]

Around weeks 3 to 4 the stretch stops and those pistil sites organise into real budlets as calyxes begin stacking. This is the most sensitive window: every bud's final position is being set, so heavy leaf removal here costs you yield. In weeks 1 to 2 plants can gain 50 to 100% in height, so keep PPFD around 800–900 μmol·m²·s¹ and feed at a moderate EC (drip EC around 2.0 to 2.6) as growth is fast.[3]

Plant height through the flower cycleFinal height locks in around week 4. Almost all of the gain is the early stretch.90121152184215flipwk1wk2wk3wk4wk5wk6wk7wk8wk9height %
Figure 4. The stretch is steep through weeks 1 to 3, then flat. Flip at half to two-thirds of your target height to leave room for it.[6]
Weeks 1 to 4 targets. Run early flower slightly warm and humid, then ease humidity down as buds set.[4]
WeekPhasePPFD (µmol·m⁻²·s⁻¹)Day / night tempRHVPD (kPa)Drip ECAction
1Stretch800–85026–28 °C (79–82 °F) / 22–24 °C (72–75 °F)65–70%0.9–1.02.0–2.2Lollipop, light defoliation
2Stretch850–90026–28 °C (79–82 °F) / 22–24 °C (72–75 °F)62–68%1.0–1.12.2–2.4Main defoliation
3Bud set90026–28 °C (79–82 °F) / 21–23 °C (70–73 °F)60–65%1.0–1.12.4–2.6Last light defoliation, then stop
4Bud set90026–28 °C (79–82 °F) / 21–23 °C (70–73 °F)58–62%1.1–1.22.4–2.6Hold steady, no more leaf removal
Defoliate early or not at all

Lollipop and do your main defoliation between roughly day 7 and the end of week 3, before bud sites lock in. Do not defoliate heavily after week 3. See the defoliation and training paper for technique.

05 · Weeks 5-10

Bulking and ripening (weeks 5–10)

Weeks 5 to 7 are peak bulking: buds swell fastest, frost (trichomes) builds, and aroma intensifies, so this is when light, feed and CO2 pay off most.[5]

From roughly week 7 onward the plant ripens: pistils darken from white to orange, trichomes shift clear to milky, and lower fan leaves yellow as the plant pulls stored nutrients into the buds. You push hardest during bulking, then ease off and lower humidity into ripening to protect the harvest. During bulking, push PPFD to about 900–1100 μmol·m²·s¹ (1200–1500 with CO2 enrichment at 1000–1200 ppm) and raise feed to peak EC.[3]

Bulking climate runs around 26–28 °C (79–82 °F) during the day, with RH down to about 55 to 62% to limit mould risk as buds densify, and VPD around 1.1 to 1.3 kPa. In ripening (week 8 and on), drop RH to 40 to 50%, keep day temps in the low-to-mid 20s °C (68–77 °F), and many growers lower or ease nutrients in the last 1 to 2 weeks. Yellowing lower leaves late in flower is normal nutrient remobilisation, not always a deficiency to chase.

Relative bud mass gain per weekPeak swelling lands weeks 5 to 7, then tapers as the plant ripens.02855821105%wk110%wk230%wk355%wk495%wk5100%wk685%wk755%wk830%wk915%wk10
Figure 5. Most of the weight goes on weeks 5 to 7. That is where light, feed and CO2 earn their keep.[5]
Target humidity falls across the cycleEC rises into bulking then eases in ripening. Humidity comes down the whole way to fend off rot.Early 55–65%Bulking 50–60%Ripening 40–50%35% RH55% RH75% RH
Figure 6. Humidity steps down stage by stage as buds get denser. The dense late canopy is where high humidity does the most damage.[7]
06 · The decision

Reading trichomes to find the cut date

Evidence limits

Trichome colour tracks maturation. Livingston measured trichome morphology and metabolite content; the study did not test perceived effects or validate a pharmacological harvest target[5]. Long plain-water flushes are traditional; controlled tests often show little taste or potency gain, so ease EC as appetite falls rather than starving the plant.

