Why is my seedling drooping?

Q: Why is my seedling drooping?

A drooping cannabis seedling is most often overwatered: a big pot or soggy plug leaves its tiny roots short of oxygen. Less often it is dry, hot or under a light that is too strong. If the stem is pinched and brown at the soil line, it is damping-off and can't be saved. Water small amounts around the stem only when the pot lightens, at 23–27 °C (73–81 °F).

Short answer

A drooping cannabis seedling is usually overwatered. Less often it's dry, hot or under too much light. A stem pinched and brown at the soil line means damping-off. Lift the pot before you water again. Supported.

Why

Roots need air. A tiny root in a soggy plug, or a big pot soaked for a seedling, sits in water with little oxygen, so the plant can't move water and droops even though it is wet.

Wet, cool media favour root pathogens. Pythium infects cannabis roots in wet conditions ^[3], and the same organisms cause damping-off in seedlings ^[1]^[2].

Damping-off is different from droop. The stem rots and collapses at the soil line, often with green leaves still on it, and the seedling can't recover ^[2].

Dry air and heat cause a quick, midday wilt. Small roots can't keep up with a strong pull from hot, dry air or an intense lamp.

The numbers

Clue	Overwatered	Underwatered	Damping-off
Pot weight	Heavy	Light	Usually heavy
Leaves	Curl down, firm	Limp, thin	Often still green
Stem base	Normal	Normal	Pinched, brown, wet
Recovers	In 1–3 days if left to dry	Within an hour of water	No

Targets: 23–27 °C (73–81 °F) air, 65–75% RH, 150–300 µmol/m²/s, medium at 21–25 °C (70–77 °F).

What to do

Lift the pot. Heavy: wait. Light: water a small ring around the stem.
Check the stem at the soil line for pinching.
Use room-temperature water, never cold ^[1].
Check air temperature and light distance.
Next time, start seedlings in a small cup or plug, or water only near the stem in a big pot.

Common mistakes

Watering a droopy seedling that is already wet.
Soaking a 15 L pot for a seedling with a 2 cm root.
Cold tap water straight onto the stem.
Sealing it under a dome to "help" an overwatered plant.

Full article: Seedling care: the first three weeks
Drooping: overwatering vs underwatering · Damping-off
Why are my seedling's leaves yellow?
Terms: damping-off · overwatering

Follow-up questions

How do I tell overwatering from underwatering in seedlings?

Lift the pot. An overwatered cannabis seedling droops with firm, heavy, dark-looking mix and leaves that curl down but feel full. An underwatered one droops with a light pot, a dry surface and limp, thin leaves that perk up within an hour of watering. Overwatering is far more common at this stage, especially in big pots.

Why did my seedling fall over at the base?

A cannabis seedling that topples with a thin, water-soaked or brown stem right at the soil line almost always has damping-off, caused by soil-borne pathogens such as Pythium and Rhizoctonia. It can't be saved. Remove it and the mix around it, then sow again in fresh, clean mix with lighter watering. An evenly thin, pale stem points instead to low-light stretch.

Can too much light make a seedling droop?

Yes, but it looks different. Cannabis seedlings under too much light or heat tend to hold their leaves stiffly up and folded ('praying') and bleach at the tips, and may wilt at midday if the air is hot and dry. Keep seedlings at 150–300 µmol/m²/s and 23–27 °C (73–81 °F). Long, floppy, pale stems mean too little light instead.

Sources

University of Minnesota Extension. How to prevent seedling damping off. extension.umn.edu/solve-problem/how-prevent-seedling-damping

NC State Extension. Damping-off in flower and vegetable seedlings. content.ces.ncsu.edu/damping-off-in-flower-and-vegetable-see

Punja ZK, Rodriguez G, 2018. Fusarium and Pythium species infecting roots of hydroponically grown marijuana (Cannabis sativa L.) plants. Canadian Journal of Plant Pathology 40(4):498–513. doi.org/10.1080/07060661.2018.1535466