How to Predetermine the Sex of Cannabis Plants
A ll of the cannabis we buy, sell, and smoke is harvested from unpollinated female plants. Male plants produce pollen instead of flowers, and pollinated females convert their metabolic energy away from growing flowers to growing healthy seeds instead. The result is smaller, less potent flowers that are full of seeds. Only unpollinated females produce the large, psychoactive flowers that earned the name sinsemilla, literally “without seeds.”
- Understanding the Anatomy of a Cannabis Plant
- How Long Does it Take to Grow Cannabis Plants?
- Are Plant Growth Regulators (PGRs) in Cannabis Cultivation Harmful?
- Sexing Marijuana Plants & Reproduction
Identifying the sex of a cannabis plant as early as possible is a critical skill when growing plants from seed. If you are growing to harvest your own bud, then not only are males useless as they do not grow smokable flower, but they also pose a threat to your female plants by pollination.
The good news is that it is possible to determine the sex of a cannabis plant by identifying its reproductive organs before the plant can use them. This article will explain how cannabis plants reproduce, and examine how to determine the sex of a cannabis plant before they are able to pollinate or be pollinated.
Note: while cannabis plants are capable of hermaphroditism — when a female plant produces its own pollen — this is most often the result of external stress or modulation. This article will focus solely on identifying the initial presenting sex of the plant.
How Cannabis Reproduces
Like most living things on this planet, cannabis plants come in one of two sexes, male and female. Male plants produce pollen which carries their DNA to female plants who receive the pollen and begin to grow seeds.
Male pollen is produced in pollen sacks that grow like clusters of grapes. Eventually, these sacks burst and pollen is released, where it floats on the wind searching for a female plant.
Where male plants grow pollen sacks, female plants will grow a calyx, which contains an ovum that could be pollinated. From the calyx, the female plant will produce fine hairs called “stigma,” and these stigma are adapted to receive pollen.
Once these stigma catch pollen from the air, the plant has been “impregnated,” and the plant’s metabolic energy is directed toward seed production, rather than flower production. The flowers that do grow, then, are smaller, less potent, and full of seeds.
How to Determine a Cannabis Plant’s Sex
A cannabis plant’s life exists largely in two stages: the vegetative stage, and the flowering stage. The vegetative stage is first, when the plant is aggressively growing stalks and leaves, and it is this stage when you will determine the sex of your plant.
Sex structures can begin to show in as few as 14 days, but the average is longer. Keep an eye out beginning the second week of the growing process, and you should catch them as they start coming in.
The plant’s sex organs will grow in the “nodes,” or the corners where the leafy stems emerge from the stalk. But sex organs will not grow from every node, so be sure to examine your plant thoroughly each day.
The male pollen sacks will grow in small bulbous clusters sometimes described as “crab claws.” They may not release pollen for a few more weeks, so you have a small, forgiving window if you miss one. However, the plant material is soft, and a slip of the finger or bump of a tool could puncture or tear them, releasing the pollen inside.
Thus, it is best to identify males as soon as possible and, if you are growing to harvest, remove them from the grow area, and probably from the property. Researchers at Michigan State University have shown that cannabis pollen can carry more than ten miles.
The nodes of female cannabis plants generally emerge later, first presenting a calyx. This is a “pre-flower,” and some growers recognize this “pre-flowering stage” between vegetative and full flowering. The calyx serves as a small sheath containing an ovum. If the plant were to be pollinated, this is where the first seeds would start being produced.
Eventually, wispy white stigma will emerge, holding a hand to the air, hoping to catch some pollen. Once the stigmas are visible, the female plant is mature enough to be pollinated, so make sure any males are far away, or sealed up someplace tight and sanitary
Predetermining the Sex of a Cannabis Plant
If you need to shave a few weeks off the process, photoperiod manipulation combined with cloning may be used to “force sex” a plant. Begin by cutting and potting a suitable clone from the plant. Once it is established and healthy, put the clone under a 12/12 light cycle. This will induce the flowering stage, even in its infancy, and reveal stigmas or pollen sacks. Since this is the same DNA as the plant it was cut from, this reveals the gender of the source plant.
