Why Is My Autoflower So Small? (The First 4 Weeks Decide It)

Watch the episode · 3:08

Why do some autoflowers stay tiny? Usually it is decided in the first four weeks. An autoflower doesn't wait until it is big enough: in a Cornell trial, autoflowering plants started to flower at four weeks, and every plant of the same type flowered on the same day. So every slow day before that comes straight off the final size.

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

Growth compounds ▶ 0:27

Young plants grow like savings earning interest. More leaf makes more sugar, and that builds more leaf. So a seedling that stalls for a few days doesn't just lose those days: it starts every day after from a smaller base, and the gap keeps widening. A regular plant can stay in its growing phase until it catches up. An auto switches to flowering on schedule, gap and all.

Four usual suspects ▶ 0:56

One: too much water. A seedling's roots need air as much as water, and soggy soil starves them of oxygen. It is the glass-of-water problem from the germination methods episode, in a pot.

Source · Voesenek et al., New Phytol. (2006)

Two: cold. In lab tests, cannabis leaves photosynthesised fastest at around 30 °C, and less in the cooler range. A chilly room slows the whole engine right when growth compounds fastest.

Source · Chandra et al., Physiol. Mol. Biol. Plants (2008)

Three: too little light. In a Canadian study of flowering cannabis, yield rose in a straight line with light, right up to the brightest level tested. Autos aren't tied to day length, so many growers give them 18 hours of light or more, to pack more growth into those few weeks.

Source · Rodriguez-Morrison et al., Front. Plant Sci. (2021)

Four: moving it. Transplanting can tear the fine root hairs, and the plant spends days repairing instead of growing: days an auto doesn't have. (Does pot size matter? covers why autos should start in their final pot.)

Sometimes it's genetics ▶ 1:54

But sometimes it isn't you. In the same Cornell trial, two day-neutral hemp lines under the same long days started flowering three weeks apart, one at four weeks and one at seven. The later one simply had three more weeks to grow.

So some autos are bred to finish small and fast, and no grow-room trick will change that. If you want a big plant, check the breeder's listed height and flowering time before you buy.

The verdict ▶ 2:22

So how do you grow them big? Sow straight into the final pot. Keep it moist, not soggy. Keep it warm. Give it long days of strong light. Then pick genetics bred for the size you want.

Autos flower on schedule, big or not, and early stalls compound. The usual suspects are soggy soil, cold, dim light and transplanting, and some autos are simply bred to stay small. For more, see the 5 autoflower mistakes.