The flower cycle, one week at a time
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Basic · Flower

The flower cycle, one week at a time

This paper shows the eight to ten weeks from the flip (the change of the light cycle) to the harvest. You will know how to examine the plant in each stage, and how to set the correct climate and feed for each phase. You will also know how to use the trichomes, and not the calendar, to select the time to cut.

Basic9 diagrams8 sources~12 min to read
In this paper
01 Purpose and scope02 Definitions03 Evidence and limitations04 The effect of the 12/12 flip on the plant05 Stretch and bud set (weeks 1 to 4)06 Bulking and ripening (weeks 5 to 10)07 Examine the trichomes to find the time to cut08 Troubleshooting09 Expected results and limitations10 References
01 · Start here

Purpose and scope

Cannabis has two large phases: vegetative growth and flowering. In vegetative growth, the plant makes leaves, stems and roots. In flowering, the plant makes the buds that you harvest. A change of the light cycle to 12 hours on and 12 hours off (the flip) starts flowering. For most plants, flowering continues for approximately 8 to 10 weeks. This paper shows these weeks one week at a time.

You can use this paper if you are a new grower. Flowering is the last phase. For typical hybrids, flowering continues for approximately 8 to 10 weeks. Indica types that are not hybrids can complete flowering in 7 to 8 weeks. Long sativa types can continue for 11 to 13 weeks.[6]

A photoperiod plant starts flowering only when the light cycle changes. An autoflower plant starts flowering because of its age. Thus this paper is about photoperiod plants.

The plant shows the stage that it is in. Thus you examine the plant (the pistils, the trichomes and the leaves) and not only the calendar. If the climate, the light and the feed are correct each week, the buds have a high density and a high potency. If the climate, the light or the feed is not correct, the buds can be loose, can have mold, or can have harsh smoke.

The two-phase lifecycle1Seed / clonestart→2Vegetative18/6 light, makethe structure→3Flipchange to 12/12→4Flowering8-10 weeks, budproduction→5Harvestcut aftertrichome check
Figure 1. Flowering is the last half of the cultivation. Before the flip, the plant makes its leaves, stems and roots. After the flip, the plant makes the buds.[6]
Who this paper is for

This paper is for a grower who will do the flip for the first photoperiod crop. The grower wants information for each week. Use this paper with the crop steering paper and with the harvest and cure paper.

02 · Terms

Definitions

This paper uses six terms of growers many times. Make sure that you know these six terms. Then the other sections are easy to read. Each term is easy after the definition.

Flip / 12/12The change of the timer of the lights to 12 hours on and 12 hours off. The photoperiod plant uses the longer dark period as a signal that winter will start. As a result, flowering starts.
PistilsThe small white hairs that extend from the bud sites. At the start, the pistils are white. During ripening, they become orange or brown.
TrichomesThe small resin glands, each with a stalk and a head, that are on the buds and on the leaves near the buds. The color of a trichome shows the chemical maturity, and it is easy to examine the color with a 60× loupe or a small microscope. A transparent trichome shows that the compounds continue to increase in quantity. A milky (cloudy) trichome shows the peak quantity, and an amber trichome shows that degradation starts. The trichomes contain most of the THC, the CBD and the terpenes. Thus the trichomes are the accurate signal for the harvest, and the color of the pistils is only a first indication with low accuracy.
Calyx, bud and colaA calyx is one small part of the flower. A calyx has the shape of a drop. A bud is a group of many calyxes. A large group of buds on the primary stem is a cola.
EC and VPDEC (electrical conductivity) measures the strength of the nutrient solution and not the volume (more dissolved minerals in the water give a higher EC). For most of flowering, a drip EC from approximately 1.8 to 2.8 mS·cm¹ is correct (a lower EC causes a deficiency, and a higher EC causes nutrient burn). VPD (vapor pressure deficit) shows how much the air pulls the moisture from the leaves of the plant. When the air is dry and warm, the VPD is high, and the plant opens the stomata and uses water at a high rate (in cool air with high humidity, the VPD is low). VPD is one value in kPa for the temperature and the humidity together (thus you can adjust the work that the plant does to keep sufficient water). Cannabis makes the best flowers at a VPD from approximately 1.0 to 1.4 kPa, and the value changes with the stage.
Defoliation and lollipoppingDefoliation is the removal of leaves to increase the airflow and the light in the plant. Lollipopping is the removal of the growth from the lower third of the plant. As a result, the plant sends its energy to the top buds.
From calyx to cola1Calyxone flower part→2Budmany calyxes→3Colagroup of buds ona stem→4Pistilswhite to orangehairs→5Trichomestransparent tomilky to amber
Figure 2. The names in the figure are for the same flower at different sizes. The pistils and the trichomes are the two signals that you monitor to select the time to harvest.
Parts of a flower (bud)the parts that you see on a budCola (group of buds)Calyx (inner part of the bud)Pistils (the hairs)Trichomes (resin)Pistils change color with maturity. Trichomes (not pistils) are the accurate signal of ripeness.
Diagram. The parts of a flower: the cola, the calyx, the pistils and the resin trichomes. The trichomes show the ripeness.
VPD: the correct climate bandVPD is one value for the temperature and the humidity together. Keep it in the band for the stage.wet airclone / vegetativefloweringdrystress0.2 kPa1 kPa1.8 kPa
Diagram. Keep the VPD in the flowering band while the canopy becomes full.
Lollipopping: keep the top, remove the bottomlight and air go to the buds with good yieldKEEPREMOVElarf and shadeRemove the bottom third (small ‘larf’ buds and shade leaves). Thus energy, light and airflow go to the top canopy.
Diagram. In flowering, lollipopping and defoliation let light and air go into the canopy.
Quality of the information

