The yellowing of normally green leaves is called chlorosis. It results from a shortage of chlorophyll, which is essential to the plant’s continued health. Like anemia in people, chlorosis can have many causes. The gardener must identify the cause to correct the problem. For instance, plants not getting enough light may develop yellow leaves and must be moved to a sunnier spot.
More common in potted plants, yellow leaves can be a sign of poor drainage. Check to see if the roots are sitting in water or very wet soil. If this is the cause, it is much easier to reduce watering in container plants than to correct serious drainage problems for plants growing in the earth. One solution might be to move the chlorotic plant to another site and plant a moisture lover in the boggy spot. Cannas, elephant ears, Louisiana iris and lobelia all like a moist environment.
Yellow spots on foliage indoors and out may be a sign of insect damage. Turn the leave over to check for aphids, mites, thrips and scale. Yuk. A good bath may be the treatment of choice. Horticultural soaps are safe for the environment, and water is not a controlled substance. Lady bugs love to eat aphids, but in their absence, you can squirt or scrape the critters off the plant. The mouth parts are left behind, so there are no survivors.
Chlorosis can result from a nutrient deficiency. If there is not enough nitrogen in the soil, older leaves may turn yellow as the plant uses what nitrogen is available to produce new growth. If the leaves have green veins while the rest is yellow, the problem is probably a lack of iron, which is essential to chlorophyll formation. The soil may be low in iron or an alkaline pH may prevent plants from using the iron that is present.
An application of liquid “chelated” iron produces rapid results. Wear gloves when handling it as it will stain your skin. If the leaves look mottled with green veins, this may be a magnesium deficiency which can be treated with epsom salts.
If the leaves look burned around their yellow edges, potassium is needed. Actually, your composted fruit and vegetable scraps are a good source of this nutrient. Most balanced fertilizers contain the essentials, but our heavy rains may have leached them from the soil.
Yellowing of leaves that have finished their life on the plant is normal. Likewise, stressed plants may develop chlorosis.
Finally, changes in leaf color may be a sign of disease. Blackspot on roses is a common example.
If the green leaves of summer begin to pale, the gardener needs to take a close look at both the leaves and the environment to identify the cause and correct the situation.
Master gardener Ann Bartlett never lets lack of familiarity with a plant stop her from trying it in the ornamental beds around her home.