Used as privacy hedges or ornamental plantings, laurels are evergreen trees or shrubs that produce fragrant flowers in shades of white, pink and purple and red. Native to Eastern United States, the ornamental plants thrive in U.S. plant hardiness zones 4 through 9. However, a number of factors adversely affect their health, causing the glossy, green leaves to turn yellow. Rectify improper growing conditions to restore the shrub back to its original health and vigor.
Pests including aphids, scales, whiteflies, thrips and lacebugs are common on laurel shrubs. While some insects sap essential juice from the leaves, causing them to turn yellow and drop prematurely, others secrete sticky honeydew that not only attracts ants but also serves as the breeding ground for sooty mold fungus. Damaged shrubs become susceptible to a number of diseases including leaf spots. Inspect your laurel shrub frequently and take prompt action against pests. Spray a jet of water from a hose over infestations, or treat infested leaves with neem oil or horticultural oil. Alternatively, spray a low-toxicity herbicide over large infestations. Read label directions beforehand for administration rates and precautions.
Improper soil conditions also cause yellowing leaves on a laurel. Laurel shrubs thrive in well-draining, acidic soils. The shrubs planted in poorly draining or alkaline soils develop chlorosis, a nutrient deficiency that causes partial or complete yellowing of foliage. Amend the soil with aluminum sulfate to provide the plant essential nutrients. Test the soil's existing pH to determine the amount of ammonium sulfate needed to correct the problem. If the treatment does not amend yellowing leaves, consult your county cooperative extension agent for additional tips regarding additional micro-nutrients lacking in the soil. Avoid over-watering the plant or causing its roots to stand in pools of water for prolonged periods.
Fungal diseases responsible for yellowing laurel leaves mostly occur in spring, during cool, warm and moist weather. Leaf spots and root rot are common diseases in laurel shrubs that cause yellowed foliage. The fungal spores of the diseases overwinter in the debris at the base of the plant and spread through irrigation and rain water. To treat the infected laurel plant, prune damaged parts with sterilized pruning shears and discard plant clippings and debris from around its base. Apply a registered fungicide to control spread.
Excessive sunlight burns the foliage of a laurel shrub, causing it to yellow before turning brown. Install a shade cloth to provide the shrub dappled afternoon sun, especially in U.S. Hardiness Zones 5 through 9. Over-fertilizing also causes the foliage to turn yellow. Excessive fertilizer burns the roots of the plant, causing its foliage to turn yellow. Ideally, the plant benefits from one fertilizer application every spring, before new growth. Feed it a fertilizer specifically formulated for acid-loving plants, following label directions for dosage rates. Space the fertilizer from the wood to prevent burn, and irrigate the soil deeply after the application.