Why Did My Onion Sets Rot in the Ground?

You planted your onion sets and waited for the row to fill in, only to discover that some or most positions show nothing — and when you investigate by pushing a finger into the soil, you find a collapsed, slimy mush where a firm set should be. Rotted sets are a common early-season disappointment and always have a clear cause. Understanding what went wrong means you can replant with confidence and prevent it from happening again.

Cold, wet soil

The primary cause of set rot is planting into cold, waterlogged soil. Onion sets are essentially small dormant bulbs — they do not have the active root system yet to absorb oxygen from the soil, and in cold, anaerobic conditions bacteria and fungi quickly break them down. The soil should be consistently above 7°C before planting, and it should drain freely — water should not pool or stand in the planting area after rain. On heavy clay soil this may mean waiting until April or even late April for the ground to warm and drain sufficiently, even if seed catalogues suggest earlier dates that apply to lighter, warmer soils.

Damaged or soft sets

Always inspect your sets before planting. Any set that is soft, spongy, discoloured, or has visible mould should be discarded immediately — it will rot in the ground and potentially spread infection to neighbouring sets. A healthy set should be completely firm all the way through when squeezed gently, with clean, dry outer skin. Large sets — those over 2 cm in diameter — are more prone to bolting and also more prone to rot than smaller sets in a range of 1.5–2 cm, which are generally considered the best size for reliable establishment. Do not plant any set that does not pass the squeeze test.

Fusarium and white rot in the soil

If you have grown onions, garlic, leeks, or other alliums in the same ground recently and experienced disease, soil-borne pathogens — particularly Fusarium species and white rot (Sclerotinia cepivorum) — may be present at levels that cause rapid rotting of newly planted sets. These diseases can persist in soil for many years. If this is suspected, rotate to a fresh bed that has not grown alliums for at least three to four years, or for white rot, considerably longer. There is no effective soil treatment available to home gardeners.

Planting too deep

Planting sets too deep puts them in colder, wetter soil and slows root development. Onion sets should be planted with the tip just visible at the soil surface — typically 2–3 cm deep with the top of the set level with or just below the soil surface. Press them in gently rather than pushing hard, which can compact soil around the base and restrict drainage. In very free-draining, light soil on a raised bed, slightly deeper planting (3–4 cm) helps retain moisture, but in heavy, moist soil, keep them shallow.