English 
Stabilizing a line turns a lucky cross into something you can repeat. Breeders talk about F1, F2 and IBL, yet the words hide a simple idea. You grow, you select, and you repeat until the offspring stop surprising you.
What F1 really means
An F1 is the first cross between two different parents. Therefore every seed carries one set of genes from each side. F1 plants often look uniform and grow fast. However, that uniformity hides a huge amount of variation underneath.
Why the F2 looks messy
Cross two F1 plants and the hidden genes separate again. As a result, the F2 generation splits into many different plants. Some resemble one grandparent. Others resemble the second. A few combine traits nobody expected. This mess is normal, and it is also your opportunity.
The goal of the work
Stabilizing a line means shrinking that variation. In short, you want most seeds to grow into similar plants. You never reach perfect sameness. Instead, you aim for a batch where the differences no longer matter to the grower.
Choose one trait first
Beginners chase everything at once. By contrast, experienced breeders pick one or two priorities. Flowering time is the easiest to judge. Structure comes next. Aroma is harder, because it changes with feeding and cure. Choose your target before you plant.
Population size matters
Selection needs choice. Ten plants give you almost none. Therefore, aim for thirty or more in the F2 if space allows. A vigorous line makes the job easier, because weak seedlings waste slots you could use for real comparison.
Making the F2 generation
Pick a male and a female from the same F1 batch. Then pollinate one branch and keep the rest clean. The seeds you collect are your F2. Label them clearly, because this batch is the foundation of everything that follows.
Selecting into F3
Grow the F2 out and watch closely. Next, keep only the plants that show your target trait. Cross two of them. The result is your F3. Meanwhile, discard the rest without guilt, since keeping favourites for sentimental reasons slows the whole project.
Rogueing: what to remove
Rogueing means pulling plants early. Remove anything that flowers far outside your window. Remove weak stems and unstable plants too. As a result, the pollen in your room comes only from plants you approve of.
F4 to F6: locking it in
Each generation narrows the gene pool a little more. By F4 the plants start to look alike. By F6 most growers can predict the outcome. Therefore, six generations is the usual rule of thumb for a working line.
When a line becomes an IBL
IBL stands for inbred line. It describes a population that breeds true for the traits you selected. In practice, a true IBL takes many generations and a lot of plants. Consequently, the term gets used loosely in seed catalogues.
Inbreeding depression
Narrowing genes has a cost. Plants can lose vigour, yield or resistance. You notice slower growth and smaller roots first. However, the effect varies, and some lines tolerate inbreeding far better than others.
Refreshing vigour
An outcross fixes tired genetics. Bring in one unrelated plant, cross it, then select back towards your line. This costs you generations. Even so, it beats watching a stable line fade year after year. A fresh unrelated variety works well for this step.
Keep records or lose the work
Write down every cross. Note the date, the parents and the reason for your choice. Photograph each keeper. Otherwise, two similar jars become impossible to tell apart, and years of work turn into guesswork.
Storing seed between generations
Each generation produces more seed than you need. Store the surplus cold and dry, in sealed jars away from light. Then, if a later generation disappoints, you can return to an earlier point instead of starting over.
How long it takes
Photoperiod plants need three to five months per generation. Six generations therefore takes two to three years indoors. Autoflowers move faster, though selection is harder because the plants finish quickly. A dependable base variety shortens the road.
Judging plants fairly
Compare plants only when they grow in the same conditions. Otherwise, you reward the pot near the light rather than the better genetics. Therefore, rotate pots weekly and feed every plant the same way. In addition, take notes on the same day of each week. Consistent conditions turn opinions into evidence, and evidence is what moves a line forward.
Keeping a mother plant
Cuttings buy you time. Take a clone from every plant you might keep, then judge the parent through flowering. If the plant impresses you, the clone is still alive and ready. Meanwhile, the plants you reject simply go to the compost. This habit saves an entire generation of waiting.
Backcrossing versus straight inbreeding
Some breeders cross back to one original parent instead of pairing siblings. As a result, the line moves quickly towards that parent. By contrast, sibling crosses keep more of the original mix. Both routes work, and many projects use a bit of each.
Common mistakes
The first mistake is a tiny population. The second is chasing too many traits at once. The third is keeping a plant because it smells nice on one particular evening. Above all, avoid skipping notes, because memory fades faster than seed does.
Selection pressure explained
Selection pressure is simply how strict you are. Keep two plants out of forty and the pressure is high. Keep twenty out of forty and the pressure is weak. Therefore, a big population lets you be strict without running out of candidates. Strict choices early save several generations later on.
Testing your seeds on other people
Your own room hides problems. Send a small batch of seed to a friend who grows in a different setup. Then compare notes on flowering time and structure. As a result, you learn whether the line performs anywhere, or only under your lights.
Naming and versioning
Give every generation a code, not a nickname. Something like Line A F3 works better than the name you invented at midnight. In addition, write the code on the jar, on the label and in your notebook. Three copies of the same detail prevent most mix ups.
Culling early frees resources
Every plant you keep consumes water, nutrients and light footprint. Keeping a poor plant to the end costs exactly the same as keeping a good one. Therefore cull early and give that space to the strong candidates. Consequently the keepers grow better and the comparison between them becomes fairer.
Do not select on a single trait
Selecting on aroma alone tends to produce fragile plants. By contrast, selecting on vigour alone tends to produce dull ones. Therefore set two priorities at most and accept being average on everything else. In fact that is how projects move forward instead of stalling.
Save seed from every generation
Each round produces more seed than the next step needs. Therefore store the surplus in a sealed jar with the generation code and the date. Consequently a disappointing F5 sends you back to the F3 rather than back to the beginning.
Legal note
Seeds are sold as collectable items. Growing rules differ from country to country and change over time. Therefore, check the current law where you live before you plan any project. This article is informational only.
Frequently asked questions
Can I stabilize a line with feminized seeds?
It is possible, but harder. Regular seeds give you males, and males are what you need for open selection.
How many plants do I really need?
Thirty in the F2 is a practical minimum. More plants mean better choices and faster progress.
Is F6 always stable?
No. Stability depends on selection pressure, not on the number alone. Sloppy selection at F3 shows up at F6.
