Why Are My Brussels Sprout Leaves Turning Yellow?

Yellow leaves on Brussels sprouts are one of the most common complaints from home growers, and the cause is rarely just one thing. The yellowing pattern — where it starts, how it spreads, and which leaves are affected first — gives you the clearest diagnosis. Old lower leaves yellowing gently is normal ageing, but widespread yellowing from the middle up signals something the plant urgently needs you to fix.

Nitrogen Deficiency

Brussels sprouts are heavy nitrogen feeders. When the soil runs low, the plant redirects what little nitrogen it has to new growth, and the older lower leaves turn pale then bright yellow. If the entire plant looks washed out and growth has slowed to a crawl, apply a balanced nitrogen feed — a liquid seaweed and poultry pellet combination works well for organics growers. Rake in a general fertiliser around the base and water it in thoroughly. Repeat every three to four weeks through the growing season.

Waterlogged or Compacted Soil

Brussels sprouts hate sitting in water. When drainage is poor, roots suffocate and can no longer absorb nutrients even if they are plentiful in the soil. The result looks exactly like nutrient deficiency — yellow leaves, stunted growth, wilting despite wet soil. Dig around the base and check whether the soil smells sour or looks grey-blue rather than brown. If it does, fork in grit or organic matter to improve structure, and consider raised beds for future crops.

Magnesium Deficiency

If the yellowing appears between the leaf veins while the veins themselves stay green, magnesium deficiency is the likely cause. This often happens in sandy, free-draining soils or after heavy rain leaches the minerals away. A foliar spray of Epsom salts (magnesium sulphate) — roughly 20 g dissolved in 1 litre of water — applied to both sides of the leaves gives quick results within a fortnight. Correct the underlying soil pH too, since acidic conditions reduce magnesium uptake.

Clubroot Infection

If the yellowing is accompanied by stunted plants that wilt on warm days even when watered, pull one plant and examine the roots. Clubroot causes swollen, distorted, club-shaped roots that cannot function properly. There is no cure once established — remove affected plants, lime the soil to raise pH above 7.2 (clubroot thrives in acid conditions), and avoid growing brassicas in that patch for at least seven years. Only buy certified club-root-resistant varieties going forward.

Natural Leaf Senescence

Do not panic at every yellow leaf. The lowest leaves on a Brussels sprout plant naturally yellow and die off as the plant matures. This is the plant recycling nutrients from old leaves to support the developing sprouts. Remove these spent leaves promptly by snapping them cleanly downward — leaving them attached can harbour slugs and fungal disease. A tidy stem with clean scars is the sign of a well-managed plant.

Checking Soil pH

Almost every yellowing problem in brassicas is made worse by incorrect soil pH. Brussels sprouts perform best between pH 6.5 and 7.0. Below 6.5, nutrient lockout becomes common and clubroot thrives. Above 7.5, trace elements become unavailable. A cheap soil test kit gives you a reliable reading in minutes. Lime to raise pH in autumn the year before planting and retest before the season starts.