Watch the episode · 6:23Cross two weak, inbred plants, and their seeds can outgrow both parents. Breeders call it hybrid vigour. It turned American corn into a crop of hybrids in just a few years, and cannabis breeders are chasing it now. So where does that extra growth come from, and what did the newest cannabis studies actually find?
The short answer ▶ 0:26
| Inbred parent | F1 hybrid | |
|---|---|---|
| Hidden faults | Paired up and showing | Covered by the other parent |
| Growth | Smaller, weaker | Often bigger than both |
Inbreeding pairs up hidden faulty genes, so the plants grow weaker. Cross two different inbred lines, and each parent covers the other's faults, so the seeds grow back stronger. In cannabis there is a catch, though, and a 2026 study ran straight into it.
Darwin's corn and the first hybrids ▶ 0:44
The first careful test came from Charles Darwin. In 1876 he grew maize in pairs, the same age, in the same pots. One plant of each pair came from a cross between two different plants; its partner came from a plant that had pollinated itself.
Source · Darwin, The Effects of Cross and Self Fertilisation (1876)
13 of the 15 crossed plants grew taller. On average they reached 20.2 inches, against 17.6 for the self-pollinated plants.
Source · Darwin (1876), Table 97: average 20.2 vs 17.6 inches
In 1908 an American botanist, George Shull, took it further. He inbred maize, and the inbred lines grew weaker and yielded less. Then he crossed two of them. The hybrid recovered at once, and often out-yielded the variety the inbreds had come from.
Source · Shull (1908), in Crow, Genetics (1998)
Farmers noticed. In Iowa, hybrid corn went from under 10% of the crop in 1935 to over 90% just four years later.
Source · Crow, Genetics (1998)
Why crossing works ▶ 1:32
So why would crossing two weak plants make a strong one? Every plant carries two copies of each gene, one from each parent. And almost every plant carries a few faulty copies, hidden, because the other copy still works.
- 01Self
the plant pollinates itself
- 02Pair up
faulty copies start to double
- 03Show
no working copy left
- 04Decline
smaller, weaker plants
Inbreeding changes that. Each round of self-pollination makes it more likely that a seed gets two faulty copies of the same gene. With no working copy left, the fault shows. Breeders call this inbreeding depression.
Source · Crow, Genetics (1998)
As the seed pack codes article explains, an F1 is the first cross between two parents. Say line 1 has two faulty copies of a gene and line 2 has two working ones. Every F1 seed gets one of each, and one working copy is usually enough. At other genes, line 1 returns the favour.
And because each inbred parent passes the same genes to all its seeds, every F1 seed gets the same mix. That is why a true F1 field looks so even.
Geneticists argued about this for decades. The current view is that covering faults explains most of it. Whether the best hybrids get an extra boost on top is still an open question.
Source · Crow, Genetics (1998)
Does it work in cannabis? ▶ 2:41
In 2020, a University of Connecticut team made feminised hemp seed two ways: by self-pollinating one plant, and by crossing it with other strains. The selfed plants grew smaller, yielded less and varied far more. Some came up with patchy, variegated leaves.
Source · Kurtz, Mahoney, Brand & Lubell-Brand, HortScience (2020)
In 2024, a team from the same university went further. They self-pollinated two cultivars for up to four generations, and inbreeding showed. In the full-size cultivar, a quarter to half of the selfed plants made 1 gram of dried flower or less. In the dwarf autoflower, up to 40% came up yellow, malformed and infertile. Then they crossed the two inbred lines.
Source · McDonald & Lubell-Brand, HortScience (2024)
| Self lines | F1 hybrids | |
|---|---|---|
| CBD in flower | 2.9-5.4% | 7.4-8.3% |
| THC in flower | 0.11-0.22% | 0.32-0.35% |
| The plants | Highly variable | More uniform |
Against the self-pollinated lines of the full-size cultivar, the F1 hybrids were heavier and gave more flower. Their CBD was over 7% in every hybrid line, against 3% to 5.4% in the inbred lines. THC was higher too, though still around a third of a percent. The hybrids were also more uniform. One parent was a dwarf autoflower, yet the hybrids grew about as tall as the full-size line. The authors put it down to hybrid vigour.
