Germination: What Really Happens Inside a Sprouting Seed?

Watch the episode · 3:24

A cannabis seed can sit in a drawer for years doing almost nothing: barely breathing, waiting. Then add water, and within a few days it is splitting its shell and growing a root. So what actually happens in between?

Inside the shell ▶ 0:21

First, a surprise. What we call a cannabis seed is technically a tiny fruit, called an achene. The hard, striped shell is the fruit wall.

Inside the shell, under a thin seed coat, lies a complete baby plant: the embryo. It has two seed leaves, called cotyledons, packed with food; a shoot tip, the future stem; and the radicle, the embryonic root.

That food is mostly oil and protein. Around a third of a hemp seed's weight is oil, a dense energy store that has to last until the seedling can make its own food from light.

Three phases: soak, repair, break out ▶ 0:59

Germination happens in three phases. Phase one is imbibition: water rushes in and the seed swells. Phase two is a quiet lag, when the seed wakes up and gets ready. Phase three is breakout, when the root splits the shell.

The first phase is pure physics. Dry tissue simply soaks up water like a sponge, and even a dead seed will swell. That is why a swollen seed is no proof of a living one.

In the lag phase the real biology starts. The embryo repairs damage to its DNA, builds fresh mitochondria to power its cells, and starts making new proteins.

Two hormones decide whether it goes ahead. Abscisic acid is the brake that keeps a seed dormant; gibberellins are the accelerator. As the seed wakes, the brake comes off and the accelerator goes down.

Finally the radicle pushes through the shell, and the stored oils and proteins are broken down to feed rapid growth. The root heads down. The shoot heads up.

What a seed needs: water, oxygen, warmth ▶ 2:03

A seed needs three things: water, to start the whole process; oxygen, because a waking seed is breathing hard; and warmth, which sets the pace.

Oxygen matters more than most growers realise. Leave a seed underwater for too long and it can run short of air just when it needs it most. (Which germination method works best was tested in the lab: see episode 21.)

And temperature? A study of hemp seeds found that germination barely starts at around 3 °C, is fastest at around 30 °C, and fails above about 43 °C. Anywhere from about 20 to 30 °C works well.

The first leaves ▶ 2:40

The first two leaves to open are not true leaves at all. They are the cotyledons from inside the seed: smooth, rounded, and still full of stored food. Only after that do the first serrated true leaves appear, and the plant starts living on light instead of savings.

The shell is a fruit wall. Inside is a complete embryo with its own food supply. Germination goes soak, repair, break out, and it needs water, oxygen and warmth. If your seedlings then shoot up tall and floppy, read why seedlings stretch.