Does Pot Size Matter for Autoflowers? (What the Research Says)

Watch the episode · 3:36

Does a bigger pot really mean a bigger harvest? Ask ten growers and you will get ten pot sizes. But the research gives a clear rule of thumb, and it isn't always "bigger is better". Autoflowers add a twist, because they can't wait for their roots to catch up.

Doubling the pot (10 L to 20 L) ▶ 0:25

A big review pooled 65 experiments on pot size. On average, doubling the soil volume, say from 10 litres to 20, made plants about 43% bigger: not twice as big. And the surprise is why. It wasn't mainly smaller leaves. The leaves themselves photosynthesised less, as if the cramped roots were holding the whole plant back.

Source · Poorter et al., Funct. Plant Biol. (2012)

The researchers even suggest a limit: once a plant's dry weight passes about 1 gram per litre of pot, the pot starts holding it back.

The autoflower clock ▶ 0:59

A regular plant flowers when the nights get long, so a grower can keep it growing until it fills its pot. An autoflower flowers by age, thanks to a single gene region called Autoflower1 (explained here). After a few weeks it switches to flowering, whatever its size. So most of its frame has to be built in those first weeks, and cramped roots early on can't be fixed later.

Source · Toth et al., Front. Plant Sci. (2022)

That is why autoflowers are best sown straight into their final pot: no transplant to recover from, and no weeks lost with roots circling a small cup.

Can a pot be too big? ▶ 1:36

Bigger pots grow bigger plants, but a tent only has so much floor. A review of cannabis yield studies compared 5 and 11 litre pots. Per square metre, the bigger pots didn't make more THC, and for one variety, White Widow, it actually fell. Fewer, bigger plants or more, smaller ones can fill the same space.

Source · Backer et al., Front. Plant Sci. (2019)

And a huge pot has a catch for seedlings. All that soil holds a lot of water, and a tiny root ball can sit soggy for days. So water a small circle around the stem at first, and widen it as the roots spread.

What about coco? Many growers raise big autos in small pots of coco, fed little and often, sometimes several times a day. There is no cannabis trial on this. But in a lettuce study, feeding more often raised yields by keeping nutrients topped up right at the root surface. So the less root space you give, the more often you need to feed.

Source · Silber et al., Plant and Soil (2003)

The verdict ▶ 2:35

Sow autoflowers straight into their final pot. Within reason, go bigger: doubling the litres adds roughly 40%, not double. Think per square metre, not per plant. And with big pots, water a small circle first.

Most growers settle on something between about 11 and 20 litres for autos. There is no magic number in the research: just roots, space and water, in balance.

Doubling the soil volume grows plants about 43% bigger, and small pots slow the whole plant down. Autos can't wait, so start them in the final pot, and think per square metre, not per plant.