Diagnosing yellow leaves – a rainbow of reasons

Why are the leaves on my plant turning yellow? This is question commonly heard at ISU Extension Offices. And although a simple, straightforward question, there is no one-size fits-all answer to the question of why a plant’s leaves turn yellow. “It depends,” may well be a universal response given, as to make a correct diagnosis of this symptom, further details are needed.

Healthy plants typically have dark green foliage. When the green chlorophyll leaf pigment normally found in leaves is lacking, abnormal yellowing develops, a condition called chlorosis. There are exceptions, as some plant cultivars, such as hosta, trees or shrubs, have healthy foliage colors that have been bred to be pale green or yellow.

Yellowing leaves may the result of a nutrient deficiency in the soil, especially nitrogen, which is needed for healthy green foliage. But over-fertilizing can also lead to yellow leaves, or leaf scorch, as plant roots can become burned when excessive amounts of fertilizer are applied.

A high soil pH can cause yellowing leaves; as nutrients, although present in the soil, are unavailable for plants to take up due to high pH conditions. Soil testing for pH and any fertilizer needs will help to prevent the overuse of chemical fertilizers, which can lead to surface water contamination.

Another cause of yellowing leaves in plants is over-watering. Water-logged soil is deficient in oxygen, which plant roots need to grow. Lack of drainage holes in container plants, or pots left standing in water-filled saucers, can also cause yellowing of foliage due to excess moisture. Over moist soils also can create an environment favorable for fungal disease, such as root-rot, creating yellowing leaves as plant roots are attacked.

A plant experiencing too hot or dry conditions may develop yellow leaves. Damage to roots, pot-bound plants, insect pests, or the presence of foliar disease, are all conditions that may be indicated by a plant with yellowing leaves.

Did you know? When bringing plant samples to the Extension Office for diagnosis, bring more than just one leaf. Provide freshly collected plant material and include the entire plant with roots; for trees, submit a branch with transition from healthy leaves to the very sick. And remember, a picture speaks a thousand words. Be sure to include a photo of the entire plant, as well as a close-up of the problem area.

Have a sick plant? For ISU horticulture advice on what to do, email photos, along with a good description of the problem to Yvonne McCormick at yvonne@iastate.edu