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how to sex your marijuana seeds

In My Grow

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.
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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.

Cannabis Sex Reversal Using Silver Thiosulphate (STS)

A lot of people have requested information about reversing the sex of a female plant to produce male pollen. The text from this post was pulled from the forums – Fet gets credit for much of the research and a kind (and currently unknown member) assembled it as it appears below.

The following is a safe, inexpensive, and successful method for reversing the sex of female cannabis plants. Individual plant responses may vary based upon strain, but I can verify that this process is fully effective in stimulating profuse staminate flower production.

This process can be used to:
A: create new feminized seeds from solitary prize mothers that you currently have
B: create interesting feminized-seed hybrids from different prize strains that you currently have
C: create feminized seeds for optimum outdoor use
D: accelerate the “interview” phase of cultivation, in searching for interesting new clone-mothers
E: reduce total plant numbers- great for medical users with severe plant number restrictions
F: increase variety, by helping to create stable feminized seedlines to be used as an alternative to clones

At the bottom of this post are some specific details about the chemicals used, their safety, their cost, and where to get them.

It is important to educate yourself about cannabis breeding theory and technique prior to using a method like this one. Check out Robert Clarke’s “Marijuana Botany”, which is a very good reference.

It is also important to use basic safety precautions when mixing and handling these chemicals, so read the safety data links provided. The risk is similar to mixing and handling chemical fertilizers, and similar handling procedures are sufficient.

Remember: nothing will ever replace good genetics, and some of your bounty should always go back towards the professional cannabis breeders out there… the ones who have worked for many generations to come up with their true-breeding F1 masterpieces. Support professional breeders by buying their seeds. Also, order from Heaven’s Stairway. Not that they need a plug from me, but they are very professional and provide very fast service worldwide.

Preparation of STS:
First, a stock solution is made. It consists of two parts (A and B) that are initially mixed separately, then blended together. Part A is ALWAYS mixed into part B while stirring rapidly. Use distilled water; tap water may cause precipitates to form.

Wear gloves while mixing and using these chemicals, and mix and use in a properly ventilated area. A mask will prevent the breathing of any dust, which is caustic. STS is colorless and odorless, and poses minimal health risks if used as described here. (See material safety data sheet links below). Note that silver nitrate and STS can cause brown stains upon drying, so spray over newspaper and avoid spilling.

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Part A: .5 gram silver nitrate stirred into 500ml distilled water
Part B: 2.5 grams sodium thiosulfate (anhydrous) stirred into 500ml distilled water

The silver nitrate dissolves within 15 seconds. The sodium thiosulfate takes 30-45 seconds to dissolve.

The silver nitrate solution (A) is then mixed into the sodium thiosulfate solution (B) while stirring rapidly. The resulting blend is stock silver thiosulfate solution (STS).

This stock solution is then diluted at a ratio of 1:9 to make a working solution. For example, 100ml of stock STS is added to 900ml of distilled water. This is then sprayed on select female plants.

Both the stock STS and the working solution should be refrigerated after use, as well as the powdered chemicals, to avoid activity loss. Excess working solution can be safely poured down the drain after use (with ample running water) with negligible environmental impact. It’s pretty cheap.

Each liter of stock STS will make ten 1-liter batches of working solution of STS. With the minimum amount of base chemicals ordered from Photographer’s Formulary (see link below), this means that each 1-liter bottle of working solution STS costs less than 9 cents, and can treat 15-20 mid-sized plants. That’s 200 1-liter batches of STS for $18. Note that the distilled water costs far more than the chemicals.

The STS working solution is sprayed on select female plants until runoff. Do the spraying over newspaper in a separate area from the flower room. You probably won’t smell anything, but ventilate anyway. You now have what I call a “F>M plant”; a female plant that will produce male flowers.

After the F>M plant dries move it into 12/12 immediately. This is usually done three to four weeks prior to the date that the target (to be pollinated) plants will be ready to pollinate. Response times may vary slightly depending upon the strain. More specific times can be determined by trial with your own individual strains. In my trials it took 26 days for the first pollen. 30-35 days seems optimum for planning purposes.

So, assuming that a target plant needs 3-4 weeks to produce fully mature seeds, a strain that takes 8 weeks to mature should be moved into flower at about the same time as the female>male plant. A target plant that finishes flowering in 6 weeks needs to be moved into flower later (10 days or so) so that it doesn’t finish before the seeds can fully mature.

