Why Are My Onion Seeds Germinating So Slowly?

Onion seeds are famously slow, even under ideal conditions. They are among the slowest vegetable seeds to germinate, and this catches out many growers who are used to the quick response of lettuce, radish, or bean seeds. Patience is always required with onion seeds, but several factors can make germination even slower than normal — or prevent it altogether. Understanding what onion seeds need removes most of the guesswork.

Temperature requirements

Onion seeds germinate most reliably at soil temperatures between 10–25°C, with the optimum around 20°C. Below 10°C, germination is very slow and unreliable — at 5°C seeds may sit for six weeks before emerging, if at all. Above 30°C, germination is inhibited. When sowing indoors in January or February for early transplants, a propagator or warm windowsill maintaining 18–20°C is essential. When sowing direct outdoors in spring, wait until the soil temperature reliably exceeds 10°C — typically late March to April in the UK. Covering the soil with black polythene for a week before sowing raises temperature by several degrees.

Sowing depth and moisture

Onion seeds should be sown at 1–2 cm depth. Deeper sowing delays emergence significantly, and very shallow sowing exposes seeds to drying out before the radicle has made contact with moist soil. Germination also requires consistent moisture — seeds need to imbibe water to trigger germination, and if the soil surface dries out after sowing, seeds that have started to swell will die rather than germinate. Water the seedbed before sowing, sow, then cover with a light scattering of fine compost and firm down gently. Keep the surface consistently moist until emergence is complete.

Old seed

Onion seed has a shorter viable life than many vegetable seeds — typically two to three years if stored well (cool, dark, dry), and often less than two years in poor storage conditions. Old onion seed germinates patchily and slowly, and heavily aged seed may fail to germinate at all. If your seed is more than two years old, test a small sample on a damp piece of kitchen paper before investing in a full sowing. If fewer than 50–60% of test seeds germinate after two weeks at room temperature, buy fresh seed. This is also why autumn-bought seed sown the following spring usually germinates better than seed held for two or three seasons.

Normal expected germination time

At 18–20°C, onion seeds typically take 7–14 days to emerge. At 12–15°C, 14–21 days is normal. At 10°C, 21–28 days or longer is possible. Direct-sown seeds outdoors in a cool spring can take four to five weeks before you see any growth. This is not failure — it is within the normal range. Resist the urge to re-sow until you have given a full four weeks at the appropriate temperature. Covering the seedbed with a cloche or fleece speeds germination outdoors by two to four degrees and can halve the waiting time.