Sexing Cannabis Plants

Rambo August 23, 2012 38
Sexing Cannabis Plants

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

Signs of a male plant

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

Pistils of a female plant

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.

Sinsemilla

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.

Caveats

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.

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38 Comments »

  1. gore August 24, 2012 at 6:54 am - Reply

    I have been able to “pre-sex” young seed starts by scent. It is not an exact science, at all, but some years I have had up to 85% accuracy using my nose alone. It’s possible that developing plants produce different terpenoids that are unique to their gender.

    This offers little practical application and is something that requires practice and a working familiarity with ones chosen genetics.

    I’m curious to know if any other growers have been able to sex plants by scent alone.

    • rYaNwidow December 5, 2012 at 8:55 pm - Reply

      yes all myn have given off potent smells from 2nd wk by rubbing stalk you can get a wiff off a semingly male plant. My moto is no smell either male or s*it genetics

  2. 45YearToker August 29, 2012 at 2:37 pm - Reply

    There are a couple of points to make here. One is that if you’re using the method of closely examining the pre-flowers with a magnifier, look to see if the flower is sitting on a small stalk and doesn’t appear to be a properly formed female floral bract, there is a 99% certainty it’s male. And with plants grown from Fem seed, the female pre-flower can look very ‘leafy’ & not all that flower like. Why sex Fem seed you ask? Because seed companies make errors in packaging, and it’s been stated that not always are 100% of the seeds created as Fems, really female. Personally, I find this hard to believe, but I’ve personally had packaging errors and Fems that grew male! I kept one such plant and used it for breeding. While I came up with some killer stuff, I have no idea of all the genetics. As the seed was not a Fem, it’s also possible that it wasn’t even the variety that it was labeled to be, either. .Also, as for the method of exposing the plant to a short light cycle until it reveals it sex, while I haven’t done it myself, folks have told me that they have been able to use this technique on single branches, covering the branch in a black bag of some sort, and that in doing so, only the branch being covered shows the sex. Anybody that can confirm this? Lastly, I seriously think that Gore has something in his ability to ‘sniff out’ males. I’ve tried it with my crop this year, and I could always detect a difference between the plans that later proved males and those that later proved female. Still, I wouldn’t want to rely solely on this technique, but rather use it to get an early idea of what I’m going to have.

  3. mike November 8, 2012 at 4:17 am - Reply

    Hi my friend has a question. He was wondering if its possible to clone two clones together?

    • Rambo November 8, 2012 at 7:25 am - Reply

      Mike, I’m not exactly sure what you are asking. I suspect you terminology is wrong. Are you asking if two plants can be grafted together or are you asking if two cuttings can be rooted at the same time. In either case this has nothing to do with the article “sexing cannabis plants.” Please ask your questions again in the forum and try to clarify exactly what you want to know. Thanks.

    • rYaNwidow December 5, 2012 at 8:50 pm - Reply

      yes there are many strains that have been multiply cloned f1,f2 hybrids

  4. rYaNwidow December 5, 2012 at 8:48 pm - Reply

    I’ve found that causing a little stress to plant (lst) also read about this gives you more chance of a fem, but not too much stress as male’s are more likely

    • Rambo December 5, 2012 at 9:02 pm - Reply

      I hate to break it to you but stressing a plant does not increase the likelihood of it being a female. The plants sex is ingrained in its DNA when the seed is formed, long before the plant is coaxed to show its sex. Stress can however cause a female plant to become a hermaphrodite.

  5. seedkingseeds December 29, 2012 at 7:49 am - Reply

    no self respecting grower smells the stem of a plant to sex it omg lmfao noobs

    • Rambo December 29, 2012 at 2:51 pm - Reply

      I certainly can’t smell the difference, but it’s likely that they do produce different odors. If a trained pig or dog can smell cancer inside someone then I suspect they could also pick up on the difference in smell between mail and female plants. My nose is certainly not skilled enough. I don’t think the comment was made as a suggestion that self respecting growers should ID plant sex with there nose, but I suspect that they do produce different terpene levels and could theoretically be identified in this way.

