Why Seeds Rot Before Germinating — and How to Prevent It

Why Seeds Rot Before Germinating — and How to Prevent It

You open the tray and find soft, browned or vanished seeds: they rotted before ever emerging. It is the most common sowing failure — and it has precise causes, all avoidable.

Contents

• Cause 1 — excess water
• Cause 2 — cold
• Cause 3 — fungi (damping-off)
• Cause 4 — contaminated substrate
• Cause 5 — suffocation
• Prevention in 6 rules
• Frequently asked questions

Cause 1 — excess water

The proven no. 1 cause: too much water rots the seed. A soggy substrate — a mud — deprives the seed of air and delivers it to fungi. The gardener must aim at the balance: moist, never soggy. Sign: a handful of substrate that lets water seep out under pressure is too wet.

Cause 2 — cold

Below ~18 °C soil temperature, seeds rot instead of germinating — especially warmth-loving species (tomato, pepper, aubergine, watermelon). A cold damp soil is spring's fatal duo: the seed rots, or if it emerges, it bolts. In open ground: nothing germinates below 7 °C.

Cause 3 — fungi (damping-off)

Damping-off is the technical name of this rot: soil fungi kill the seed before emergence or the seedling just after — it wilts, collapses at the collar, dies. The conditions that favour it, all proven:

• Sowing into cool damp soil.
• Contaminated containers — not disinfected.
• Substrate carrying pathogens — garden soil or poorly sanitised homemade compost.
• Humid confined atmosphere, crowded seedlings, careless watering.
• Excess nitrogen in a soggy substrate — encourages fungi.

Cause 4 — contaminated substrate

A homemade compost can be poorly sanitised — risk of damping-off and weed invasion. Garden soil almost always contains pathogens: it has no place in a seed tray. The solution: a sterile, soil-free seed-starting mix.

Cause 5 — suffocation

Seeds breathe: buried too deep, in soggy or compacted soil, or submerged too long in soaking water, they suffocate — rot follows. Same for an airtight pre-germination bag.

Prevention in 6 rules

• Sterile, soil-free substrate — bought or disinfected seed-starting mix.
• Clean containers — soaked in 10% bleach, air-dried.
• Right temperature — sow when the soil is warm enough for the species; a heat mat helps indoors.
• Just-moist level — wrung-out sponge, not mud; bottom watering.
• Sparse sowing — air circulates, fungi retreat.
• Drainage — no pot without holes in the bottom.

Frequently asked questions

Why do my seeds rot?

In order: excess water + cold soil (the most frequent duo), fungi on contaminated substrate or dirty containers, or suffocation from lack of air.

What is damping-off?

A group of soil fungi that attack the seed or the young seedling — soft seeds before emergence, or seedlings collapsing at the collar after emerging. Prevention comes through hygiene (sterile substrate, clean pots) and controlled moisture.

Can a rotting sowing be saved?

Remove affected seeds or plants at once to prevent contagion, ventilate, stop watering, raise the temperature. If the rot is confirmed, re-sow on fresh sterile substrate.

A germination kit with fresh sterile pots and a controlled lid removes most rot causes — the safest starting point.

Want your home sowings to succeed? Germie's reusable germination kits get your seeds started in record time — and the Gardiscan analyses your soil for a perfectly calibrated start.