This board is for growers who make their own seed instead of buying it. It covers choosing parent plants, making a cross and selecting the best plants over several generations. It also covers feminized seed made with silver thiosulfate (STS). STS is a silver solution sprayed on a female plant so that it grows male flowers.
The board also deals with drift and degradation. Drift means traits shift or disappear by chance when only a few plants are kept each generation. Problems like this often show up in a line two generations after the cross. The causes and fixes are covered in Cannabis Strain Degradation.
Breeding is mostly selection, and selection is mostly record keeping
Pollinating a plant takes one afternoon. Deciding which of the offspring should become a parent takes a full grow cycle. To decide well, you need honest notes on every plant. You also have to be ready to throw away almost all of them.
The threads on this board cover that working habit as much as technique. Each plant you grow has its own phenotype, meaning how it actually grows, looks and smells. Choosing one keeper from many seeds is explained step by step in Phenotype Hunting: From a Packet of Seeds to One Keeper Mother.
What to do: before your first cross, give every plant a numbered tag. Keep one notebook or spreadsheet for each cross. Write down the parents, the sowing date and a short weekly note for each plant: vigour, smell, pests, flowering time and any male flowers. Then you choose parents from written records and not from memory.
Two things to understand before you start
Feminized seed and regular seed do different jobs
To make feminized seed, you spray a female plant with STS. The chemical makes it grow male flowers, and you use their pollen on another female or on the same plant. Both parents are female, so the seeds carry the genetics of two females and grow into female plants.
A hermaphrodite is a plant that carries both female and male flowers. A regular cross uses a true male and a true female. If female offspring from that cross grow male flowers in normal conditions, you know the tendency is in the genetics. Then you can drop that parent.
A feminized cross cannot give you this information. Its pollen parent is a female that a chemical forced to make male flowers, and its offspring include no males to select from. So feminized seed cannot be used to check a line for hermaphrodites the way a regular male can.
Both methods are useful, and one cannot replace the other. Feminized seed gives you a crop of females from a known plant. Regular seed gives you males to select from and a way to check for hermaphrodites.
What to do: if you plan to keep a line for several years, also keep a regular (male and female) version of it. That way you can still select males and check the line for hermaphrodites later.
The first generation is uniform; the second shows the variation
A stable parent is a line whose seeds grow into plants that look and behave alike. F1 is the first generation from crossing two parents. F2 is the seed you get by crossing two F1 plants. Seed-label terms like F1, F2 and BX are explained in Cannabis Genetics Decoded: The Breeder Vocabulary Every Grower Misreads on Seed Labels.
When both parents are stable, the F1 plants are usually uniform. In the F2, the parents’ traits mix in new ways, so plants begin to differ from each other. Most home breeders never grow out an F2. As a result, they never see what their cross really carries.
What to do: for any cross you want to keep, save some F1 seed and grow an F2 from it. Grow as many F2 plants as your space allows, and record how much they differ. This shows you which traits are already fixed and which still split.
Backcrossing, outcrossing and stabilising a line
| Method | What you do | Why you do it | Main risk |
|---|---|---|---|
| Backcross (BX) | Cross the offspring back to one of its parents | Bring the line closer to a plant you want to keep | The line narrows, and the trait you wanted from the other parent can be lost |
| Outcross | Cross with an unrelated line | Add a new trait or more vigour | Uniformity breaks, and you need more generations of selection |
| Stabilising | Cross selected siblings for several generations | Make seeds grow into plants that are alike | Drift and weaker plants if too few plants are kept |
Backcrossing step by step
A backcross usually goes back to a female you want to preserve, often a kept mother clone. Breeders call this plant the recurrent parent. You need to keep it alive as a clone for the whole project, so read Mother Plants & Cloning first.
- BX1: cross an F1 plant with the recurrent parent.
- BX2: select the best BX1 plant and cross it with the same parent again.
- BX3 and BX4: repeat the same step with the best plant from each round.
With each round, roughly half of the genes that still come from the other parent are replaced. On average, BX1 carries about 75% of the recurrent parent’s genes, BX2 about 87.5%, BX3 about 93.75% and BX4 about 96.9%. These are expected averages; real plants vary around them. To make each cross, you pollinate the mother clone with a selected male offspring. Another way is to reverse one of the two plants with STS.
What to do: in every round, keep only the plants that show the trait you wanted from the other parent. The aim is to get most of the mother’s character plus that one trait. If you skip selection in one round, the trait can be lost.
Outcrossing
Outcrossing means crossing your line with a plant from an unrelated line. Use it when your line lacks something it needs, such as vigour or tolerance to a problem you see on your farm. The F1 may look good. But the following generations will vary again, so plan for several rounds of selection.
Stabilising a line
To stabilise a line, you grow F2, F3 and later generations by crossing selected siblings. In each generation, you keep only plants that show your target traits. You stop when the seeds grow into plants that are alike.
Crossing close relatives is called inbreeding. If you keep very few plants each generation, chance decides which traits survive, and that is drift. Close inbreeding can also make plants weaker over time. This is one reason a line can degrade two generations after the cross.
What to do: before you start, write down two or three target traits. Grow as many plants per generation as your space allows. Where possible, keep more than one male and one female as parents. This lowers the risk of losing good traits by chance.