Knowing how to sex a plant is important for both breeders and home-growers hoping to harvest their own bud.
Begin scouting for sex organs after the first two weeks of the plant’s life. They will grow from the intersection where the stems grow out from the stalk, called “nodes.” Male plants will produce pollen sacks that look like grapes or balls, and females will produce thin, wispy hairs called stigma.
Once the female’s stigmas have emerged, the plant is mature enough to be pollinated. Any pollen they receive after that point will “impregnate” the plant. After a female plant has been pollinated, flower production is sacrificed for seed production, effectively ruining your harvest. Fertilized cannabis flowers are smaller, less potent, and full of seeds.
To avoid this whole ordeal, some growers purchase feminized seeds: seeds that are guaranteed to produce a female plant.
How have you determined the sex of your plants? Share your wisdom in the comments below!
Cannabis Pre-Flowers: Identify Sex of a Plant as Early as 3 Weeks Old (with pics!)
The female plants will soon produce pistils. Wispy white hairs are a sure sign that you’re looking at female pre-flowers.
How to Determine the Sex of a Young Cannabis Plant
What are cannabis “pre-flowers?” They are little versions of adult flowers that appear on your marijuana plants relatively early in the vegetative stage.
When I first started growing weed, I learned (incorrectly) that there is no way to determine a cannabis plant’s sex until the flowering stage. But I’ve since learned that pre-flowers can reveal the plant’s sex while it’s still in the vegetative stage! Cannabis plants grow pre-flowers as young as 3-4 weeks from germination for male plants, and 4-6 weeks from germination for female plants.
Cannabis Pre-Flowers Are Small Versions of Adult Flowers. These reveal a plant’s sex.
Knowing the plant’s sex is helpful because most hobbyist cannabis growers would like to identify and remove male plants from the grow room early in the growing process. This is because only female plants make potent buds/flowers, while male cannabis plants make non-potent pollen sacs where female plants would grow buds. Additionally, female buds need to avoid pollen from male plants in order to make the highest quality cannabis (sinsemilla or “no seeds”).
Cannabis pre-flowers appear at the base of leaves when male plants are about 3-4 weeks old, and female plants are 4-6 weeks old.
Even if you’re not 100% sure about every plant from looking at the pre-flowers, it’s nice to know which plants you need to watch closely and which are definitely female. However, if precision is very important…
Chemical Leaf Tests Can Determine Sex & Potency for plants as young as 1-3 weeks
Chemical leaf testing is getting less expensive every day and can be used on cannabis seedlings with just a few sets of leaves to test for sex and future potency.
These tests only require a tiny amount of plant tissue (for example a small punch-out from a leaf, or a single cotyledon leaf), so it won’t hurt or slow down your seedlings to take a test sample!
In general, the tests are available for seedlings as young as 1-3 weeks. Sex testing uses a real-time polymerase chain reaction (qPCR) test, and potency tests use Gas Chromatography with a Flame Ionization Detector (GC/FID) or High-Pressure Liquid Chromatography with a Diode Array Detector (HPLC) for testing.
Although testing can be done as early as week 1 from germination, waiting until week 3 to conduct testing on seedlings can increase accuracy, and some companies won’t conduct testing until week 3.
There are many reasons growers would like to know plant sex as early as possible, as well as be able to estimate the overall THC/CBD ratios of future buds!
Did You Know? There are Chemical Leaf Tests that Can Definitively Determine Both Plant Sex & Future Cannabinoid Ratios of Very Young Marijuana Seedlings!
But for those of us using our eyes…
(these turn into buds)
This female pre-flower hasn’t released a wispy white pistil quite yet
When starting with “feminized” seeds (which you can usually only get from a breeder), all your seeds should end up being female, so determining male from female isn’t very important. Learn more about buying seeds (including feminized seeds) from breeders online.
But for growers starting with “regular” (non-feminized) seeds, about half of the plants can turn out to be male. And unfortunately, there’s no way to look at a seed and be able to tell what sex it is.
