For Release On Or After 02/21/14
By Dan Gill
LSU AgCenter Horticulturist
Yellow leaves have been showing up on a wide variety of evergreen shrubs since the past fall, and this has caused many Louisiana gardeners some concern. Although the condition may look alarming, in most cases the yellowing leaves do not indicate a problem.
Here in the Deep South we tend to use a large number of broadleaf evergreen trees and shrubs in our landscapes. These plants – such as gardenia, azalea, Southern magnolia, cherry laurel, Indian hawthorn, camellia, hollies, ligustrum, sweet olive, banana shrub, cleyera, viburnum and others – do not lose all of their leaves during winter and are green year-round. This allows our landscapes to look green and attractive even in the depths of winter.
Leaves on these plants, however, eventually grow old, die and are dropped from the plant. Before old leaves drop, they often turn bright yellow, orange or red. The affected leaves are the ones located farthest from the tips of the branches. This makes sense because the newest leaves would be those closest to the growing point at the tips of branches, and the oldest leaves would be those located farther away from the growing points.
As many as one-third of the leaves on a plant may be dropped at one time. This dropping of old, worn-out leaves generally occurs sometime during the period from November through May, depending on the type of plant. Azaleas generally drop leaves December through February, while hollies and cherry laurels do it in March or April, just as or before new growth appears. Gardenias and Southern magnolias yellow and drop leaves in April or May. As startling as it is to see one-third – or even more – of the foliage on a plant turns yellow, orange or red and drops during this period, it is usually normal.
This leaf drop is not uniform from year-to-year or from plant-to-plant. One year, leaf drop may be fairly extensive on a plant and the next year very minor. In a single bed, some of the plants may be dropping more leaves than other plants of the same type. Do not let this variation confuse you; it is quite typical. Stressful weather conditions – such as the severe drought during the previous summer – may cause more leaf drop some years. But the dropping of older leaves is still no cause for concern. Once the leaves have dropped, the plants may look a little naked, but the condition is generally temporary. The plants will look fuller when new growth appears later.
Do not fertilize these plants in an effort to correct what you may have initially seen as a problem. Fertilizer will not stop or prevent normal leaf drop. And if applied in fall or winter, fertilizer will not be efficiently used by the plants. Even worse, with mild winter weather, fertilizer applications in the fall may stimulate growth that will be susceptible to winter freezes. Fertilization is appropriate in late February or March, just before or when plants begin to grow. New growth then will help plants that lost a lot of leaves look fuller.
This yellowing of leaves is sometimes confused with a disease called iron chlorosis, especially in the case of azaleas and gardenias. In the case of iron chlorosis, the symptoms appear because the plant is not getting enough iron, an essential element for plants.
Unlike the dropping of older leaves, the symptoms of iron chlorosis are most noticeable on the youngest leaves located at the ends of the branches. The leaves become yellow-green to yellow while the veins of the leaves stay green. This creates a very distinctive green vein pattern on a yellow-green to yellow background. In severe cases, the new growth may be a pale yellow-white with brown edges.
This condition is common in areas of Louisiana where soils are alkaline. In alkaline soils, plants that prefer an acid soil have difficulty obtaining iron. A few commonly grown acid-loving plants that may show this problem include azaleas, gardenias, willow oaks (and some other oaks), camellias, banana shrub, pines, blueberries, hollies, starbush and cleyera. The solution is to fertilize the plants with iron in a form readily available to plants, treat the soil with an acidifying agent or, preferably, do both.
If the plant is severely iron chlorotic, it should be treated with a fertilizer rich in chelated iron, such as Liquid Iron or Ironite. Chelated iron is a form of iron that is readily absorbed by the plant’s leaves and roots. It will not stimulate growth and so may be used now. Continue to treat every two to four weeks until the foliage is a normal green.
To make the iron in the soil more readily available, the soil should be treated with an acidifying agent such as sulfur, iron sulfate (copperas), aluminum sulfate or a liquid soil acidifier. Sulfur is the slowest-acting and longest-lasting. Using one of these products achieves a better long-term solution than just treating with chelated iron. Soil acidifiers may need to be reapplied periodically over the years to maintain an acid soil condition. Follow package directions carefully.
Yellowing leaves and nutrient deficiencies may also indicate problems such as drought stress, poor growing conditions, poor drainage or damage to the root system. Unless something suggests otherwise, however, the yellow, orange or red leaves you may see on broadleaf evergreens now through late spring will probably be normal.