Seed lists are full of codes like F1, F2, BC1 and OP, along with crosses written as “A × B.” Some sellers use H1 and H2 to mean the same thing as F1 and F2. Here's what they mean and, more importantly, what they tell you about the seedlings you'll get.
What a hybrid is
A hybrid is the offspring of two genetically different parents. In cacti that usually means two different species (Astrophytum asterias × capricorne), two different named clones of the same species, or occasionally two different genera. Hybrids are written with a ×, and by convention the seed parent (mother) comes first and the pollen parent (father) second.
The generation codes
- P — the original parents.
- F1 (sometimes H1) — the first generation from crossing two parents. F1 seedlings from two consistent parents tend to be fairly similar to each other, often somewhere in between the parents. They're also often more vigorous than either parent, an effect called hybrid vigor.
- F2 (sometimes H2) — seedlings from crossing two F1 plants together. This is where things get interesting: the traits of the original grandparents get reshuffled, so F2 seedlings vary widely. Some look like one grandparent, some like the other, and some like neither. Breeders love F2s because that's where unusual combinations turn up.
- F3, F4, and so on — each further generation of crossing within the line. With selection at every step, the line slowly becomes more uniform.
- BC1, BC2 (backcross) — a hybrid crossed back to one of its original parents. Used to pull the line back toward that parent while keeping a trait from the other.
- OP (open-pollinated) — the mother is known, but the bees chose the father. OP seed from a specimen plant can be great, but the seedlings are a gamble.
- Sib cross — two seedlings from the same batch crossed together.
Why seed doesn't “come true”
A seed always carries genes from two parents. Even if you buy seed harvested from a spectacular plant, the seedlings are new individuals, not copies of it. That's why named plants are reproduced by cuttings or grafts (which are genetic copies), while seed gives you something new every time.
Most cacti can't pollinate themselves — they're self-incompatible — so in practice nearly every cactus seed is a cross between two plants. True selfed lines, common in vegetables, are rare in cacti.
What this means when you buy seed
- Species seed from two plants of the same species gives you plants that look like that species, with natural variation.
- F1 hybrid seed gives fairly consistent seedlings that blend the parents.
- F2 and OP seed gives a wide spread. Grow a tray, then keep the standouts.
- Cultivar seed (like Super Kabuto or variegated Astrophytum) gives a share of seedlings with the trait, not all of them. Some traits take years to show.
No seller can promise that every seedling will look like the parent — and the honest ones won't try. The fun of growing from seed is that the best plant in the tray might be one nobody has ever seen before.