Unfortunately, you can’t tell a cannabis plant’s sex for sure by looking at the seeds
How to Figure out Sex of a Cannabis Plant by Examining Pre-flowers
Vegetating plants usually reveal their sex when they’re just 3-6 weeks old from seed, but you have to know where to look.
What you’re looking for is “pre-flowers.” These are tiny versions of adult sex parts, and when you see them you can tell what sex the plant is going to be. They usually show up in the upper parts of the plant, closer to the lights, but sometimes you’ll search the whole plant and only find a pre-flower on a random branch lower down on the plant.
Vegetating cannabis plants reveal their sex with “pre-flowers” that usually appear 3-6 weeks from when the plant first germinated.
Although these are the general shapes of male and female pre-flowers, if you continue looking through the pictures below, you’ll see there’s quite a bit of variation on what pre-flowers look like from strain to strain.
Most male plants have grown a pre-flower by week 3-4 from seed, while female plants don’t show until week 4-6. Basically, all vegetative plants will have revealed their sex by about the 6th week from seed.
So, without further ado, here are pictures showing what you’re looking for when it comes to pre-flowers. Remember, pre-flowers are found at the V where stems meet a main stalk. But pre-flowers don’t usually show up all over the plant. Make sure to look around in different places, especially near the top of the plant and closer to the lights
Note: Pre-flowers show up most often near the top of the plant and closer to the lights but could be anywhere on the plant. There may be just one on the whole plant so you may have to search all over!
Male pre-flowers tend to have a “spade” shape, like the spades from a deck of playing cards. Male cannabis plants often (but not always) reveal their sex sooner than female plants.
Male pre-flowers tend to be shaped somewhat like a spade
This male plant was only 3 weeks when it made its first pre-flower. Notice how tiny it is compared to the giant-sized thumb! Often it’s unclear what the sex is when a pre-flower is this small (unless you’ve got a lot of experience) so if you’re not sure, it’s a good idea to wait and see how it develops, just in case.
Just to give you an idea how small these can be when they show up…
This is the exact same picture as above, but with the pre-flower made bigger so you can see it. Pretty tiny, isn’t it?
Male pre-flowers are basically immature pollen sacs. When the plant starts flowering, they will grow and turn into bunches that almost look like grapes.
I’ve also noticed that sometimes (though not always!) the stipules on male plants seem more “leafy” and less “pointy” than stipules on female plants (the stipules are the green hair-like growths near where pre-flowers show up). However, this is just a generality, and should be used together with other factors to determine if a plant is male! There are definitely male plants with pointy stipules and vice versa, but it’s sort of a general difference.
This particular pre-flower is really tough to determine. However, in the end, it was a male plant. The little “stem” is one clue it may be male
Just like the above male plant, sometimes you get almost what looks like two tiny little leaves that the pre-flower pollen sac “unfurls” from. In the above picture the pollen sac is still mostly hidden, while in this next picture, the tiny growths have opened up to fully reveal the pollen sac. This can be confusing because these extra growths don’t appear on all plants, and are not a pre-flower or a stipule.
Here’s another male pollen sac pre-flower that’s on a little “stem”
A single male pre-flower appears
Once you see multiple pollen sacs and no white pistils, you can be confident it’s a male plant
Although this plant ended up being male, the stipules are long, pointy and crossed as you’d normally see with a female plant. That’s why you need to confirm sex with the pre-flowers and not just look at other factors on the plant!
Sometimes the pollen sacs look a little unusual when they first start growing in, but you know it’s male when you see several pre-flowers without any pistils stacked on top of each other like bunches of grapes
If you click the following picture and zoom in close, you can see pollen sacs scattered among the leaves
This is what male pollen sacs look like when the plant actually starts flowering
This male cannabis plant has gotten further along in the flowering stage
This is what a male plant looks like at maturity when it’s starting to spill its pollen
Another example of pollen spilling onto a nearby leaf
For those who’ve never seen a male cannabis plant in its full glory
Ok, now that you know what male pre-flowers look like, what do female pre-flowers look like?
Female pre-flowers tend to be longer and narrower than male pre-flowers, sometimes with a fat bottom. They also usually (but not always) have 1-2 white hairs (pistils) sticking out from the top. Sometimes it takes a few extra days for the pistils to appear.
