Why Are My Broccoli Leaves Turning Yellow?

Yellowing leaves on broccoli or cauliflower are one of the most common problems home growers encounter, and the cause is almost always one of four things: nitrogen deficiency, waterlogged soil, clubroot disease, or downy mildew. Getting the diagnosis right is important because the remedies differ completely — more fertiliser will not help a waterlogged plant, and better drainage will not fix a nitrogen shortage.

Nitrogen deficiency — the most common cause

Brassicas are hungry plants and the most frequent cause of yellowing is simply not enough nitrogen. The yellowing starts on the oldest, lowest leaves first and works its way up the plant as the nitrogen the plant does have gets redirected to new growth. The rest of the plant looks reasonably healthy but pale. Fix this by applying a high-nitrogen fertiliser such as dried blood, fish meal or a liquid feed high in N. On very hungry soil, a second application two weeks later is worthwhile.

Waterlogging and poor drainage

Broccoli and cauliflower sitting in saturated soil cannot take up nutrients even when nutrients are present. The roots are essentially suffocating. Yellowing from waterlogging tends to be more uniform across the plant rather than starting at the base, and the soil around the plant will be visibly wet or compacted. Improve drainage by raising beds, adding grit to heavy clay, and avoiding walking on the bed surface. On already-yellow plants, ease off watering and wait for recovery before feeding.

Clubroot

If the yellowing is accompanied by wilting in warm weather and pulling the plant reveals swollen, distorted roots, clubroot is the cause. This soil-borne disease cannot be cured once established. Affected plants should be removed and binned (never composted). The soil pH should be raised to 7.5 with garden lime — clubroot thrives in acid soils. Avoid planting brassicas in the same spot for at least seven years.

Downy mildew

Downy mildew causes irregular yellow patches on the upper leaf surface with corresponding grey-purple fuzzy growth on the underside. It spreads in cool, damp conditions and is common in autumn. Remove affected leaves promptly, improve air circulation by spacing plants well, and avoid overhead watering. There is no cure once a plant is severely infected, but removing affected material slows the spread to other plants.