Hemp or Marijuana? It's One Number (And It Was Arbitrary)

Watch the episode · 6:11

Hemp and the plant in your tent are one species, split by one number. That number is 0.3% THC. At or under it, the plant is hemp; over it, in law, it is marijuana. The botanists who picked that number said they chose it arbitrarily. And on 11 December 2026 the US redraws the line, in a way that reaches seeds. Here is where 0.3% came from, the gene behind it, and what is changing.

The short answer ▶ 0:30

Hemp and drug-type cannabis are one species, Cannabis sativa. The big chemical difference comes down mostly to one gene: one version makes THC, the other makes CBD. And legally, it is just a lab test: 0.3% THC or less, by dry weight. But that test isn't as simple as it sounds, and the new US version reaches seeds.

Source · de Meijer et al., Genetics (2003); 7 U.S.C. 1639o

Look at a fibre hemp crop: tall, thin plants, sown close together, grown for stems and seed. A drug-type plant is grown wide and bushy, for its flowers, packed with resin glands. Different crops, bred for different jobs. Same species.

Where 0.3% came from ▶ 1:12

In 1937, a US tax law covered every cannabis grower. Even hemp farmers needed a tax stamp as a "producer of marihuana". There was no THC line at all: THC itself wasn't even identified until 1964.

Once it could be measured, scientists could sort plants by it. In 1976, two botanists, Ernest Small and Arthur Cronquist, proposed 0.3% THC as the line between two groups of cannabis. Canada wrote it into law in 1998, and the US in 2018. It is still the line that decides what counts as hemp.

Source · Gaoni & Mechoulam (1964); Small & Cronquist, Taxon (1976)

And here's the twist. In their own paper, they wrote that they adopted it arbitrarily. It was a handy cut-off for sorting plants, measured in young leaves, not a safety limit. Today the US test samples the flowering tops, where THC is much higher.

Not every country agrees on it, either. The US and Canada use 0.3%. The EU raised its limit from 0.2% to 0.3% in 2023. And Switzerland draws its line at 1%.

Source · 7 U.S.C. 1639o; Canada IHR; EU Reg. 2021/2115; Swiss FOPH

One gene switch ▶ 2:21

So what decides which side of the line a plant lands on? Both THC and CBD are made in the resin glands, from the same raw material, called CBGA. An enzyme turns it into THC's acid form. A slightly different version of that enzyme turns it into CBD's acid form instead. Which versions a plant carries comes down, mostly, to one gene.

In 2003, a Dutch and Italian team crossed plants to test this. Each plant carries two copies of the gene, and both copies show. Cross two mixed plants and the offspring split about one in four THC type, two in four mixed, and one in four CBD type.

Source · de Meijer et al., Genetics (2003)

But hemp isn't just a drug plant with one gene switched. A 2015 study read the DNA of 124 samples. Hemp and drug-type cannabis differed right across the genome, not only at the THC gene. Even so, no single DNA letter separated every hemp plant from every drug plant. Same species, different family lines.

Why hemp crops run hot ▶ 3:24

So why do careful hemp farmers still fail the test? It's that enzyme again. CBD hemp is bred to carry the CBD version, but that enzyme isn't perfect. For roughly every 20 parts of CBD, it also makes about 1 part of THC. In one study, the ratio sat near 20 to 1 in 13 of 14 varieties.

Source · Arsenault et al., Plants (2024)

So when a CBD crop reaches 0.3% THC, it holds only about 6 to 8% CBD. With today's genetics, CBD hemp with zero THC isn't possible.

Source · Suchoff et al., NC State Extension (2022)

And it happens fast. A Connecticut study tested 14 CBD hemp varieties every week, over three seasons. In early September, every one was under or near the limit. Within one to two weeks, all of them were over it. Harvest a little late, and a legal crop can fail the test.

Source · Arsenault et al., Plants (2024)

One more catch. A growing flower holds mostly THCA, the acid form, not THC itself. So the US test counts 87.7% of the THCA as THC. That is called total THC.

What changes on 11 December ▶ 4:32

Now, the part that matters for seeds. Here is what changes in the US on 11 December 2026, unless Congress delays it again. THCA now counts in full, so high-THCA flower sold as hemp is out. Finished THC products are capped at 0.4 mg per container. And seeds: until now, the DEA treated a seed as hemp if the seed itself tested under 0.3%, and seeds carry very little THC. The new law looks at the parent plant instead.

In the law's words, viable seeds from a plant above 0.3% are not hemp. That covers seeds from essentially any drug-type plant, because those plants sit far above the line. Fibre and grain hemp are kept. Bills to delay or repeal the change are sitting in committee.

Source · Public Law 119-37, sec. 781

This is the picture as of 7 November 2026, and it may still change. It is not legal advice: laws differ by state and by country, so check yours.

Hemp and drug-type cannabis are one species, split by one number, and that number was picked, in the botanists' own words, arbitrarily. One gene mostly decides which side a plant lands on, and from 11 December the US line reaches seeds.