Short answer
Single-finger, rounded leaves usually mean a cannabis plant is revegging: going from flower back to vegetative growth because its nights got too short or were broken by light. The common causes are light leaks or timer faults in flower, outdoor plants set out under short spring days, and clones cut from flowering plants. Supported.
Why
Photoperiod cannabis reads the night. Flowering starts once the uninterrupted dark is long enough, and it stalls or reverses when nights shorten or are broken [1].
Very little light can break a night. A hemp study reported flowering delays from stray light at about 2 µmol/m²/s [1], and a 2024 conference report saw delays in the most sensitive cannabis cultivars at 0.01 µmol/m²/s [2].
A mid-night pulse of light is used on purpose. Growers keep mother plants vegetative on 12/12 with a 1-hour night break [3], which shows how well a short interruption works. An accidental leak does the same thing to a flowering plant.
Revegging leaves look distinctive. New growth comes in as single blades or 3-finger leaves with smooth or rounded edges, sometimes poking out of the buds.
The numbers
| Situation | Threshold |
|---|---|
| Dark-period stray light | As near 0 as you can measure; below 0.01–2 µmol/m²/s [1][2] |
| Eye test | Nothing visible after 5 minutes with eyes adjusted |
| Flower schedule | 12/12 or 13/11 (house-endorsed), uninterrupted dark |
| Reveg recovery | About 2–4 weeks before normal leaves return (grower practice) |
What to do
- Sit in the dark room for 5 minutes. Tape over indicator LEDs, zipper ends and duct ports.
- Check that the timer and controller logs match the schedule, especially after a power cut.
- Make sure no light reaches the room from a veg area or a door gap.
- Outdoors, plant out photoperiod cultivars once the days are long, or add light in early spring.
- Take clones from vegetative mothers, not flowering plants.
- Once you've fixed the cause, give the plant time. Don't add stress on top.
Common mistakes
- Opening the tent during the dark period.
- Trusting a mechanical timer that drifts.
- Putting photoperiod plants outside in early spring.
- Mistaking a seedling's first single-blade leaves for reveg.
Related
- Full article: Photoperiod and light leaks
- Answers: how to reveg a plant · lights stayed on in flower · opening the tent at night · when to plant outdoors
- Terms: re-veg · light leak