Wispy white pistils are a sure sign that you’re looking at female pre-flowers
This pre-flower doesn’t have a pistil sticking out at first, but the shape helps tell you it’s a female plant. If you’re not sure about sex after spotting a pre-flower, it’s a good idea to wait and see for a little while, just to see if a white hair appears (which means it’s definitely a girl)
Another example of female cannabis pre-flowers that haven’t revealed their pistil yet
Here’s a picture that shows a pistil right as it’s emerging from the calyx!
If the pre-flower is very pointy and thin like this one on the right, it is often a female pre-flower
Some of the time the stipules (green hair-like growths near where pre-flowers show up) will cross each other on female plants. This certainly doesn’t always happen, as you can see from the pics of female pre-flowers on this page, but while girls can go either way, male plants rarely have stipules that cross each other. So although crossed stipules cannot be used definitively as a way to identify female plants, it can be a small clue to help guide you when you’re not sure. For example, the following female pre-flower doesn’t have a pistil, but the long thin shape combined with the crossed stipules help indicate that this plant is a girl. Whenever in doubt, wait a week and look again!
This female plant has a long, thin calyx and crossed stipules, which are typical female plant features
In this pic, you can see white pistils emerging from the calyxes. Female pistils are white and wispy, never green.
Here’s another female pre-flower that doesn’t have a white hair yet, but you can tell it’s female because it’s long and narrow, instead of spade-shaped
One last female pre-flower without a pistil yet. The long narrow shape is the only thing that gives the sex away until pistils begin to emerge
Super close-up picture of a female cannabis pre-flower
Female cannabis calyxes with pistils, under an LED grow light
Did you know that pre-flowers/calyxes/flowers are actually what holds seeds if your plant gets pollinated? Once pollen touches the white pistils, the pollen gets delivered to the inside and a seed starts forming!
Variability of Cannabis Plant Sex – How to Increase Ratio of Female Plants with Regular Seeds
In fact, to this day scientists are still not sure exactly what causes certain plants to be one sex or another after sprouting. We’ve identified several factors that predict the overall likelihood of male/female plants (for example feminized seeds always produce female plants no matter what), but sex seems to be somewhat fluid in cannabis plants when they’re first germinated.
Certain conditions such as excessive heat, stress, unusual light periods and nutrient problems can cause a greater percentage of plants to produce male flowers.
You may be able to increase the percentage of female plants with regular seeds during the first few weeks of life
On the flip side, the following factors may possibly increase the ratio of female plants with regular seeds (learn more):
- Healthy Mom – Only grow seeds from a vigorous, healthy mother plant who never showed any signs of herming or male pollen sacs (seeds are more likely to grow pollen sacs if the mom plant had a tough start in life, or hermed during the flowering stage)
- Cool Temperatures – Give seedlings slightly cool temperatures (65-75°F day and night) and avoid excessive heat
- High Humidity (50-70% RH)
- Short but not too short days. Keep consistent day and night periods with no light interruptions at night, and days should be 14-18 hours long (between 14/10 and 18/6) for the first few weeks
- Blue light. Always start seeds under a vegetative grow light (something with plenty of blue like a Metal Halide or a 6500k CFL/T5/fluorescent)
- Avoid Deficiencies – Make sure to provide plenty of Nitrogen and don’t let seedlings become nutrient-starved or run into other types of deficiencies
- Prevent Stress, especially heat or light stress during the first few weeks
- Happy Roots – Avoid over (and especially) under watering
Once a cannabis plant is about 3 weeks old, its sex is pretty much completely set and can be determined either by visual inspection or by chemical leaf test.
Unfortunately, due to the fact that different environmental conditions during the first part of life can alter the sex, you can’t look at seeds and definitively know one way or the other whether the plant will end up being female because even the plant doesn’t necessarily “know”.
For example, say you take a clone of a seedling before it’s 3 weeks old. It’s possible that one clone will be male, and the other clone will be female. However, if you take a clone after week 3, the sexes of clones will always match each other. This is further evidence to indicate that the environment can affect sex expression in some cases.