A hardy member of the cabbage family, fresh, healthy kale (Brassica oleracea) has vibrant bright to deep green leaves. But when kale is attacked by pests or wilts after too little water or too long in the refrigerator, the leaves turn an unappetizing yellow. While it is safe to eat the yellowed leaves, they taste bitter and are best thrown out or added to the compost pile.
Kale loses nutrients as the leaves yellow, so you won't get the same nutritional punch. Green kale is rich in vitamins A, C and K, along with the mineral manganese and plenty of fiber. Steaming kale makes more of the nutrients available when it's fresh and green.
Annual kale is more resistant to pests than many leafy vegetables, but some pests can cause the leaves to turn yellow. Black rot can cause leaves to yellow and drop off the stems. Aphids can yellow and curl leaves. The fungus fusarium yellows causes older leaves to discolor. Cold weather, not enough water and nitrogen deficiency can also cause leaves to turn yellow and unpalatable.
Growing Greener Kale
Kale grows best in well-draining, loamy soil enriched with organic matter. Enrich the soil with nitrogen to enhance growth and avoid yellowing of leaves. Manure-enriched compost and peat moss can help boost nitrogen levels. The soil should be in a pH range of 5.5 to 6.8. Mulch can also help kale grow as its roots run horizontally, just inches under the soil. Kale grows best in full sun, but will tolerate partial shade.
Keeping Leaves Green
Pick kale leaves from the outer edges of the plant when you need them, rather than storing them in the refrigerator, where they can wilt and turn yellow. When harvesting, look for crisp, tender green leaves. The smaller, greener leaves have a milder flavor. The flavor becomes stronger and more bitter as the leaves yellow. Once picked, store unwashed kale in a plastic bag in the refrigerator.
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