Why Are My Zucchini Leaves Turning Yellow?

Zucchini are vigorous, greedy plants that grow at a sprint, and yellow leaves are usually a sign that the plant's fast pace has outrun one of its needs. Because zucchini push out enormous leaves and crop heavily, they show stress quickly. The good news is that the colour and position of the yellowing point you straight to the cause. Before you do anything, take a moment to look at which leaves are yellow and how.

Soggy roots: the most frequent reason

Zucchini have big leaves that wilt dramatically, which tempts gardeners to keep watering — and overwatering is the most common cause of a yellow zucchini. When the soil stays waterlogged, the roots cannot draw in oxygen and they begin to fail, so the plant cannot take up nutrients and the leaves pale and yellow across the whole plant. Dig a finger into the soil near the base. If it is wet and heavy, hold off watering and let it dry. Zucchini prefer a deep soak when the top inch has dried rather than constant damp.

A hungry plant running low on feed

If the oldest, lowest leaves yellow first while the crown stays green, the plant is most likely short of nitrogen. A zucchini producing a steady stream of fruit drains the soil fast, pulling nutrients out of its old leaves to feed the new growth and the developing squash. A balanced feed restores the colour. There is a related cause worth knowing: yellowing between the veins while the veins stay green usually signals a magnesium shortage, common on sandy soils and easily eased with a dilute Epsom-salt foliar spray.

Pests and the borer check

Turn a yellow leaf over. Sap-sucking pests such as aphids, whiteflies and spider mites drain the plant and yellow the leaves, and they hide on the undersides — clear those and the colour recovers. But zucchini have a more sinister possibility: if the whole plant yellows and wilts and the trouble seems to come from the base, inspect the lower stem for a small hole oozing sawdust-like waste. That is the squash vine borer, a grub tunnelling inside the stem and cutting off the plant's water — a problem you must treat at the stem, not the leaves.

Run the quick check

Work through it in order: is the soil waterlogged (let it dry and ease watering)? Are the oldest leaves going first with no spots (feed it)? Is the yellow between green veins (try magnesium)? Are there pests on the undersides (treat them)? Is the whole plant wilting from a holed, oozing base (suspect vine borer)? Most zucchini yellowing is simply water or feeding, and those plants green back up fast once corrected — letting your zucchini get back to its astonishing production.