Cannabis is a dioecious plant, meaning it has two sexes. The males produce the pollen and the females produce the buds which accept the male pollen and produce seed. Unless you want seeded bud, you need to find the male plants and destroy them before they flower.
Seeded cannabis used to be a major problem with imported outdoor crops from Mexico. It was easy for a lone male plant to escape detection in a massive ganja field and pollenate many female plants unknowingly. Thus the notorious Mexican brick pack weed of days gone by.
Thankfully, things have improved dramatically with increased legality and better cultivation techniques. Sensimilla (bud without seed) is now the gold standard and all but the lowest quality schwag is going to be seedless.
However the fear of a male plant contaminating an entire grow is still very much a concern in the back of many a grower’s mind. So much so that feminized seed (seed that has been genetically altered to produce only female plants) has now become the norm. Unfortunately, this has created an all new problem of weak genetics that are highly prone to hermaphroditism (female plants that mutate and produce male and female flowers).
9 times out of 10, if you get a bag of weed that has a seed in it, it is due to hermaphrodite pollen contamination, and not a true male accidentally pollenating a crop. This is also why it is generally not advisable to grow “bag seed” as these plants will also be prone to mutation.
I’ll not make this a debate on feminized vs non-feminized seed, you're free to choose whichever you like. I will say that Lucky 13 Seed Company does not, and will not ever produce feminized seed, and I'll leave it at that. But that means if you're growing cannabis from natural organic seed, you’ll need to know how to identity female plants and get rid of the males.
This can be done quite easily in the first ten days of the flowering cycle. But if you have limited plant counts and want to identify your keeper females early and not waste time, money, and space on plants that may turn out to be male, this is how to do it while your plants are still small and nowhere near bloom time.
Step One: Sprout twice as much seed as you intend on keeping
Sexual differentiation in cannabis is somewhat of a sliding scale. At certain developmental points in the plant's growth, the environment can skew your plants to produce slightly more male or female plants depending on variables like temperature, nitrogen content of the soil etc. But all things being equal, out of one hundred cannabis seeds germinated, roughly forty will be male, and sixty will be female.
Therefore if you’re wanting to end up with four female plants, you would want to germinate at least twice that many seeds. For instance, if you germinate a full pack of Lucky 13 Seeds, which contains thirteen seeds, this will give you around 6-8 females. You can then further reduce this down to the four biggest, strongest, and fastest growing females while the plants are still small seedlings and you haven't invested in large amounts of soil, nutrients, and power to grow out many large plants.
How many plants you grow will depend on where you live and what your legal plant limits are. I am certainly not advising you to grow more plants than you are legally allowed, but many people choose to germinate two to three times as many seeds as they wish to end up with. They then discard the weaklings, slow growers, and males, to ultimately end up with their final select plants that they will grow out in full.
Step Two: Grow without topping until the plant is 16” tall and/or has 10 nodes
Allow your seedlings to grow naturally without topping or training until they have developed eight to ten nodes (sets of leaves). Cannabis produces paired leaf sets in an alternating fashion until it reaches the point of sexual maturity; which for many varieties occurs around the tenth node or roughly when the plant is 16 inches tall. At this point the plant will begin to spiral upwards producing nodes on a fibonacci-like spiral arrangement. This is also the stage where you can place the plant into 12 hours of light and 12 hours of darkness to begin blooming. Contrary to popular belief, germinating seeds in a 12/12 light cycle will not induce flowering immediately. The plant must reach sexual maturity and begin the spiralling leaf pattern before it will bloom.
Around the tenth node, or approximately 16” tall, is when the pre-flowers that will allow you to identify plant sex will differentiate. However, if you top your plant (removing the main stem to encourage branching) this will delay pre-flowering and you will not be able to sex the plant until it has grown much larger. This is why I recommend allowing the plant to grow until you have identified the pre-flowers. Once you have identified the sex you can then begin topping and training the plant as you so choose.