Evidence and limitations

Three types of information

We want each paper to be correct. Thus we examine each paper for information that is not solid. This information comes from the view of one grower, from a small number of sources, or from grower methods that have no controlled test. We identify this information, and we do not show it as solid.

Frequently there is no paper for the decision that you must make. Then we use the reports of other growers and the methods that work in the rooms that we know. This information can help you, but it is not the result of a laboratory test. Use the methods that work for your plants, for your room and for your meters. If a table does not agree with your crop, use the data from your crop and record the difference.

Solid
Tests of plants, specifications or many sources that agree show that the information is correct
  • Long nights are necessary for photoperiod cannabis. A light leak can cause problems for flowering.
  • Trichomes and pistils are signs of maturity. A calendar is a weak sign when it is the only sign.
  • In the last weeks of flowering, the risk of Botrytis increases when the canopy has a high density and the humidity is high.
Grower method
Many growers use this method in their rooms. Start with it, then change it for your room.
  • Week-by-week tables for the 8 to 10 weeks of flowering of hybrids, and climate ranges for each stage.
  • The estimates of stretch are very different for each cultivar and each light spectrum.
Weak
The view of one grower, a small number of sources, one test, or a method that works in one room
  • Amber trichomes, then CBN, then couch-lock, as one clear sequence. Many causes have an effect. The data on CBN in persons are small.
  • A long period in which you flush the plants with only water to increase smoke quality. Controlled tests frequently show a small effect.
  • A phone LED or a very small indicator light as a sure hermaphrodite trigger. Light leaks at night are important. Genetics and many stresses have the largest effect.

If you find an error, make a report. We will correct it. Make the report on GitHub, with the name of the paper and the information that is incorrect. If you have a source, include it: Make a report of an accuracy problem. The regulations, labels and licenses of your area always override each procedure in these papers. The label weak in a paper shows the points where the risk of an error is highest.

03 · The mechanism

The effect of the 12/12 flip on the plant

Photoperiod cannabis measures the length of the dark period. The plant uses this length to find if the time of the year is spring (growth) or autumn (reproduction). When you give the plant 12 hours of continuous darkness, the plant uses this as a signal that the days become shorter. As a result, the plant changes to the production of flowers.[2]

The dark period must have no light. Continuous darkness is necessary to start flowering and to keep flowering. A light leak in the 12-hour night can cause stress in the plant, also when the leak is small. The plant can start vegetative growth again, or the plant can make seeds (hermaphroditism).[4] An in vitro test shows that photoperiod plants start vegetative growth again when light comes into the dark period.[8]

A dark period of twelve hours is the safe value to use. Investigations show that nights that are longer by a small value can also be correct for some cultivars.[1] Most cultivars show the first pre-flower pistils in 7 to 14 days after the flip. Do the flip when the plant is in good condition. The plant must have approximately half to two-thirds of the height that you want at the end of flowering. The plant will have a large stretch after the flip.

