Why Are My Leek Seedlings Growing So Slowly?

Leek seedlings are genuinely slow-growing compared to most vegetables — this is normal. A tray of leeks sown in January or February will produce pencil-thin, grass-like seedlings that seem barely to grow for the first six to eight weeks. This is not failure; it is simply the nature of the plant. However, leek seedlings that are still thread-thin and grass-like by April or May — when they should be approaching pencil width and ready to transplant — are being held back by something specific.

Low temperature

Leek seedlings grow very slowly below 10°C. A tray on a north-facing windowsill in January in an unheated room may be sitting in temperatures of 5–8°C on cold nights, and even the daytime temperature on a cloudy January day provides limited warmth. Active leaf growth in leeks requires temperatures above 12–15°C. If your seedling tray is in a consistently cold location, the seedlings will simply mark time, remaining alive but barely growing. Moving the tray somewhere warmer — or waiting until late February or March to sow — will make a significant difference to growth rate.

Low light

Leek seedlings raised on windowsills in winter are often light-limited. Short days (eight to ten hours of daylight) and low light intensity through glass in January and February slow photosynthesis and therefore growth. Seedlings in low light develop longer, paler, weaker leaves as they stretch toward the light source. The solution is to move them to the brightest available windowsill — ideally south-facing — and rotate the tray a quarter turn daily so all sides receive equal light. If you have grow lights, supplemental lighting for twelve to fourteen hours per day dramatically accelerates seedling growth from January sowings.

Overcrowding

Leek seeds sown too thickly produce a dense mass of seedlings competing with each other. Each individual seedling is shaded by its neighbours and root competition limits growth. Thin heavily at the seedling stage if growth is clearly inhibited by density, or prick out into individual modules at the two-leaf stage. Seedlings given their own module or cell will grow noticeably faster than those competing in a dense clump.

When slow is normal

Seedlings sown in January that are still small in early March are most likely simply behaving normally. Leeks sown in late March or April under better light and temperature conditions will overtake January-sown seedlings by transplanting time (May–June), making early sowing less advantageous than it seems. If your seedlings are growing — even slowly — and are healthy green in colour, they are fine. The main criterion for transplanting readiness is stem diameter of approximately pencil width (5–6 mm), not calendar date.