Why Are My Onion Leaves Turning Yellow?

Yellowing onion foliage is a symptom shared by several different problems, ranging from the completely normal to the seriously concerning. The pattern, timing, and progression of the yellowing — along with what is happening to the bulb below — tells you most of what you need to know. Diagnosing the cause correctly before acting prevents you from feeding a plant that is actually ripening, or waiting too long when a real problem is developing.

Natural ripening — the most common cause

As onion bulbs reach maturity in mid to late summer, the lower outer leaves naturally begin to yellow and die back. This progresses from the outermost leaves inward and from the base upward over two to four weeks. If the yellowing is happening in summer on plants that have been growing for four to five months, and the bulbs are visibly swollen at the base, this is natural senescence and your cue to prepare for harvest. The tops will fold over naturally and dry — the classic sign the onion is ready. Do not feed a ripening onion in this state; it is wasting your fertiliser and time.

Nitrogen deficiency

If yellowing appears mid-season on plants that should still be actively growing — particularly if the whole plant turns a pale, uniform yellow-green rather than specific outer leaves — nitrogen deficiency is likely. This is most common in sandy, free-draining soils where nitrogen leaches quickly after rain, or in ground that was not prepared with any organic matter before planting. A liquid nitrogen feed (diluted chicken manure pellet extract, or a balanced liquid fertiliser) applied two or three times in early to mid-season corrects mild deficiency. Do not feed late in the season when bulbs should be ripening.

Waterlogging

Onion roots need oxygen. In waterlogged soil the roots fail and the plant above turns yellow, often with a dull, slightly blue-green cast before full yellowing. If your soil has been persistently wet and the yellow plants are concentrated in the lower, damper areas of the bed, drainage is the issue. Improve the bed structure with organic matter or sharp grit before the next season, or grow onions on a raised bed where drainage is reliable.

White rot and other root diseases

White rot (Sclerotinia cepivorum) causes yellowing and collapse of the foliage with fluffy white mould visible at the bulb base. If pulling a yellow plant comes away easily with a rotten, mould-covered base, white rot is the cause — and it is a serious problem because the sclerotia (tiny black resting bodies) persist in the soil for 20 years. Remove and destroy all affected plants immediately, do not add to compost, and do not grow any alliums in that bed for as long as practically possible. There is no chemical treatment available to home gardeners.