Chlorosis

Chlorosis is simply the technical word for leaves losing their healthy green and turning pale yellow. The green comes from chlorophyll, the pigment that lets a plant turn sunlight into food, so a chlorotic leaf is one that isn't making enough of it. It is a symptom rather than a disease — a sign that something is wrong with the plant's nutrition or growing conditions — which means the cure is to find and fix the underlying cause, not to reach for a spray.

Interveinal yellowing — read the pattern

The shape of the yellowing tells you a great deal. The most useful pattern to learn is interveinal chlorosis: the leaf goes yellow between the veins while the veins themselves stay green, leaving a fine green network on a yellow background. Where it shows up first is the clue:

• On the oldest, lowest leaves — the plant is shuffling a mobile nutrient up to its new growth. This usually means a shortage of nitrogen (whole leaf pales evenly) or magnesium (classic interveinal yellowing on older leaves).
• On the youngest leaves at the tips — the plant can't move the nutrient around, so the newest growth suffers. This points to iron chlorosis, very often caused by the wrong soil pH rather than a true lack of iron.

The common causes in a UK garden

• Nitrogen shortage. Hungry crops in old or poor compost go pale and slow, oldest leaves first. Common in containers and late in a long growing season.
• Magnesium shortage. Tomatoes and potatoes in pots are prone to it — older leaves yellow between the veins. Watering with rainwater on light soils makes it more likely.
• Iron shortage from wrong pH. This is the big one for acid-loving plants. Blueberries and other ericaceous plants can only take up iron when the soil is acidic. In most UK gardens the soil is neutral to alkaline, and hard tap water nudges potted compost the same way, locking up iron even when plenty is present. The leaves go yellow with green veins and the plant stops cropping.

Because pH controls what a plant can absorb, a soil test is often more useful than feeding blind — see pH.

How to fix it

Match the fix to the cause. For nitrogen, feed with a balanced general fertiliser or a liquid feed and the colour returns within a couple of weeks. For magnesium, water in Epsom salts (magnesium sulphate), about a tablespoon per watering can, every couple of weeks. For iron chlorosis on blueberries, the answer is rarely more iron — it's correcting the pH: grow in bagged ericaceous compost, feed with an ericaceous fertiliser, and water with collected rainwater rather than hard tap water. Top up the compost each spring to keep it acidic. Get the conditions right and green, productive growth follows.