Day and night: vegetative cycle and flower cycleThe flip is the change from the top cycle to the bottom cycle. The 12h night block must have no light leaks.18h light6h night12h light12h night, sealed0h12h24h
Figure 3. Vegetative growth has long days of light. Flowering has equal periods of light and darkness, and the dark period is the trigger. If light comes into the dark period, the plant can start vegetative growth again or make seeds.[4]
The dark period must have no light

Before you do the flip, put tape on the indicator lights and seal the room. In the dark period, light from chargers, timer LEDs or door gaps can start vegetative growth again. The light can also cause hermaphroditism.

Photoperiod: the light cycle changes the growth stagehours of light and dark in 24hVegetative 18/618h light6h darkFlower 12/1212h light12h dark12 hours of continuous darkness cause flowering and keep it in progress. A light leak can cause stress or vegetative growth.
Diagram. The change to 12/12: 12 hours of dark in one period causes flowering and keeps it in progress.
04 · Weeks 1 to 4

Stretch and bud set (weeks 1 to 4)

In the first 2 to 3 weeks after the flip, the plant increases in height (the stretch). The height frequently becomes almost two times larger. The plant makes the structure that holds the buds, and white pistils start to show at the nodes.[6]

At approximately weeks 3 to 4, the stretch stops. The pistil sites become small buds, and the calyxes start to be in groups. In this period, each bud gets its position. Thus the plant is most sensitive to the removal of leaves. If you remove many leaves, the yield decreases.

In weeks 1 to 2, the height of the plant can increase by 50 to 100%. Thus keep the PPFD at approximately 800–900 μmol·m²·s¹. Apply feed with a moderate EC (drip EC of approximately 2.0 to 2.6), because the growth is fast.[3]

Plant height during the flower cycleThe height is maximum at approximately week 4. Almost all growth is in the first stretch.90121152184215flipwk1wk2wk3wk4wk5wk6wk7wk8wk9height %
Figure 4. The height increases at a high rate in weeks 1 to 3, and then it does not change. Do the flip when the plant has half to two-thirds of the height that you want. Thus the plant has space for the stretch.[6]
Target values for weeks 1 to 4. At the start of flowering, keep the temperature and the humidity moderately high. Then decrease the humidity in small steps during bud set.[4]
WeekPhasePPFD (µmol·m⁻²·s⁻¹)Day / night temperatureRHVPD (kPa)Drip ECTask
1Stretch800–85026–28 °C (79–82 °F) / 22–24 °C (72–75 °F)65–70%0.9–1.02.0–2.2Lollipopping and small defoliation
2Stretch850–90026–28 °C (79–82 °F) / 22–24 °C (72–75 °F)62–68%1.0–1.12.2–2.4Primary defoliation
3Bud set90026–28 °C (79–82 °F) / 21–23 °C (70–73 °F)60–65%1.0–1.12.4–2.6The last small defoliation. After this, do not defoliate.
4Bud set90026–28 °C (79–82 °F) / 21–23 °C (70–73 °F)58–62%1.1–1.22.4–2.6Keep all values stable. Do not remove more leaves.
Defoliate in the first weeks, or not at all

Do the lollipopping and the primary defoliation from approximately day 7 to the end of week 3. Do this work before the buds get their position. After week 3, do not do a large defoliation. For the method, read the defoliation and training paper.

05 · Weeks 5 to 10

Bulking and ripening (weeks 5 to 10)

Weeks 5 to 7 are the peak of bulking. In this period, the buds increase in size at the highest rate, the quantity of trichomes increases, and the aroma becomes stronger. Thus the light, the feed and the CO2 have the largest effect at this time.[5]

From approximately week 7, the ripening of the plant starts. The white pistils become orange, the transparent trichomes become milky, and the bottom fan leaves become yellow. The plant moves nutrients from the leaves into the buds.

Set the light, the feed and the CO2 to the highest values during bulking. Then, in ripening, decrease these values and decrease the humidity, to prevent damage to the harvest. During bulking, increase the PPFD to approximately 900–1100 μmol·m²·s¹ (1200–1500 with CO2 enrichment at 1000–1200 ppm). Increase the feed EC to the peak EC.[3]

In bulking, the climate during the day is approximately 26–28 °C (79–82 °F). The RH decreases to approximately 55 to 62% to decrease the risk of mold while the density of the buds increases. The VPD is approximately 1.1 to 1.3 kPa.

