Yellowing leaves on a plant warrant a thorough inspection to determine underlying causes and correct them immediately. Plant foliage turns yellow due to a variety of reasons including improper cultural control, insect infestation, disease and improper growing conditions. Determining the cause helps you fix the problem so the plant regains its health, its foliage returns to its natural color and the problem doesn't recur.
Although water is essential for growth and maintenance, plants are sensitive to the amounts they need. Over-watering causes root problems such as root rot, which manifests itself as yellowing plant foliage. The problem is exacerbated when the plant grows in heavy clay or poorly drained soils. Allowing the roots of the plant to sit in water for prolonged periods prevents them from absorbing oxygen and supplying it to other parts, thus increasing stress and causing yellowing leaves. Upon inspection, rotted roots appear slimy and black and pull out of the soil easily. Transplant your outdoor plant to a well-draining spot to prevent root problems. Similarly, a potted plant whose roots outgrow their container can suffer yellowing foliage. If the roots of your potted plant are pressed against the sides, transplant it to a larger container immediately to provide room for growth and spread.
Improper cultural practices increase stress on a plant, causing leaves to appear mottled or yellow. Over-watering a plant is as harmful as under-watering it. Fluctuations in watering schedule, or allowing a considerable amount of time to elapse between irrigation, causes the foliage of the stressed plant to turn yellow. The problem is exacerbated during periods of drought or dry weather, when the soil dries out sooner, requiring frequent irrigation. Although different plants require different amounts of water, irrigate your plant when the soil surface appears dry. Use a soaker hose that provides a steady and controlled amount of water to the soil as opposed to overhead watering with a hose.
Over-fertilizing a plant burns the roots and causes the foliage to appear yellow. Follow label directions for dosage and frequency when feeding your plant a high-nitrogen fertilizer. Conversely, iron or nitrogen deficiencies caused by improper fertilizing practices manifest themselves as yellowing foliage. While a nitrogen deficiency causes the leaves to turn yellow, leaves with an iron deficiency appear yellow but feature green veins. Apply an iron chelate foliar spray on the plant to treat iron deficiency. Stop fertilizing the plant when it becomes dormant.
Pests such as spider mites, aphids and nematodes damage plants and cause yellowing foliage. The pests chew foliage and roots with sucking mouthparts, causing plants to appear stressed and foliage to wilt and turn yellow. Stressed plants become susceptible to diseases including leaf blights and spots. Treat pests with horticultural oil or insecticide to control spread. Excessive sunlight exposure, transplant shock or sudden exposure to cold winds also can cause a plant's foliage to turn yellow. Other factors responsible for yellowing plant foliage include aging, dormancy, physical damage or declining roots.