Selecting males, collecting pollen and pollinating by hand
Choosing males
Only regular seed gives you males. Grow the males next to their sisters and compare them on vigour, branch structure and how they cope with heat and pests. The most reliable test of a male is his offspring. Make a small test cross with each candidate, grow the seed and keep the male whose daughters are best.
Pollen travels on air, on clothes and on hands. Flower your males in a separate room, away from flowering females. Change clothes and wash your hands before you go from the male room to the flowering room. This prevents unwanted seeds in your crop.
Collecting and cleaning pollen
When the male flowers start to open, cut a few branches. Let them drop pollen onto clean, smooth paper in a still, dry room. Sieve the pollen through a fine mesh to remove flower parts and plant debris. Clean, dry pollen keeps longer than pollen mixed with plant material.
Storing pollen
Pollen loses its strength faster when it is warm or damp. Dry it, then put it in small airtight vials with a desiccant such as silica gel. Label each vial with the male’s tag number and the date. Keep vials in the refrigerator for short-term use or in the freezer for longer storage.
Before you open a cold vial, let it reach room temperature. This stops water from condensing on the pollen. How long pollen and seed stay viable is covered in Storing Seeds and Pollen.
Pollinating by hand
Choose a female in early flowering with fresh white pistils. Pistils are the hairs on the flowers that catch pollen. Switch off the fans in the room first. Brush pollen onto one chosen branch, or cover the branch with a paper bag and shake pollen inside it.
Tie a label to the branch with the pollen source and the date. Pollinating only one or a few branches keeps the rest of the plant seed-free. Seeds then need several more weeks on the plant to mature.
Translator’s note: pollen loses viability quickly once it is shed, and warm, humid grow rooms in the rainy season make storage harder. Collect and sieve pollen in your coolest, driest room, such as an air-conditioned room. Split it into several small vials so you only open one at a time. Pack each vial with desiccant so a short power cut does not let moisture in.
Translator’s note: test one vial on a single branch before you rely on a batch. That tells you whether the pollen survived storage. Seeded buds stay on the plant longer. In the rainy season, dense buds raise the risk of Botrytis (grey mould), so keep air moving and humidity down until harvest.
Translator’s note: using STS safely in Thailand
STS is mixed from silver nitrate and sodium thiosulfate. The full method is in the STS topic linked at the top. Silver nitrate stains and burns skin and can damage eyes. Wear gloves and safety glasses while mixing and spraying. Spray in a ventilated space away from your other plants.
Do not pour leftover solution down the drain or onto soil. Keep it in a closed, labelled container and ask your chemical supplier or local authority how to dispose of it. Treat the sprayed plant only as a pollen donor and do not harvest its flowers for use. To buy the chemicals, ask a laboratory chemical distributor in Thailand. Ask them what paperwork, if any, they need from you. You can also ask other members in this board. A method for making feminized seed without chemicals is discussed in Rodelization (AFT).
Translator’s note: where your parent plants come from
Start with plants you have grown and recorded yourself. Clones bought from other farms can bring pests, the wrong strain or Hop Latent Viroid (HLVd), a disease that stunts plants and reduces yield. HLVd is reported to pass through seed, as covered in Hop Latent Viroid. Keep any new clone apart from your plants until you have checked it. If you start from bought seed, read How to Vet a Thai Seed Vendor first.
Translator’s note: Thai law
Cannabis flower is a controlled herb under the Notification of the Ministry of Public Health Re: Controlled Herbs (Cannabis), B.E. 2568. That notification has been in force since 26 June 2025. Flower may only be sold with a PT 33 prescription for up to 30 days. Advertising and online sale of flower are prohibited, so do not offer flower from your breeding runs on this forum.
The Ministerial Regulation on Controlled Herbs (No. 2) B.E. 2569 has been in force since 30 April 2026. It tightens the licence criteria for cannabis flower. Under Section 78 of the Thai Traditional Medicine Protection Act B.E. 2542, unlicensed sale or processing of a controlled herb carries up to 1 year in prison, a fine of up to 20,000 baht, or both.
The rules for breeding and selling seed are a separate question. Read Buying Cannabis Seeds in Thailand in 2026 and Cannabis as a Controlled Herb in Thailand before you give away or sell any seed. The draft Cannabis Control Act is not law yet. The Cabinet approved the draft on 22 September 2026 and sent it to the House, so check for changes before each project.
Where to start
Read the five topics linked in the sections above in this order:
- Cannabis Genetics Decoded: The Breeder Vocabulary Every Grower Misreads on Seed Labels
- Making Feminized Seeds: The Silver Thiosulfate and Colloidal Silver Methods, Step by Step
- Phenotype Hunting: From a Packet of Seeds to One Keeper Mother
- Cannabis Plant Sex: Male, Female, Hermaphrodite and DNA Testing
- Cannabis Strain Degradation: Causes, Consequences and Solutions
Ask your question under this topic
Post your question or project as a reply below. Include the names of both parents, the generation (F1, F2, BX1…), how many plants you grew and what you are selecting for. Add photos and the relevant lines from your notes. With these details, other breeders can give you specific answers.