Mint is a fast-growing perennial herb that takes over the entire garden when left to spread. Its natural vigour makes it a no-fail plant for people looking for a tough ground cover or potted herb. The leaves, when added to savoury soups, salads or other dishes enhance flavour and aroma. If you notice the foliage of your mint plant turning yellow, take steps to correct the underlying problem immediately so you can continue harvesting mint all season.
Pests such as aphids, mint cutworm and spider mites cause the leaves of mint to curl, turn yellow and drop from the plant. The tiny insects suck essential juices from the plant's foliage, causing them to appear wilted or curled and stunt plant growth. Left untreated, severe infestations can kill the potted herb. Spray pest infestations with a jet of water from a hose to dislodge them, or apply neem oil or horticultural oil over them. Alternatively, pour a solution of insecticidal soap into a spray bottle and spray evenly over pest infestations.
Disease viruses prevent the plants from making chlorophyll so the foliage appears yellowed or mottled. Caused by the fungi Puccinia menthae, symptoms of rust include yellow pastules on the stem and leaves of mint in late spring or early summer that deepen in colour by the end of summer. Verticillium wilt causes leaves to turn yellow at the margins before curling up and dying. Prune infected leaves on the mint plant with sterilised pruning scissors and apply a sulphur spray to control disease. Rotate the plants frequently to reduce chances of disease and practice good cultural control to reduce plant stress.
The leaves on a mint plant turn yellow if they receive too much or too little sunlight. Ideally, mint requires full sunlight to partial shade. Exposing the plant to plenty of light in the summer causes the foliage to sun scald, appearing scorched or yellowed. Move the potted mint to a shadier spot if exposed to full sunlight, especially in the afternoon. Exposing the mint plant to dry winds, frost or a heatwave also causes foliage to turn yellow that generally recover when the environmental condition stabilises.
Over or under watering and fertilising causes the foliage of the mint to turn yellow. If container-growing mint, use a pot with adequate drainage holes through its base and fill it with a mix of quality potting soil and compost. Water the soil when the top 1 to 1 1/2 inches feels dry. Over watering mint in poorly draining soil causes the roots to rot. Feed the plant a well-balanced fertiliser every spring, according to label directions. Avoid over fertilising the mint plant as it makes it susceptible to disease.