Pistils are a rough early signal but they lie. Trichomes are the true clock, and you read them with a cheap jeweler's loupe or pocket microscope.[5]

Harvest when most trichomes have turned from clear to milky or cloudy, with a small fraction going amber. The 80 to 90% milky and 5 to 15% amber split is a grower convention, not a validated effect target. Clear, milky and amber ratios alone do not reliably predict an energetic or heavy experience. If the intended chemical profile matters, compare harvest samples by cannabinoid and terpene analysis.[5]

Wait until pistils are mostly darkened and curled in (roughly 70% or more), then switch to checking trichomes for the real call. Use 60× or higher magnification on actual bud, not sugar leaves, and check several spots, since maturity varies across the plant. Do not harvest on the calendar alone: a week 9 strain can need week 10 depending on conditions and phenotype.

Trichome colour over the ripening weeksAs clear trichomes vanish, milky peaks and amber creeps in. The shaded band is the common harvest window.harvest window0255075100wk6wk7wk8wk9wk10wk11% clear
Figure 7. Clear trichomes fade out as milky takes over and a little amber appears. Cut inside the window as a maturity cue; later cuts often feel heavier, genotype still dominates.[5]
Read trichomes on bud at 60× or more. Pistil colour is only a first hint.[5]
SignalWhat you seeReadinessEffect if cut now
PistilsStill mostly white, sticking outToo earlyThin, harsh, low potency
TrichomesMostly clearToo earlyUnderdeveloped
Trichomes80–90% milky, 5% amberPeak window (early)Common early-cut target
TrichomesMilky with 10–15% amberPeak window (late)Balanced
Trichomes30%+ amber, leaves yellowOver-ripeOften denser/heavier feel; genotype still dominates
07 · Avoid these

Troubleshooting

Most first-grow failures in flower come from a few repeatable errors: light leaks, over-defoliating, chasing the calendar instead of the trichomes, and letting humidity stay high as buds get dense. Each is easy to avoid once you know it. The single most expensive mistake is harvesting too early because you ran out of patience.

The five mistakes that sink most first grows, and the concrete fix for each.[7]
PitfallThe fix
Light leaks in the dark periodTape over LED indicators and seal door gaps. Bright or repeated night interruptions can stall flowering; genetics and multi-stress drive most herms
Over-defoliating or doing it lateStop heavy leaf removal after week 3. Leaves are the plant's sugar factory during bulking
Humidity too high in late flowerPull RH down to 40–50% from week 7. Dense buds plus 60%+ RH invites bud rot (botrytis)
Nutrient burnDrop feed EC and give enough water volume per feed. Burnt tips mean the feed is too strong
Harvesting too early or by calendarLet the trichomes decide. A few extra days to proper maturity beats hitting an exact week
Watch the late canopy for rot

High humidity (60% and up) in dense late-flower canopies sharply increases bud rot risk. Botrytis starts inside the bud where you cannot see it, then collapses a whole cola. Keep air moving and humidity low through ripening.[7]

08 · Reality check

Expected results and limitations

A typical photoperiod indoor grow runs about 8 to 10 weeks of flower on top of veg time, and your first harvest will rarely be your best. Yield and quality climb as you learn to read the plant. Strain genetics set the outer limits, while your light, climate and feed decide how close you get to them. Breeder-stated flowering times are estimates, and the actual finish varies with conditions by a week or more in either direction.[6]