Step Three: Use strong magnification
Depending on the strain, the pre-flowers that you will use to identify the plant sex can be very tiny. Unless you have really good vision, I strongly recommend getting a cheap USB microscope or jewellers loupe with 50x magnification to inspect the plants. This will also come in handy come harvest time when you will be inspecting the glandular trichomes for ripeness.
Step Four: Know Where To Look
The pre-flowers are located at the nodes next to the junction between the main stem and the petiole (leaf stem). This is where is can be a little tricky because there will be four different protuberances emerging from this location. The first, and most obvious, is the leaf petiole. The second is the developing side shoot or “sucker shoot”. This will grow out into another stem off the main stem. The third is the pre-flower sheath, which is shaped like a single blade of grass. This is often confused as the pre-flower, but if you look behind this sheath, which acts as a shield for the developing pre-flower, this is where you will see the true sex organ of the plant; allowing you to discern male from female.
Step Five: Process of elimination
Male cannabis plants do not reliably produce definitive pre-flowers at this early stage of growth. Therefore the quickest and easiest way of sexing cannabis is a process of elimination whereby you confirm the female plants and thereby assume the others to be male.
The reason for this is that an underdeveloped male pre-flower can look almost identical to an underdeveloped female pre-flower. Often times males will never reveal themselves completely until they have begun blooming. However a fully developed female pre-flower is unmistakable and easy to confirm.
Using your magnifying tool, look behind the sheath at the junction of the petiole, main stem and secondary shoot. You will see a very small structure shaped like a teardrop.
If the teardrop has one or two silvery-white hairs emerging from it, the plant is female. Under strong magnification you will see that the little hairs are very spikey and translucent. The male plants have no structure that resembles this feature, so once you see that white-silver spikey hair thingy (called a pistil) then you can label the plant as female and move on. Done and Done.
Step Six: What about the males?
As I mentioned, the male pre-flowers are much harder to determine. On many occasions I have pre-sexed my plants and labeled them as male or female, only to have some of the males turn out to actually be females. This doesn’t happen with the females because if you see that little hair, you know for sure it’s a girl. The male pre-flowers that do decide to reveal themselves look like a little ball on a stick. As they develop further they begin to take on the appearance of a small artichoke.
The only definitive way to know for sure if a plant is a male is to put it under twelve hours of light and twelve hours of uninterrupted darkness for a week to ten days. The pre-flowers will rapidly develop into a small flower cluster that resembles a bundle of grapes. At this point you know for sure you’ve got a male and you can chose to cull it or allow it to produce pollen if you’re wanting to make seed.
What if you can’t find any female pre-flowers?
As already stated, most female plants will begin to show their pre-flowers by the tenth node or around sixteen inches tall. But there are some strains, particularly long flowering sativas, that may take a little longer to show. But with patience, eventually the pre-flowers will emerge. Some will be a little earlier than others, but once you see at least one plant with pre-flowers, the others won’t be far behind. Just be patient, and keep checking until you’ve found enough females to fill your plant limit.
The alternative is to take cuttings from each plant, root them, and put them under a 12/12 light cycle until they show sex. This does work, and is a great method if you have the time, but you are also essentially doubling the number of plants you have (legality may be an issue) and very often clones fail to take root, resulting in a much longer timeframe to sex your plants if you have any mishaps.
What if accidentally miss one of the males? Won’t it pollenate my crop and ruin it?
Unless you’re a guerrilla grower with a crop out in the bush that you only visit occasionally, this shouldn’t be much of a concern. Once you’ve initiated flowering, the males will begin blooming much sooner than the females. This gives you plenty of time to positively identify the males and remove them long before the females are developed enough to be fertilized by the males. As long as you can inspect your plants once every two days, you will be able to find and remove the males with time to spare.
One Final Tip:
Lighting and background makes all the difference when searching for your females. Yellow HPS lighting and 'blurple' LED lighting makes it nearly impossible to see the silvery translucent hairs on your ladies. In order to make those little threads stand out, use 6500k "daylight" CFLs or bright white LEDs (I use a bright white LED headlamp) and place your plants in front of a black backdrop for contrast.