Damping Off: What Is It and How to Prevent It?

Damping off is the reason a tray of healthy looking seedlings can be lying flat the next morning. It is caused by soil dwelling fungi and water moulds that attack seed and very young seedlings, and it is driven far more by conditions than by bad luck. Wet, airless, crowded, cold trays are what it wants. Clean containers, fresh mix, thin sowing and air movement are what stop it.

What it looks like

There are two forms, and they look quite different.

Before emergence, the seed or the sprout rots underground and simply never appears. You are left blaming the seed, the weather or yourself, with nothing to look at. Patchy, uneven germination in a tray that was watered the same all over is often this.

After emergence, the seedling comes up normally and then collapses. The tell is at the base: a water soaked, pinched, dark or shrivelled section of stem right at soil level, thin enough that the stem can no longer hold the plant up. The seedling topples while its leaves still look green and fresh, which is what makes it so disheartening.

There is a variant worth recognising. On brassicas in particular, Rhizoctonia can produce what growers call wire stem, where the stem base goes brown, dry and constricted but tough, so the plant stays standing while stunted and hardened. Those plants rarely amount to anything and are best pulled.

What causes it, accurately

Damping off is caused by a group of soil organisms, chiefly the water moulds Pythium and Phytophthora, and the true fungi Rhizoctonia solani and Fusarium species. Pythium and Rhizoctonia are the usual offenders in home seed trays.

Two points are worth being clear about, because they are often stated wrongly. Damping off is not caused by viruses or bacteria. And it is a disease of seed and young seedlings only. Once a stem has firmed up and the plant is past the first true leaves, it is no longer really susceptible, so damping off will not wipe out established plants. If something is killing mature plants, look for a root or crown rot, a wilt, or a watering problem instead.

Not every failed germination is damping off, either. Old seed, seed sown too deep, mix that dried out at a critical moment and soil that was simply too cold for the crop all look similar from above.

The conditions it needs

The organisms are common and often already present in soil, compost and reused trays. Whether they cause a problem depends almost entirely on conditions:

• Mix that stays saturated, so roots run short of oxygen
• Poor drainage, or trays sitting in standing water
• Seed sown too thickly, so seedlings crowd and stay damp
• Still, humid air, especially under a lid left on after germination
• Soil temperature wrong for the crop, so germination is slow and the seedling sits vulnerable for longer
• Too much nitrogen, which pushes soft, sappy growth

Slow germination is the underrated one. A tomato seed in 14 degree mix might take three weeks instead of six days, and every one of those extra days is time for a water mould to find it.

How to prevent it

Prevention works. Treatment, once a tray has gone, mostly does not.

• Start clean. Wash and dry reused punnets and trays before the season. Use fresh seed raising mix rather than leftover potting mix or garden soil.
• Sow thinly. Crowded seedlings hold moisture against each other and make it easy for the disease to spread from one to the next.
• Water from below. Stand trays in shallow water until the surface darkens, then take them out and let them drain. This keeps the surface and the stems drier than overhead watering does.
• Let them breathe. Take the lid or dome off once most seed is up. A small fan on low, or simply a more open spot, makes a real difference.
• Water in the morning so the surface dries before the cooler night.
• Match the temperature to the crop so germination is quick. This is where a heat mat genuinely helps with warm season seed, and where sowing cool season crops without one is fine.
• Do not overfeed seedlings early.

If a tray starts to go, remove the affected seedlings and the mix immediately around them, stop watering until the surface dries, and improve the airflow. Expect to lose some. Resow rather than nursing a badly affected tray.

What about treated seed?

Some seed is sold with a fungicide coating, commonly thiram, specifically to protect against damping off and other seed and soil borne rots during germination. It is widely used in commercial production, and it is a legitimate tool.

Most home gardeners, and anyone gardening organically, prefer untreated seed, and the cultural measures above are usually enough at home. If you do use treated seed, handle it as the label directs: wear gloves, wash your hands afterwards, keep it away from children and pets, and never eat treated seed or feed it to animals or poultry. Use only products registered for the purpose in Australia and follow the label.

Frequently asked questions

Can I save a seedling that has damped off?

No. Once the stem base has collapsed the plant cannot recover. Remove it so it does not spread and protect the rest of the tray.

Does cinnamon or chamomile tea work?

These are popular home remedies and the evidence for them is weak. They are not a substitute for clean trays, thin sowing and drier, better ventilated conditions.

Will damping off spread to my garden beds?

The organisms are already in most soils. The practical concern is losing seedlings, not introducing something new. Do not tip affected mix onto beds where you are about to sow.

Is it safe to reuse the mix from a tray that damped off?

Do not reuse it for seed. Spread it on an established bed or add it to a hot compost instead.

My seedlings are leggy but not collapsing. Is that damping off?

No, that is a light problem. Leggy seedlings need stronger light or to be closer to it, and often less warmth.

Related growing guides

• A step by step guide to starting seeds indoors
• The benefits of using a seedling heat mat
• Sow to the Seed, Not to the Spade: How Deep Seeds Really Want To Go
• Organic pest control in Australia

Ready to plant? Browse our Vegetable Seeds.

Causes and symptoms follow Business Queensland and Penn State Extension guidance on damping off.