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Gardening glossary

Damping off

Damping off is the most heartbreaking failure in seed starting. One day a tray of seedlings looks fine; the next morning whole rows have collapsed at soil level, the stems pinched and dark, the cotyledons still attached and intact. Damping off is fast — symptoms can appear in 24 hours, and by 48 hours the trays are typically beyond saving.

The culprits. Several different soil pathogens cause damping off, sometimes alone, sometimes together:

- **Pythium spp.** and **Phytophthora spp.** — water moulds (oomycetes). Most active in cool, wet, poorly-drained conditions. - **Rhizoctonia solani** — a true fungus that thrives in slightly drier, warmer conditions and at the seedling stem just above the soil line. - **Fusarium spp.** — favoured by warm soils. - **Sclerotinia, Botrytis, Alternaria** — secondary or occasional causes.

All of these pathogens can be present in unsterilised soil, recycled seed trays, contaminated water, or even on the seed itself.

Two phases of attack:

1. **Pre-emergence damping off.** Seeds rot in the soil before they ever germinate. Easy to misdiagnose as "old seed" or "cold soil." 2. **Post-emergence damping off.** Seedlings emerge, look fine for a few days, then collapse at the soil line.

Prevention is everything:

- **Use a sterile seed-starting mix** — fresh, pasteurised, never reused. - **Sanitise seed trays** between uses with a 10% bleach solution or hot soapy water. - **Water with clean, room-temperature water** — never use stagnant rainwater for seedlings. - **Bottom-water, do not top-water.** Keeping the seedling stems and crown dry alone prevents many infections. - **Maximise airflow.** A small fan running 24/7 over seedling trays dramatically reduces damping off. - **Avoid overwatering.** Soggy soil is the single biggest risk factor. - **Light cover of milled sphagnum, vermiculite, or chamomile tea spray** on the soil surface has a long folk-medicine reputation; chamomile especially has shown mild antifungal activity in trials. - **Adequate light.** Etiolated, leggy seedlings are far more vulnerable than stocky ones.

If damping off appears in one corner of a tray, immediately isolate or discard the affected cells, increase airflow, reduce watering, and hope the rest survive. Once a tray is heavily infected, dispose of the soil (not in the compost) and start over with fresh material.

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Related terms

Germination

Germination is the process by which a seed wakes from dormancy and grows into a seedling. It requires water, oxygen, the right temperature, and for some seeds, light or a cold period. Time from sowing to first true leaves typically ranges from 3 days (radish) to 4+ weeks (parsley).

Leaf spot

Leaf spot is a general term for discrete, often circular lesions on leaves caused by fungal or bacterial pathogens. Spots may be brown, black, tan, or yellow-haloed and often expand or merge in humid conditions.

Mildew

Mildew is a catch-all term for two distinct fungal-type diseases: powdery mildew (white surface dusting that thrives in dry warm air) and downy mildew (yellow leaf-top patches with grey-purple fuzz beneath in cool wet conditions).

Vascular wilt (verticillium and fusarium)

Vascular wilt diseases are soil-borne fungal infections — most commonly Verticillium and Fusarium species — that invade a plant's xylem and block water transport. Symptoms include sudden one-sided wilting, yellowing leaves, brown streaks inside cut stems, and rapid collapse. There is no cure once infected.