A seeded individual branch can be left to mature on a plant for a bit longer, while harvesting the other seedless buds if they finish first. Just leave enough leaves on for the plant for it to stay healthy.

Within days I noticed a yellowing of the leaves on the F>M plants. This effect persisted for two weeks or so; after this they became green again, except for a few of the larger fans. The plants otherwise seemed healthy. No burning was observed. Growth stopped dead for the first ten days, and then resumed slowly. No stretch was ever seen. After two weeks the F>M plants were obviously forming male flower clusters. Not just a few clusters of balls, but complete male flower tops. One plant still formed some pistillate flowers, but overall it was predominantly male.

It is strange indeed to see an old girlfriend that you know like the back of your hand go through a sex change. I’ll admit that things were awkward between us at first.

When the F>M plants look like they may soon open and release pollen, ( 3-1/2 to 4 weeks) move them from the main flower room into another unventilated room or closet with lighting on a 12/12 timer. Don’t worry too much about watts per square foot; it will only be temporary.

When the pollen flies, move your target plants into the closet and pollinate.

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A more controlled approach is to isolate the F>M plants in a third remote closet (no light is necessary in this one, as they are releasing pollen now and are nearly finished anyway). In this remote other closet the pollen is very carefully collected in a plastic produce bag or newspaper sleeve and then brought back to the lighted closet, where the target plants are now located. If this is done, be careful to not mix pollen types by letting the F>Ms dust each other. Avoid movement, or use yet another closet.

Take special care to not let pollen gather on the outside of this bag- a static charge is sometimes present. Drop small open clusters of blooms inside and then close the bag at the mouth and shake. Important: next, step outside and slowly release the excess air from the bag, collapsing it completely, so that pollen doesn’t get released accidently. Point downwind; don’t let it get on your hands or clothes.

This collapsed pollinated bag is now very carefully slipped over only one branch and is then tied off tightly at the mouth around the branch stem with a twist tie or tape, sealing the pollen inside. Let the bag inflate slightly with air again before sealing it off, so the branch can breathe. This technique keeps the entire plant from seeding. Agitate the bag a bit after tying it off to distribute the pollen. Don’t forget to label the branch so you know which seeds are which. Other branches on this same plant can be hit with different pollen sources.

If no lighted closet is available, the plant can be moved back into the main room, but- be very carefulollen is sneaky. After 4-5 days, the bag is gently removed and the plant completes it’s flowering cycle.

Yet another method has worked well for me. I position the target plants in a non-ventilated lighted closet, and then I collect pollen on a piece of mirror or glass. This is then carefully applied to the pistils of one pre-labeled branch by using a very fine watercolor paintbrush. Care is taken to not agitate the branch or the pollen. No sneezing. The plant needs to be in place first; moving it after pollination can shake pollen free and blow this technique.

Regardless of technique, at completion you will have feminized seeds. Let them dry for 2-4 weeks.

About the chemicals:
Silver nitrate is a white crystalline light-sensitive chemical that is commonly used in photography. It is also used in babies’ eyes at birth to prevent blindness. It can cause mild skin irritation, and it stains brown. Avoid breathing. I didn’t notice any smell or fumes, but ventilation is recommended. Be sure to wash the spray bottle well before you use it elsewhere; better yet: devote a bottle to STS use. A half gram is a surprisingly small amount; it would fit inside a gel capsule.

Here are links to some safety data. A Google search will bring up more information if needed.

For a realistic hazard level comparison, here is a link for the safety and handling data for Ammonium Nitrate, or common fertilizer:

Sodium thiosulfate is also a white crystalline chemical commonly used in photography; it is used in photographic fixers. Same general cautions apply, minus the staining. This formula uses the anhydrous type. Non-hazardous.

Have fun experimenting with this technique. Use it responsibly. There are a few good threads here at CW that go into the pros and cons of transsexual agents and feminized seeds. Read them. And most importantly, use STS with quality F1 strains developed by professional breeders for the most consistent results.

A huge thanks to Fet from Spice Brothers Seeds for his help and advice in using this technique. I simply brought together available information from previous posts and tried my own recipe. I’m thrilled to share the results. Future tests will be done to adjust the formula so the molar ratios of the chemicals are correct, as specified by Gobgoober (thanks, Gob) but the formula posted here is completely effective.