  6. seedkingseeds December 29, 2012 at 7:52 am - Reply

    you guys do realize theres such a thing as dual sex plant right? there not common, but i had 3 skunk #1 from delta 9 that were self pollinating.. in other words the top of the plant had male characteristics, while the lower branches were females with pistols.. I didnt believe it either until I started studying botany at university

    • Rambo December 29, 2012 at 3:46 pm - Reply

      Interesting. I have seen plants do that but I always just assumed they were herms. How are the two differentiated?

  7. seedkingseeds December 29, 2012 at 7:55 am - Reply

    and yes you CAN graft two weed plants together… while it serves no purpose other than to look cool, you cn even graft it onto a hops plant, as many people dont realize how closely related weed and hops are…. the only real use I can see out of grafting a weed plant is you could graft a indica onto a larger and older sativa plant maybe if you lived in a dry climtae, since sativa have longer and bigger roots systems, you could tap into that with the roots of your indica, therefore having to feed it less and water it less, just make sure that both the plants are fert tolerant!

  8. seedkingseeds December 29, 2012 at 7:56 am - Reply

    f1 hybrids is not a clone… its a first generation seed that hasnt been stabilized aka IBL.. the farther away you go from a f1 seed, the genetics dont get weaker, but you WILL lose the uniformity and common phenotypes

  9. bnofoshodoe May 1, 2013 at 10:29 am - Reply

    Rambo, thank you SOO much for this post. I’m a firm believer of growing outdoors under ‘organic’ light but recently I was forced to purchase some clones, something I’m not very fond of but the seeds I had were underdeveloped. These clones started showing their sex pretty early and it worried me, but after reading about blue light sexing I was able to figure out that my fluorescent light isn’t giving them the full spectrum they need. I have since fixed my problem and are well on their way to going outside by mid-May.THANK YOU. I wish I could shake your hand but a virtual one will just have to do.
    Keep up what you’re doing here, it’s extremely informational. If you were to write a book, The Cannabis Grow Bible will have a run for its money!

    • Rambo May 1, 2013 at 10:48 pm - Reply

      Thank you, It’s always nice to hear when someone enjoys your work.

  10. cris June 17, 2013 at 7:39 am - Reply

    hello , I’m a beginner grower and I would like to know if the male plants smell the same as the females.

    • Rambo June 17, 2013 at 11:19 am - Reply

      As a beginner you won’t be able to smell a difference before they are sexed. Some very experienced growers say they can, but I’m skeptical about that. A well trained dog might be able to tell the difference but probably not the human nose

  11. Bonnie July 7, 2013 at 7:09 pm - Reply

    An old timer told me that you can sex a plant by the position of the stems to the trunk. Males stems grow on the stem at the same point. Females will be one above the other. Anyone hear this or know if it’s true? I’m a first time grower n could use some help. Thanks!!

    • Rambo July 9, 2013 at 12:57 pm - Reply

      My experience would say this is a complete myth. Most plants will veg with the leaves equal on both sides and then begin to alternate once they begin to flower.

  12. calli August 22, 2013 at 8:37 pm - Reply

    Also new you have been very helpful. Thank you

  13. diane September 2, 2013 at 12:54 am - Reply

    Hello, im not real new to growing but this is my first attempt at clones. I have had success using plain water and good soil for my first 2, the ones growing or lack of growing in coco coir fiber stuff look horrible. My ph is right and keeping nutes low, using a dome but it’s not looking good. Any suggestions

    • Rambo September 3, 2013 at 12:21 pm - Reply

      I’ve never tried coco or talked to anyone who has. Rockwool or rapid rooters are pretty easy and shouldn’t give you any problems.

  14. steve September 5, 2013 at 4:34 pm - Reply

    Q. If buds begin to form after using 12:12 light cycle manipulation for a week, is it safe to return to 16:8 or is it too late?

