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

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 the 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 turns it hermaphrodite. The most susceptible plants already have a predisposition to hermaphrodism. See Chapter Sixteen, “Breeding,” for more information.

This drawing shows the main parts of a female cannabis plant.

The green calyx supports two very small pistils on this pre-flowering ‘Flo’ from DJ Short.

The pre-flower on this ‘Mr. Bubble’ female is very easy to spot with the naked eye.

Pre-flowers on this ‘Puna Budder’ from THSeeds are nearing the end of the pre-flowering stage that lasts about two weeks.
    There are several ways to promote male or female plants during seedling growth. (See “Grow More Female Plants from Seed” in Chapter Two.) 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” (see Chapter Three). For a complete discussion, see Chapter Sixteen, “Breeding.”
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 pistils 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 an 18/6 day/night photoperiod will usually show pronounced pre-flowers before plants 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 soon as you can distinguish pre-flowers as male or female, plants can be induced to flower with a 12/12 day/night photoperiod.
    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 plant. Such stress could cause strange growth, and plants may grow into 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 pre-flowers to develop.

‘Chocolate Chunk’ in early flowering

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 on 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 one-half inch (6-12 mm) fuzzy, white hairs, called pistils that form a “V.” The set of pistils is attached at the base to an ovule, which is contained in a light green pod, called a 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 harvest weight as they swell during the last two or three weeks of growth. Pure sativas, including Thai varieties, can flower for four