The Easy Way to Sex a Cannabis Plant.


by Alex Robles

Yes good job, after about three weeks you no longer have a delicate looking seedling.  The stem has thickened up and is now able to support itself more easily and should have at least 3 sets of cannabis leaves showing.  That plant has clearly entered the vegetative (veg) growth stage.  At this point I would keep the light cycle to a healthy schedule which is an 18-6 in my opinion and that means 18hrs of lights and 6hrs of dark.  Cannabis can be vegged with 24hrs of light, but I’m of the opinion that every living thing needs rest to thrive.  This is the stage where the plant starts to get taller, bushier and hungrier.   But this is also when I can find out if it’s a boy or a glorious girl.

We can make a plant in veg start to flower by changing the photo-period (light cycle) to a 12-12 schedule which means 12hrs of light and 12hrs of darkness.

Sexing a whole plant (a.k.a. The long way)

Now like any good parent, you’re going to get excited to find out the sex of your plant as soon as possible.  You may catch yourself daydreaming of flipping the light cycle (photoperiod) on it or wondering how tall it should be when you do this.  When a plant has at least four sets of cannabis leaves it’s mature enough to start making a flower or pollen sacs if the photoperiod is changed.  So if I have to sex a whole plant, my rule is “the younger the better” for one reasons, I don’t want to spend resources caring for a  male I don’t need.

The signs that I’m looking for that tell the sex are either “hairs” for females or “pollen sacs” (imagine grapes) for males.  Here’s what usually happens when I sex the whole plant.

  • Indoors the plant takes about 8 days to show a sex under artificial light after the photoperiod is switch to flowering, depending on the strain.  If I put it outside in the spring or fall when the days are shorter, it has take upto two weeks to show signs.
  • As soon as the sex is determined and if it’s a female, I’ll switch the light cycle on her back to an 18-6.   I do this because I want to put her back into a veg as soon as possible so I can grow her out to the size I want her to be, she’s still pretty small at this point.
  • Switching the light cycle back and forth is going to stress the plant out a bit and will cause her to throw out some funny looking new leaves for about two weeks or until she stops protesting and gets use to the new schedule, that’s expected.
  • To help speed up this change I’ll carefully clip off as much of the baby flowering buds as I can.  This helps the plant use energy to make stems and leaves instead of using it to keep flowers healthy.  Since most the of flowering in a young plant is taking place at the top of plant I’ll just “top” it (cut off the top).  I’ll usually just cut about 4in and use it for a clone.

This sexing method could add up to four weeks to my grow schedule and that’s why I rarely use it to figure out  the gender.

Growers Note:  We’ve all been told that one of the signs of a male plant is a tall plant with large internodal spacings (gaps between leaf sets), which is true.  But there are other reasons why a plant might stretch and have large spacing.  Lack of light or a crowded grow space could make this happen and when a plant gets root bound it could also stretch.  What I’m saying is if I’m not familiar with a strain I’ll wait until it’s showing clear signs of a sex before I act.  I’ve used seeds from the same mother and have had a brother and sister look exactly alike until they matured enough to show a sex.

The more Zen way

Instead of putting the plant through all of that and robbing it of precious veg time, I do things a little different.  I’ve learn to wait until the plant starts to grow branches to start sexing it.  As soon as a branch has at least three sets of leaves it’s mature enough to make a flower.

  • I’ll cut a branch and prepare it like I would a clone.  My goal isn’t to make a root, it’s to keep the cut alive and healthy long enough for it  to make a flower or pollen sacs show itself.  If I make a root and it’s a female, that’s a bonus but not what I’m focusing on.
  • I’ll put the cut in a cup of water under a light that’s setup to turn on and off every 12hrs.
  • I make sure to check the water level daily and change the water every other day.  I like to use the water from a fish tank or fish bowl because it has some very light nutrients that a cut can use to stay healthy and I’m confident that I won’t starve or burn it.
  • After about eight days under a light it should start to show signs of a sex, depending on the strain.\

Growers Note:  In the spring or fall I’ll put the cup on a window sill that gets sun all or most of the day if I don’t have the space or an extra light to dedicate to flowering.  This way does take longer than under a light, up to three weeks.

Like I said, this is the way I sex my plants because it seems to take less time and puts less stress on them.    I hope this helps you find out what you have.

Don’t forget to check out the podcast “In My Grow Show” (Click here to link and listen).

Grow, Learn, Teach

One thought on “The Easy Way to Sex a Cannabis Plant.

  1. zoe May 16, 2020 / 10:14 pm

    i have found its actually more stressful to take clones from a baby plant, rather than to flip it until you see the first sign. then flip back. There is a window of time ~12 hours at betwee 7 to 9 days from the change you will see the sex organs. As soon as this happens flip everything back to 18. Even though you flip it the plants are lagged in there photoperiod about 8 days or so. These clones you take to just sex are usually thrown out which is lost biomass and nutrients. Without lab testing its a 2 week process either way, Again its all about whats the least stressful. If you were a plant, would you rather loose one of your few limbs, or just take in a few hours less of sun each day. i dunno about you but i would pick keeping my arm. 🙂 nice article thank you for sharing


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