A few months ago we published an article about Light Cycles and Flowering Cannabis that generated a lot of questions regarding how to sex cannabis plants. For those of us who have been growing for decades, we often take basic skills like this for granted. There is however a bit more to sexing cannabis plants then simply keeping an eye out for reproductive organs. I’d like to share with you some more advanced techniques for sexing cannabis plants. Perhaps some of our more experienced readers may benefit as well.
First, for all the young ones in this industry, we need to talk about the birds and bees.
Banana and Condom Lecture
Male Flower Characteristics
Males flowers resemble balls hanging from a slender stem. The pre-flowers buds are about the shape of a rugby ball and blunted at the end. When the male flowers begin to mature, they transition from hanging to a more erect position on their stem. When the flowers open, they reveal five yellow or white petals and a central stamen that releases pollen into the breeze.
Female Flower Characteristics
Female cannabis flowers are identified by the presence of their pistils. They have no flower petals like those of the males, but instead produce two long slender stigmas that range in color from white to pink or lavender. These stigmas collect pollen released into the air by the males, and then deliver it internally to the ovaries which pollinate the egg and produce the embryo. Once the stigma has done its job, the embryo begins to grow into a seed. The stigma is no longer needed and dry up within a few days. The ovary at the base of the pistils begins to grow as it produces a seed.
Hermaphrodite Flower Characteristics
Cannabis plants are fundamentally either male or female, however both have the ability to turn hermaphroditic. This means that a female cannabis plant can grow male sex organs and produce pollen and that male plants can grow female pistils. Hermaphroditic male plants are of little concern because males are generally removed from the garden. On the other hand, female plants that grow male sex organs can pollinate themselves and other nearby female plants. The female plants can then produce seeds which degrades the quality of the harvestable cannabis. Hermaphroditism can be triggered by plant stress, chemical treatments, erratic light cycles, or by genetic predisposition. Seeds created by hermaphrodite pollination should be discarded as they are more likely to pass on this trait.
Female cannabis flowers allowed to reach maturity without being pollinated by males are referred to as sinsemilla. Sinsemilla flowers are preferred because they grow larger, contain higher cannabinoid levels, and do not produce seeds. Because cannabis plants are dioecious, the male and female flowers do not occur on the same plant. This means the sexes, once identified, can easily be separated to prevent pollination. Usually male plants are eliminated completely unless seed production is intended.
Plant Sexing Methods
The pollen from a single male flower can pollinate dozens of female plants, turning a garden of sinsemilla into a seed riddled harvest. Obviously this is a nightmare scenario that growers want to prevent at all cost. Telling the difference really isn’t that difficult, you just need to know what to look for and how to go about it.
Method Al Naturale
Many growers simply wait for the plants to begin flowering before they attempt to sex the plants and cull out the males. No extra effort is required for this method and it’s pretty darn easy once you know what to look for. Different strains will begin flowering at different times depending on the amount of sativa or indica in the breed. When grown outdoors under natural sunlight most hybrids will tell their sex and begin to flower around August 1st.
Indica strains are adjusted to the natural light cycles of the northern parts of the world and will usually begin flowering when exposed to between 8 and 10 hours of darkness. In California this means possibly sexing and flowering as early as late June.
Sativas developed nearer the equator and require longer dark periods to begin flowering. Some sativas will continue vegetative growth until they are exposed to a full 12 hours of darkness. Outdoor and with natural light this means some sativas in the Northern Hemisphere may not begin flowering until late September.
There is nothing wrong with this method but it does present a few drawbacks that can be avoided with early detection methods. Time caring for unwanted male plants should be kept as short as possible because males require extra labor, space, water and fertilizer. They also double your plant count which can lead to getting caught and bigger trouble if you do.
Early Visual Detection
Sexing cannabis plants is more difficult while they are still fully in the vegetative growth phase, but it’s not impossible. Often a solitary flower will appear two to four nodes down from the top of the plant. These are usually very small and can be difficult to identify without magnification. The drawback to this method is that males usually show before females and growers often misidentify and cull out females accidentally.
Light Cycle Manipulation
Cannabis plants at any stage of vegetative growth can be forced to show their sex through light manipulation. By exposing your plants to a long night light cycle of 12 on 12 off for about a week, the plants will quickly reveal their sex. After a week the light cycle should be returned to 18:6 or for outdoor, back to natural light so the plants will return to vegetative growth. This method is failsafe, but does have a drawback. Indica dominant plants don’t always want to return to the vegetative phase and can produce some bizarre and unwanted growth or go into full bloom. This method is very easy in grow rooms but requires covering the plants or bringing them in if they are outside.
Clone and Sex
A great method for determining plant sex is to root cuttings from each plants and then sex the cuttings. This allows you to know the sex of each plant in as little as two weeks, without manipulating the light of the plant itself. Simply take a cutting or two from each plant you intend to sex and carefully label the cuttings and the plant they came from. Once the cuttings have rooted, you can switch the light down to 12:12 and force the clones into flowering. Once you have identified the sex of the clone, you will know the sex of the mother or father it was taken from. You can then discard the male plants and replace them with the younger female clones at a 18:6 light cycle. This method does require careful labeling and can be more difficult with large numbers of plants. It also requires that you are at least somewhat proficient at cloning.
I suspect that you could drop the light cycle to 12:12 as soon as the cutting are taken and may be able to identify the sex before they even grow roots. I’ve never tried this but would love to hear if anyone has.
Blue Light Sexing
The flowering response in cannabis plants is triggered by plant hormone PFR changing to PR during a long dark cycle. This PR is returned to inactive and the flowering response is mitigated when the dark is interrupted by red light around 660 nm. Blue light also has the ability to retard flowering response but is less effective. When Blue LED or fluorescent light is the only light used on an 18:6 light cycle, the plants will slightly begin flowering but will continue vegetative growth. After sexing they can be returned to a full spectrum light at 18:6 to produce more robust vegetative growth and halt the flowering process. This method works well for high plant count gardens that would make using the clone and sex method a bit tricky. It can also be used with a full 24 hours of continuous light.
Hole Punch Lab Testing.
Many cannabis testing laboratories will soon be offering DNA testing which can determine a plant’s sex from a leaf sample no larger than your fingernail. Coupled with testing to verify genetics and likely cannabinoid ratios, this will surely become an invaluable resource for many commercial growers and breeding programs.
For instance, many strains like Cannatonic and RX which are prized for their high CBD content, only create a high CBD plant from about 1 in 4 seeds. Growers can now know in early spring which of their seed starts will produce the CBD to THC cannabinoid ratio they are looking for. Clones can then be taken and the less desirable plants discarded.
As with most things there are a few exceptions. Male cannabis plants originating in far northern areas are not photosensitive. This means you can’t trigger flowering by manipulating light. Instead, age and the developmental stage of the plant plays the major role in when the male displays sex and begins to flower.
Excluding some equatorial sativas, most cannabis males will display their sex eventually even under 24 hour light, usually 3 to 9 months. They do however show their sex sooner under 18 hours of light.
Ruderalis strains are also not photosensitive and will begin flowering even under 24 hours of light within a few weeks of sprouting. Many auto flowering strains contain these ruderalis genetic.
I hope this has shed some light on a few of the finer points of sexing cannabis plants and will help you all towards another bountiful and seedless harvest. If anything needs clarification or if you have other methods for identifying plant sex, please post in the comments section below. For any questions not directly related to the article please post them in the forum.
Sexing Cannabis Plants,