A week-by-week map, not a rulebook. Let the trichomes have the final word.
WeekPhaseWhat the plant is doingPPFDDay/nightRH / VPDAction
1–2StretchDoubling in height, first pistils800–90027 °C (81 °F) / 23 °C (73 °F)65% / 1.0 kPaLollipop, defoliate
3–4Bud setStretch ends, budlets form90027 °C (81 °F) / 22 °C (72 °F)60% / 1.1 kPaStop defoliating
5–7BulkingBuds swell fastest, frost builds1000–110027 °C (81 °F) / 22 °C (72 °F)58% / 1.2 kPaPeak feed, push light
8–10RipeningPistils darken, trichomes go milky90024 °C (75 °F) / 20 °C (68 °F)45% / 1.2 kPaLower RH, ease EC, read trichomes
The flower cycle at a glance1Stretchwk 1-3→2Bud setwk 3-4→3Bulkingwk 5-7→4Ripeningwk 8-10→5Trichome readthe call→6Harvestcut
Figure 8. The whole arc on one line. Each stage hands off to the next, and the trichome read gates the harvest.
Where the weight goes by stageBulking is where most of the final mass is laid down.01632496510%Stretch20%Bud set55%Bulking15%Ripening
Figure 9. Plan your effort around bulking. That is where light, climate and feed move the harvest most.
Three rules
  1. Conditions move the finish date. Genetics set the range, but your climate and feed decide where in it you land, often by a week or more.
  2. Keep simple notes. Week, height, climate, feed, what you removed. This is the fastest way to make each grow better than the last.
  3. The last weeks feel slow. Resist cutting early, because that is where a lot of weight and potency finalise.

Get the flip clean, ease humidity down as buds densify, and let the trichomes decide. From here, read the crop-steering paper to fine-tune feed and dryback, then the harvest, dry and cure paper to protect everything you just grew.

Related papers
Coco & crop steering
Steer the plant with water
Defoliation & training
Train the canopy for yield
Harvest, dry, trim, cure
The whole post-harvest run

References

  1. Ahrens A, Llewellyn D, Zheng Y. Is Twelve Hours Really the Optimum Photoperiod for Promoting Flowering in Indoor-Grown Cultivars of Cannabis sativa? Plants. 2023;12(14):2605. https://doi.org/10.3390/plants12142605
  2. Spitzer-Rimon B, Duchin S, Bernstein N, Kamenetsky R. Architecture and Florogenesis in Female Cannabis sativa Plants. Front Plant Sci. 2019;10:350. https://doi.org/10.3389/fpls.2019.00350
  3. Llewellyn D, Golem S, Foley E, Dinka S, Jones AMP, Zheng Y. Indoor grown cannabis yield increased proportionally with light intensity, but ultraviolet radiation did not affect yield or cannabinoid content. Front Plant Sci. 2022;13:974018. https://doi.org/10.3389/fpls.2022.974018
  4. Eichhorn Bilodeau S, Wu B-S, Rufyikiri A-S, MacPherson S, Lefsrud M. An Update on Plant Photobiology and Implications for Cannabis Production. Front Plant Sci. 2019;10:296. https://doi.org/10.3389/fpls.2019.00296
  5. Livingston SJ, Quilichini TD, Booth JK, Wong DCJ, Rensing KH, Laflamme-Yonkman J, Castellarin SD, Bohlmann J, Page JE, Samuels AL. Cannabis glandular trichomes alter morphology and metabolite content during flower maturation. Plant J. 2020;101(1):37-56. https://doi.org/10.1111/tpj.14516
  6. Hesami M, Pepe M, Jones AMP. Morphological Characterization of Cannabis sativa L. Throughout Its Complete Life Cycle. Plants. 2023;12(20):3646. https://doi.org/10.3390/plants12203646
  7. Mahmoud M, BenRejeb I, Punja ZK, Buirs L, Jabaji S. Understanding bud rot development, caused by Botrytis cinerea, on cannabis (Cannabis sativa L.) plants grown under greenhouse conditions. Botany. 2023;101(4):200-231. https://doi.org/10.1139/cjb-2022-0139
  8. Moher M, Jones M, Zheng Y. Photoperiodic Response of In Vitro Cannabis sativa Plants. HortScience. 2021;56(1):108-113. https://doi.org/10.21273/HORTSCI15452-20

Citations marked in-text as [n] map to this list. Primary literature and official guidance except where noted. Cannabis tissue culture is strongly genotype-dependent, verify dilutions, hormone doses and local regulations against the primary sources before relying on them.