In ripening (from week 8), decrease the RH to 40 to 50%. Keep the day temperature from the low to the middle 20s °C (68–77 °F). Many growers decrease the nutrients in the last 1 to 2 weeks. In the last weeks of flowering, bottom leaves that become yellow are usual, because the plant moves nutrients from these leaves to the buds. The yellow color is not always a deficiency that you must correct.

Growth in bud mass each weekThe growth is at the peak in weeks 5 to 7, then decreases during ripening.02855821105%wk110%wk230%wk355%wk495%wk5100%wk685%wk755%wk830%wk915%wk10
Figure 5. The buds get most of their weight in weeks 5 to 7. In this period, the light, the feed and the CO2 have the largest effect.[5]
Target humidity decreases during the cycleThe EC increases into bulking, then decreases. The humidity decreases in all stages to prevent rot.First stage 55–65%Bulking 50–60%Ripening 40–50%35% RH55% RH75% RH
Figure 6. The humidity decreases in steps, one step for each stage, while the density of the buds increases. The canopy in the last stage has a high density. In this canopy, a high humidity causes the most damage.[7]
06 · The decision

Examine the trichomes to find the time to cut

Limits of the data

The color of the trichomes changes with the maturation. Livingston measured the morphology of the trichomes and the content of metabolites. The investigation did not measure the perceived effects, and it did not validate a harvest target for a pharmacological effect[5]. A usual method is to flush the substrate with only water for a long time. Controlled tests frequently show that a long flush increases the flavor or the potency by only a small value. Thus decrease the EC when the nutrient uptake of the plant decreases, but do not cause a nutrient deficiency.

The pistils are a first signal with low accuracy, and they can give incorrect information. The trichomes are the accurate indicator of the time, and you examine them with a loupe or a small microscope that has a low cost.[5]

Harvest when most of the trichomes are milky or cloudy, and a small fraction of the trichomes are amber. The ratio of 80 to 90% milky trichomes and 5 to 15% amber trichomes is a value that growers use. It is not a validated target for an effect. The ratios of transparent, milky and amber trichomes are not sufficient to show if the crop gives an energetic effect or a heavy effect. If the chemical profile that you want is important, compare harvest samples with an analysis of the cannabinoids and terpenes.[5]

Wait until approximately 70% or more of the pistils are orange or brown and bent to the bud. Then examine the trichomes to make the correct decision.

Use a magnification of 60× or more. Examine the trichomes on the bud and not on the sugar leaves. Examine some areas of the plant, because the maturity is not the same in all parts of the plant. Do not use only the calendar to select the time to harvest. A cultivar with a flowering time of 9 weeks can continue until week 10, because of the conditions and the phenotype.

Trichome color during the ripening weeksTransparent trichomes decrease. Milky is at the peak and amber starts. The light green area is the usual harvest period.harvest period0255075100wk6wk7wk8wk9wk10wk11% transparent
Figure 7. The transparent trichomes decrease while the milky trichomes increase, and a small quantity of amber trichomes starts. Cut in the harvest period, as an indicator of the maturity. If you cut after this period, the heavy effect is frequently stronger. The genotype continues to have the largest effect.[5]
Examine the trichomes on the bud with a magnification of 60× or more. The color of the pistils is only a first indication.[5]
SignalCondition that you seeMaturityEffect if you cut at this time
PistilsMost are white and extend from the budBefore the harvest periodThin buds, harsh smoke, low potency
TrichomesMost are transparentBefore the harvest periodLow maturity
Trichomes80–90% milky, 5% amberPeak period (first part)Usual target when you cut at the start of the period
TrichomesMilky with 10–15% amberPeak period (last part)Balanced effect
Trichomes30% or more amber, yellow leavesAfter the harvest periodFrequently a stronger heavy effect. The genotype continues to have the largest effect.
07 · Problems to prevent

Troubleshooting

Most of the problems in flowering in a first crop are from a small number of errors. These errors occur frequently: light leaks, too much defoliation, and high humidity when the buds have a high density. A harvest time from the calendar, and not from the trichomes, is also an error. It is easy to prevent each error after you know it. The error with the highest cost is a harvest before the correct time, because you cannot wait.

