Making STS Spray To Produce Feminized Cannabis Seeds

Silver Thiosulfate or STS for short, is an ethylene blocker. Spraying it on a plant will inhibit the ethylene production and cause female plants to produce males flowers. These male flowers however will only produce “female pollen” as they only carry X chromosomes.

Most home growers and breeders use Colloidal Silver (CS) whereas most professional breeders resort to STS in order to produce feminized pollen. The pros love it because it is cheaper, more convenient to apply and more effective than CS.
STS recipes are widely available throughout the internet. However, the ratios and concentrations can vary greatly so we have decided to share the recipe that gave us the very best results throughout the years. Let’s make some STS!

Storing The Solutions

STS spray is simply a mix of 3 different products. Silver Nitrate, Sodium Thiosulfate and distilled water. When these are mixed up together, the STS (Solution AB) only stores for about a month.
However, you can store Solution A and Solution B separately for at least 6 months in the fridge.
Then you can simply mix these 2 stock solutions when your females are ready to be sprayed.


Just a word of caution before we get started, some of these products can burn the skin so you should always wear gloves and protective glasses when handling these.
Whenever you spray STS onto your plants, also wear a mask in order not to breath in the STS droplets.
Last but not least, the plants which have been directly sprayed are unfit for consumption.

Solution A:

  • Fill up a light proof glass bottle with 1000ml of distilled water/reverse osmosis water.
  • Slowly poor 1 gram of Silver Nitrate into the distilled water and gently swirl the bottle around for 30 seconds to dissolve the crystals.
  • Label it “Solution A” and store it in a fridge.

Solution B:

  • Fill up a light proof glass bottle with 1000ml of distilled water/reverse osmosis water.
  • Slowly add 5 grams of Sodium Thiosulfate Anhydrous or 7.8 grams of Sodium Thiosulfate Pentahydrate into the distilled water and gently shake the bottle for 1 minute to dissolve the crystals.
  • Label it “Solution B” and store it in a fridge.

Solution AB (STS Reversal Spray):

STS spray has a short lifespan of about 1 month. Breeders need to store it in a cool and dark place. Keep in mind that the fresher your STS reversal solution (Solution AB), the better. Therefore, it is great to make your STS right before spraying the plants.

4 Simple Steps To Making STS Spray Solution:

1) Poor 100ml of solution B into a cup using a big clean plastic syringe.
2) Using another clean syringe, slowly poor 100ml of solution A into the solution B while stirring with a plastic stirrer.
3) Poor the AB solution into a clean light-proof sprayer.
4) Add 600ml of distilled water and gently shake the sprayer.

There you have it!!

If the solution turns brownish gray, it usually means that the reaction failed as the different ingredients didn’t mix properly.

Mixing up the solution with too much water would cause the STS spray to be too weak and less effective. At the same time, if you do not dilute it enough, the spray solution with burn your plants.
Therefore, diluting 1 part of mixed solution (AB solution) with 3 parts distilled water is the sweet spot.

How and When To Apply STS?

Knowing how and when to apply your STS spray solution is just as important, if not more important than knowing how to make STS.

Spray on the nodes and bud sites, not on the leaves.
Spraying on the leaves is useless and is more likely to burn the plants. It will not prevent the reversal but it will cause you to waste a lot of spray solution and time.

The first application should occur when the plants are still in their vegetative cycle. Spray your females 1 day before flipping them to flower. Reversed females take longer to start producing pollen than regular males. Therefore, flip the donor plants about 1 to 3 weeks before flipping the receiver plants to flower. This will help ensure that you don’t miss the optimal pollination window.
Then, spray every 4 days until you see the first male flowers forming. Pure Sativas require 1 application every 3 days whereas you can spray the Indicas just once every 5 days.

STS Spray Bottle

Increase The Odds

Not all strains respond to STS the same way. Some may produce empty pollen sacks. That is why it is highly recommended to spray several female plants in order to increase the odds.
Good luck!