How Feminised Seeds Are Made (The Science, Explained)

Watch the episode · 3:33

Plant ten ordinary cannabis seeds and about half of them will grow up male: no flowers worth having, just pollen. So how do breeders sell seeds that come up female almost every time? The answer involves chromosomes, a plant hormone, and a female plant that is tricked into acting male.

Boys and girls: X and Y ▶ 0:26

Most flowering plants carry both sexes on a single plant. Cannabis is one of the few that usually doesn't.

Like only about 6% of flowering plant species, cannabis grows as separate males and females. And, a bit like us, it uses X and Y chromosomes: females are XX, males are XY.

Growers want the females, because they produce the big, resin-covered flowers. Males produce pollen, and once that pollen reaches a female, she puts her energy into making seeds instead.

The 50/50 problem ▶ 1:01

Cross a female with a male and every seed gets an X from its mother. From the father it is a coin toss: an X or a Y. So regular seeds come out roughly half female and half male.

Growers have to raise regular seedlings for weeks before they can tell which is which, then pull out the males. That is a lot of wasted time, space and seed.

Tricking a female: ethylene and silver ▶ 1:25

Here is the trick. Inside the plant, a hormone called ethylene helps steer flowers towards female. Block its signal and a female plant starts growing male flowers.

Breeders do this with silver, usually a solution called silver thiosulfate (STS), sprayed on a chosen female before she flowers. Within a few weeks she grows pollen sacs, and her pollen carries only X chromosomes. Breeders collect it and use it to pollinate another female.

She looks like a male plant, but every cell of her is still genetically female. Growers call her a reversed female.

Two X's: why the seeds come out female ▶ 2:02

Now the maths changes. The mother gives an X. The pollen gives an X. Every seed gets two X chromosomes, so it grows up female almost every time.

That is a feminised seed. And if a reversed female pollinates herself, the seeds are called S1, a selfed line. What S1, F1 and BX mean on a seed pack is the subject of episode 2.

A short history of feminised seeds ▶ 2:25

In 1998 the Dutch seed company Dutch Passion became the first to sell feminised seeds. Those first seeds were made by stressing plants until they produced pollen, and the results were unpredictable. Today silver-based methods are the industry standard.

The catch ▶ 2:45

Feminised seeds are only as good as the mother plant. If she tends to throw male flowers under stress, her offspring can inherit that too, so good breeders select stable mothers and test their lines. (Why even a well-made feminised plant can still make pollen is the story of episode 13 on hermaphrodites.)

And because no true male is involved, breeders still rely on regular seeds, and real males, to create new genetics.

Block ethylene with silver, turn a female into a pollen donor, and because her pollen carries only X, every seed gets two X chromosomes. That is a feminised seed.