The five errors that cause problems in most first crops, and the correct task for each error.[7]
ProblemTask
Light leaks in the dark periodPut tape on the LED indicators and seal the gaps at the doors. A bright light in the night, or a light that occurs many times, can stop flowering. Genetics and more than one stress together cause most hermaphrodite plants.
Too much defoliation, or defoliation after the correct timeAfter week 3, do not remove many leaves. The leaves make the sugar for the plant during bulking.
Humidity too high in the last weeks of floweringFrom week 7, decrease the RH to 40–50%. Buds with a high density and an RH of 60% or more increase the risk of bud rot (botrytis).
Nutrient burnDecrease the feed EC. Apply a sufficient volume of water for each feed. Tipburn shows that the feed EC is too high.
A harvest before the correct time, or a harvest that you select with the calendarUse the trichomes to select the time. It is better to wait a small number of more days for the correct maturity than to harvest in the week that the calendar gives.
In the last weeks of flowering, monitor the canopy for rot

Make sure that the air moves in the room. Keep the humidity low during ripening. In the last weeks of flowering, a canopy has a high density. In this canopy, a high humidity (60% or more) increases the risk of bud rot very much. Botrytis starts in the bud, where you cannot see it. Then it causes damage to the full cola.[7]

08 · Results and limits

Expected results and limitations

A typical photoperiod crop in a grow room has the time of vegetative growth and approximately 8 to 10 weeks of flowering. Only in a small number of crops is your first harvest your best harvest. The yield and the quality increase when you know how to examine the plant.

The genetics of the cultivar set the limits. Your light, your climate and your feed change how near the crop comes to these limits. The breeder gives the flowering times as estimates. The finish can be a week or more before or after the estimate, because of the conditions.[6]

This table shows each week. It is not a procedure that you must obey. The trichomes make the last decision.
WeekPhaseChanges in the plantPPFDDay/night temperatureRH / VPDTask
1–2StretchThe height becomes two times larger. The first pistils start to show.800–90027 °C (81 °F) / 23 °C (73 °F)65% / 1.0 kPaLollipopping and defoliation
3–4Bud setThe stretch stops, and small buds start.90027 °C (81 °F) / 22 °C (72 °F)60% / 1.1 kPaStop defoliation
5–7BulkingThe buds increase in size at the highest rate. The quantity of trichomes increases.1000–110027 °C (81 °F) / 22 °C (72 °F)58% / 1.2 kPaPeak feed and high light
8–10RipeningThe pistils become orange or brown, and the trichomes become milky.90024 °C (75 °F) / 20 °C (68 °F)45% / 1.2 kPaDecrease the RH and the EC. Examine the trichomes.
The stages of the flower cycle1Stretchwk 1-3→2Bud setwk 3-4→3Bulkingwk 5-7→4Ripeningwk 8-10→5Trichome checkthe decision→6Harvestcut
Figure 8. The figure shows all the stages in sequence. After each stage, the next stage starts. The trichome check gives the decision for the harvest.
The weight added in each stageThe buds get most of their mass in bulking.01632496510%Stretch20%Bud set55%Bulking15%Ripening
Figure 9. Do most of your work in bulking. In this stage, the light, the climate and the feed have the largest effect on the harvest.
Three important items
  1. The conditions change the finish date. The genetics set the range of the finish date. Your climate and your feed change the position in this range. The position frequently changes by a week or more.
  2. Keep a short record. Record the week, the height, the climate, the feed and the leaves that you removed. A record is the fastest method to make each crop better than the crop before it.
  3. The last weeks are slow. Do not cut before the correct time. The buds get a large part of their weight and potency in the last weeks.

Make sure that the flip is correct. Decrease the humidity while the density of the buds increases. Use the trichomes to select the time to harvest. Then read the crop steering paper to adjust the feed and the dryback. Then read the harvest, dry and cure paper to prevent damage to the crop.

Related papers
Coco and crop steering
Use water to control the plant
Defoliation and training
Canopy training for more yield
Harvest, dry, trim, cure
All the steps after harvest

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

Each citation in the paper, for example [1], refers to an item in this list. Each item is a primary source or an instruction from an authority, unless the item shows a different type. The results of cannabis tissue culture change much with the genotype. Before you use the dilutions, hormone doses and local regulations, make sure that they agree with the primary sources.