    • Rambo October 2, 2013 at 9:17 pm - Reply

      Magic eight ball says “Cannot predict now”

  15. pikey October 3, 2013 at 9:38 am - Reply

    Yes dual sex is different then herm…. Herm is both sexes at once, pollen pods growing in with the flowers. I just had a female plant about 3 weeks into flower all of a sudden have 6 lower branches grow male, with no female flowers on the new male branches… Just cut the male branches off and she kept growing as female. Very rare but it does happen.

  16. aor November 21, 2013 at 12:49 am - Reply

    Hi Mr Rambo.

    I have just been through some of your posts and i thank you a lot for all the help and info you provide for new growers. I am newto growing cannabis and i live in a tropical place where it’s 12/12 during summer. I’ve started a plant from a random seed outdoors. It’s been like 2 months and 3 weeks now and it’s still not showing sex. It is about 35cm tall. I’ll post 2 pictures, maybe with your expert eyes, you can detect whether it’s male or female? Thanks a lot.

  17. aor November 21, 2013 at 12:50 am - Reply

    I dunno where to post pictures. lol

  18. aor19 November 21, 2013 at 5:18 am - Reply

    Hi Mr Rambo.

    I have just been through some of your posts and i thank you a lot for all the help and info you provide for new growers. I am newto growing cannabis and i live in a tropical place where it’s 12/12 during summer. I’ve started a plant from a random seed outdoors. It’s been like 2 months and 3 weeks now and it’s still not showing sex. It is about 35cm tall. I’ll post 2 pictures, maybe with your expert eyes, you can detect whether it’s male or female? Thanks a lot.

  19. Sugar Iman November 25, 2013 at 5:21 pm - Reply

    I lost my first plant due to mistakes on my part but it was far enough into the flowering stage that I was convinced it was a female plant. I clipped it and started a clone that I use as a mother plant. I have started another plant that is well into the flowering stage and just now started developing seeds. Will this pollinate the mother plant and / or the clones I have taking root, even though they not in the flowering stage?

  20. blacklion February 14, 2014 at 8:03 pm - Reply

    I live in a part of the west coast where it’s legal to grow a certain number of plants. I’ve been growing about 4 years now and am starting a new crop with a varied assortment of seeds I had kept in containers through the seasons. They were all worthy enough to warrant the time to put them away so I know the donor buds must have been fire! My problem is that I’m planning on starting about 30 of them to hopefully end up with about 10 to 12 females. Since they will be outdoors, I was wondering about your cutting/cloning method. I will be starting them at the same time and wondered if I bought 1 250-400 watt HPS lamp, would I be able to find out how many girls I have? I was thinking about using a walk in closet. Please excuse the length of my post, but so far, you seem to be the most knowledgeable, and most giving of knowledge I’ve run across. Thanks

  21. blacklion February 14, 2014 at 8:09 pm - Reply

    And BTW, the seeds I will start in a dome with peat pellets before I transplant them either into 1 gallon pots, or into the ground which is another question without knowing exactly what I’m starting with.

  22. eleetdojah February 17, 2014 at 2:01 pm - Reply

    Good info @rambo. I have been using an aerocloner for over a year now. I have used practically every other cloning method out there and I have to say the areocloner has by far produced the best, most consistent, and fastest roots. I never use rooting hormone, just plain water and a few drops of cal-mg+. I realize this post is originally about sexing, I was just responding to the few comments about clones. I have had such success with this method I like to share my experience.

  23. Stoned sour February 18, 2014 at 1:18 pm - Reply

    We have had a couple of crops now, we were given some seeds and decided to give them a go. They have come up great and are nearly ready to go. However we found a pollen sack on the least developed one and on a much closer inspection there are a very few male flowers on some of them. We found recently that our light timer had broken and that the light was on constantly for a few days, we put the timer right and it’s all back on track. What should we do with the plants , like I said they are within a week or so of being ready. Should we cut them now, will the buds just be a bit seedy or will they ruin everything. HELP !

  24. Newbi wan qunewbi February 23, 2014 at 6:31 pm - Reply

    I need help i think i messed.up i started to grow for the first time from seeds i had saved .im also using the hempy system. i started at 12/12 what risk do i run ..also my plants are small and bushy .. three weeks in hempy but were germiated in soil…

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