Source · McDonald & Lubell-Brand, HortScience (2024), Table 1
In 2026, a team in Australia pushed it as far as it goes: up to six rounds of self-pollination in 18 months, keeping one plant each round, to make truly pure parent lines.
Source · Garcia-de Heer et al., Hortic. Res. (2026)
The cost showed in the flowers. In one line, abnormal flowers rose from 0.4% to 13.7% by the sixth round. And from the fourth round on, every line set less seed.
Then they crossed the pure lines. The F1 hybrids were more uniform than both the inbred lines and the original lines. And their seed yield beat the average of their two parents by anywhere from about 4% to 155%.
Source · Garcia-de Heer et al., Hortic. Res. (2026)
The catch ▶ 4:30
| vs INBREDS | vs ORIGINALS | |
|---|---|---|
| More biomass? | 4 of 5 hybrids | Only 2 of 5 |
| More even? | Yes | Yes |
In that Australian study, the hybrids beat their own inbred parents: four of the five grew bigger. But against the original lines the inbreds came from, only two of the five made more biomass. They were still more even, though.
Source · Garcia-de Heer et al., Hortic. Res. (2026)
The authors' explanation is that the original lines were already very mixed genetically, so they already had much of that vigour. And that is the catch. The big boost comes from crossing two pure lines. Most cannabis strains are mixed, so a cross between two of them is an F1 on paper, but the plants can still vary, and the extra boost isn't guaranteed. A Cornell study of 23 hemp families agreed: for even growth, at least one parent has to be inbred.
Source · Garcia-de Heer et al. (2026); Carlson et al., J. Exp. Bot. (2021)
The Connecticut team adds a warning about S1 seed, from a plant crossed with itself. It is often sold as a copy of the mother plant, but their results suggest S1 seed can be highly variable, just like ordinary open-pollinated seed.
Source · McDonald & Lubell-Brand, HortScience (2024)
One more thing: the vigour lasts one generation. Seeds saved from F1 plants are F2s. The genes reshuffle, some faults pair up again, and the evenness is gone. That is why hybrid corn seed is made fresh in seed fields every year.
Inbreeding pairs up hidden faults and weakens plants; crossing two pure lines covers them, and the seeds grow stronger and more even. In cannabis it works too, but only when the parents are pure, and only for one generation. For how breeders get from parents to a finished strain, see how a new cannabis strain is made.
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Sources
- Darwin, C. (1876). The Effects of Cross and Self Fertilisation in the Vegetable Kingdom. London: John Murray. Zea mays, Table 97 (15 pairs; crossed 20.19 in vs self-fertilised 17.57 in). Text: Project Gutenberg #4346.
- Crow, J.F. (1998). "90 years ago: the beginning of hybrid maize." Genetics 148(3): 923-928. doi:10.1093/genetics/148.3.923 (Shull 1908; Iowa hybrid share under 10% in 1935 to over 90% four years later; the dominance explanation).
- Kurtz, L.E., Mahoney, J.D., Brand, M.H. & Lubell-Brand, J.D. (2020). "Comparing Genotypic and Phenotypic Variation of Selfed and Outcrossed Progeny of Hemp." HortScience 55(8): 1206-1209. doi:10.21273/HORTSCI15061-20
- McDonald, M.M. & Lubell-Brand, J.D. (2024). "F1 Hybrid Seed Can Enhance Cannabis Crop Uniformity and Yield." HortScience 59(12): 1795-1799. doi:10.21273/HORTSCI18197-24 (Table 1: CBD and THC of self-lines vs F1 hybrids)
- Garcia-de Heer, L., Mieog, J., Burn, A., Nolan, M., Liu, L., Manansala-Siazon, S. & Kretzschmar, T. (2026). "Weeding out variability: a proof-of-concept for producing uniform F1 hybrid Cannabis sativa L. using single-seed descent." Horticulture Research 13(5): uhag038. doi:10.1093/hr/uhag038
- Carlson, C.H., Stack, G.M., Jiang, Y. et al. (2021). "Morphometric relationships and their contribution to biomass and cannabinoid yield in hybrids of hemp (Cannabis sativa)." Journal of Experimental Botany 72: 7694-7709. doi:10.1093/jxb/erab346
Every claim in this article comes from the episode script, which is checked against these papers. Prices on Seedscanner are dated observations; this page is not growing or legal advice.
