by Nebula Haze
Table of Contents
Intro: Use Pre-Flowers or Testing to Find the Sex of Young Plants & Seedlings
- Leaf Sample Testing (Bonus) – Identify sex in just 1-3 weeks from germination
- Male Pre-Flower Pics – Appears 3-4 weeks from germination
- Female Pre-Flower Pics – Appears 4-8 weeks from germination
Variability of Plant Sex (How to Increase Ratio of Female Plants)
Example of Male & Female Cannabis Pre-Flowers
The Cannabis Grower’s Shortcut to Pre-Flowers: For regular seeds about half of seeds grow into male cannabis plants, so you must check for pre-flowers. But if you start with feminized seeds, every plant makes buds. No checking pre-flowers required.
A female plant soon produces stigmas. These wispy white hairs are a sure sign of 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 4-8 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”).
Why Do Only Female Cannabis Plants Make Buds?
Cannabis pre-flowers appear at the base of leaves when male plants are about 3-4 weeks old, and female plants are 4-8 weeks old.
Male Pre-Flower
Female Pre-Flowers
(these turn into buds)
This female pre-flower hasn’t released a wispy white hair quite yet (no stigma/pistil can be seen). Just wait a few days and it will appear.
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” seeds (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 🙁
Learn more about why it’s hard to determine sex from looking at seeds
Quick Note About Terms – Female Bud Anatomy
I quickly wanted to share this botanical terminology, since these terms cause a lot of confusion when talking about cannabis pre-flowers. It’s not uncommon to hear different words refer to the same parts of the plant.
(What is the purpose of “stipules”?)
Stigma – The white hairs are technically called stigmas.
Grower Lingo: Why are stigmas usually called “pistils” by cannabis growers?
- In plant botany, a pistil refers to the stigma/hair and the parts it’s attached to.
- Pistil = the entire reproductive unit with ovary + style + stigma.
- I just call them “hairs” to avoid confusion.
Bract – “Buds” are made of many pod-like bracts, stacked on top of each other. Bracts are modified leaf structures that get covered in resin and trichomes. They enclose the female reproductive structure.
Grower Lingo: Why are bracts usually called “calyxes” by cannabis growers?
- In a female cannabis flower, the true calyx is tiny and concealed at the base of the bract.
- Calyx = small leaf-like sepals that surrounds the ovary.
- Whether called bracts or calyxes, what’s most important is these are what buds are made of!
Learn more about cannabis flower anatomy.
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, pre-flowers are the way to go!
How to Figure out Sex of a Cannabis Plant by Examining Pre-flowers
Vegetating plants usually reveal their sex when they’re just 3-8 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 3-8 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 4-8. The time is based on the strain, but basically all vegetative plants of any strain will have revealed their sex by about the 8th 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 hairs, 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 hairs, 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 (stigmas / pistils) sticking out from the top. Sometimes it takes a few extra days for the hairs to appear.
pic by GypsySpirit WhiteEagle
Wispy white hairs (at the joints) 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 stigma yet
Here’s a picture that shows a stigma 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 stigma/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 stigma yet. The long narrow shape is the only thing that gives the sex away until stigmas begin to emerge
Super close-up picture of a female cannabis pre-flower
Female cannabis calyxes with stigmas/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.
Jump to…
What’s the Difference Between Male & Female Cannabis Plants?
Male Plants, Bananas & Hermies
Where can I buy feminized seeds?
What do I need to get started growing indoors?
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