Chapter 4: Flowering

Marijuana Horticulture: The Indoor / Outdoor Medical Grower’s Bible

by Jorge Cervantes

Cannabis must flower and produce seeds to successfully complete its annual life cycle. Marijuana is a dioecious plant, being either male (pollen producing) or female (ovule producing). However, hermaphrodite (bisexual) plants with both male and female flowers can occur.

In nature, cannabis flowers in the fall, after the long hot days of summer. The long nights and short days of autumn signal marijuana to start flowering. Plants are normally either male or female. Cannabis produces male or female pre-flowers after four weeks of vegetative growth.

Growth patterns and chemistry change during flowering: stems elongate; leaves grow progressively fewer blades; cannabinoid production slows at first then accelerates; and flower formation is rapid at first then slows. Nutrient needs change as growth stages change. Plants focus n flower production rather than vegetative growth. Green chlorophyll production, requiring much nitrogen, slows. Phosphorus and potassium uptake increase to promote floral formation. Shortly before the flowering stage, growers change to a super bloom fertilizer formula with less nitrogen and more potassium and phosphorus.

induce flowering in greenhouse, outdoors, and indoors by giving plants mre hours of total darkness and fewer hours of light. Give cannabis 12 hours of uninterrupted darkness and 12 hours of light to induce visible signs of flowering in two weeks or less. This program is effective in all but the latest blooming pure sativa strains. Growers with a vegetative room illuminated 18-24 hours a day and a flowering room with 12-hour days and 12-hour nights, create environments that mimic the photoperiod in summer and fall. With this simple combination, growers crank out a crop of outstanding buds every six to ten weeks all year long.

inducing flowering in cannabis grown from seed with a 12/12 day/night photoperiod will cause pants to show sex, male or female. Once the sex of the plant is guaranteed, males are almost always harvested before they shed pollen, and females are coaxed into higher yields. Once the photoperiod is set, disrupting it will cause plants to suffer stress. If they suffer enough stress, hermaphrodite tendencies increase.

Water intake of flowering plants is usually somewhat less than in the vegetative stage. Adequate water during flowering is important for plants to carry on internal chemistry and resin production. Withholding water to stress a plant will actually stunt growth and diminish yield.

Removing large fan leaves to allow more intense light to reach small buds or to stress plants is crazy! Large leaves are necessary to keep plants healthy. Indoors and in greenhouses where the hours of darkness are controlled, cannabis flowers for six to ten weeks or longer. This is a very short time. Hacking off branch tips to initiate more budding sites diffuses floral hormones and retards growth. Remove only leaves that are 50 percent or more damaged by disease , pests, and cultural practices.


Pre-flowers, described by Robert Clarke in Marijuana Botany as “primordial”, are the first indication of a plant’s sex. The pre-flowers grow at branch internodes just behind the leaf spur or stipule about the fourth week of vegetative growth, when the plant is six to eight weeks old. This is the point of sexual maturity, the first sign a plant is preparing for flowering – the next stage in life.

You can see pre-flowers with the naked eye, but a 10 to 30X magnifier will make viewing easier. You can accurately determine plant sex after eight weeks. Using this method, you can distinguish sex before inducing flowers.

Male Pre-flowering

Male pre-flowers are normally visible when plants are six to eight weeks old, after the fourth week of vegetative growth. The pre-flowers emerge behind the stipule at the fourth to fifth branch internodes and generally do not turn into full flowers.

Always wait to induce flowering until after pre-flowers appear. Inducing flowering iwth 12 hours of uninterrupted darkness and 12 hours of light before pre-flowers develop will stress the plant. This stress could cause peculiar growth, and plants might develop into hermaphrodites. Inducing flowering before pre-flowers form will not expedite flowering. In fact, flowering will occur at about the same time as if yu had waited for pre-flowers to show!

Plants grown from seed under a 24/0 photoperiod wil generally show pre-flwers after plants that are given a 18/6 day/night photoperiod. Once pre-flowers are distinguishable as male or female, plants can be induced to flower with a 12/12 day/night photoperiod.

Male Flowering

When given a 12/12 day/night photoperiod, male cannabis reaches maturity and flowers one to two weeks before females. However, male plants do not necessarily need a 12/12 day/night photoperiod to dawn flowers and shed pollen. Males can flower under long days and short nights as well, but they generally produce fewer flowers. Once male calyxes show, pollen develops quickly and can disperse within a very short time. There is always an early opener that shed pollen, often within 24 hours or less! To avoid pollination problems, remove males as soon as they are distinguished. If growing male plants, always isolate them from females, so they will not be pollinated.

Males continue flowering and shedding yellowish, dust-like pollen from bell-shaped pollen sacks well into females’ flowering stage, which ensures pollination. If you are making seeds, pollinating females too early, before the girls have developed many receptive female pistils, will result in a small seed crop.

male flowers are about one quarter inch long and pastel green to yellowish in color. Flowers first develop near the top of the plant. Pollen sacks develop on a short spike and hang in clusters at the base of branches. Gradually, flowers develop towards the bottom of the plant. After two to six weeks of the 12-hour photoperiod, fully formed floral sacks split open and shed pollen.

Males are usually taller than female and have stout stems, sporadic branching, and fewer leaves. In nature, wind and gravity carry pollen from taller males to fertilize (pollinate) receptive females. Male plants produce fewer flowers than females, because one male plant can pollinate many females. Males also contain less THC and overall lower cannabinoid levels.

Males fertilize females, causing them to stop high THC production and start seed formation. Remove and destroy males, except those used for breeding, as soon as their sex has been determined. The instant they show sex, separate male plants used for breeding from females. Do not let them shed pollen. Premature pollen sacks often form and open early or are hidden under foliage and go unnoticed until it is too late. If growing from seed, take special care to ferret out male flowers and plants.

Growers have reported that bouncing photoperiod around and dynamically raising or lowering the temperature have the effect of producing more male plants. Note that each stimulus involves creating a climate that causes plants to suffer stress. Also, the stressful environment does not necessarily turn the entire plant male; it turn it hermaphrodite. The most susceptible plants already have a predisposition to hermaphrodism.

There are several ways to promote male or female plants during seedling growth. During vegetative growth you can get a good idea of a plant’s sex from its genetic background and growth characteristics. The most dependable way to deduce sex is “Cloning for Sex”.

Female Pre-flowering

Near the end of normal vegetative growth, plants grown from seed develop pre-flowers. This is when female calyx formation initiates, and it is not contingent upon photoperiod. It occurs when a plant is old enough to show signs of sexual maturity, about the fourth week of vegetative growth, or six to eight weeks from germination. The pre-flowers emerge behind the stipule at the fourth to fifth branch internodes.

A pre-flower looks like a regular female flower; most have a pair of white fuzzy pistils. Pistils normally form after the light green seed bract part of the pre-flower has formed. Wait until pistil have formed to ensure the plant is a female and not a male. The pre-flowering stage lasts from one to two weeks. A little patience is in order now!

Plants grown from seed under a 18/6 day/night photoperiod will usually show pronounced pre-flowers before pants given a 24/0 day/night photoperiod. And under a 16/8 day/night regimen pre-flowers show more quickly and are often more pronounced. As son as you can distinguish pre-flowers as male or female, plants can be induced to flower with a 12/12 day/night phtoperiod.

Wait to induce flowering until pre-flowers have appeared. Inducing flowering with 12 hours of uninterrupted darkness and 12 hours of light before pre-flowers set will stress the pant. Such stress could cause strange growth, and plants may grow int hermaphrodites. Inducing flowering before pre-flowers develop does not make plants flower faster. Plants will flower about the same time as if you had waited for the pre-flowers to develop.

Female Flowering

Female cannabis is prized for heavy, potent resin production and weighty flower yield. Ideal female plants grow squat and bushy with branches close together on the stem and dense foliage and branches. In most strains, the first signs of female flowers appear one to three weeks after inducing flowering with the 12-hour photoperiod. Female flowers initially appear near the top of the terminal bud and gradually develop on lower branches starting at the tips and moving downward. Flowers have two small one quarter to half inch fuzzy, white hairs, called pistils that form a “V”. The set of pistils is attached at the base an ovule, which is contained in a light green pod, called calyx. Pistil-packed calyxes form dense clusters or buds along stems. A cluster of buds is often called a top or cola. The masses of calyxes develop rapidly for the first four or five weeks, after which they grow at a slower rate. Buds put on much of their weight as they swell during the last two or three weeks of growth. Pure sativas, including Thai varieties, can flower for four months or longer! Once the ovule has been fertilized by male pollen, rapid calyx formation and resin production slow, and seed growth starts.

When females’ flowering is at their zenith, pistils swell and swell. Soon they change in color, most often from white to amber and, eventually, to reddish brown.


Sinsemilla (pronounced sin-semiya) is derived from two Spanish words: “sin” = without and “semilla” = seed. Sinsemilla is the word that describes flowering female cannabis tops that have not been fertilized by male pollen.

Highly prized sinsemilla buds are the most potent part of any strain, with a proportionately large volume of THC per flower bud, and it’s all smoke, no seeds! Unpollinated female plants continue to flower until calyx formation and resin production peak out, six to ten weeks after turning the lights to 12 hours. During six to ten weeks of flowering, calyxes develop and swell along the stem, yielding more high quality buds than pollinated, seeded flowers.

Make any female marijuana sinsemilla by removing male plants as soon as they are identified. Removing males virtually guarantees that male pollen will not fertilize succulent female pistils. Sometimes a few early grains of pollen are shed by premature male flowers. Pollen dispersed from wild or cultivated male cannabis plants could also be floating in the air. Sometimes a hermaphrodite with a few male flowers will sprout on a predominately female plant.

Marijuana Horticulture

2 comments to Marijuana Horticulture Chapter 4: Flowering

  • Anonymous

    You’re really pushing some points here that have no validity. Simply observing nature proves that male and female plants can flower side by side through the entire stage and produce perfectly viable seeds. Until I see a video where male plants uproot themselves, walk far away, and return to the females when they’re good and ready to be pollinated, I can only take what you’re saying as personal opinion.
    “The pre-flowers grow at branch internodes just behind the leaf spur or stipule about the fourth week of vegetative growth, when the plant is six to eight weeks old”.
    This sentence makes zero sense. since day one of vegetative growth equals day one of the plants life, where are you getting the extra two to four weeks of age from?
    Lastly, we can put the “don’t flower until pre-flowers appear” comment to bed. Again, look to nature. The seasons do what they do and cannabis follows suit, not the other way around. There is no waiting for pre-flowers in parts of the world where cannabis flowers early and those plants thrive none the less.
    I appreciate anyone’s personal preferences and opinions about growing, as long as they are stated as such.

  • shaikh salman

    I like all information which is published by you on flowering, but sir I also want to know in detail about the chemical changes occurs in developing “BUD TO FLOWER”. If you have any kinds of information plz inform me…..


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