Fertilizer Damage

• Improper fertilizer application causes the leaves on boxwood to turn yellow. The evergreen plants benefit from annual fertilizer applications, preferably in early spring before new growth. Direct fertilizer contact with the trunk stresses the plant, causing young roots to die, leaves to turn yellow and branches to rot. To reduce the risk of fertilizer injury, apply the selected fertilizer beyond the drip line. Always read label directions for dosage rates and precautions to prevent over-fertilizing the plant.
  
  

Winter Injury

• Improper cultural practices make boxwoods vulnerable to winter injury, a process that causes leaf tips to turn deep yellow, yellow-orange or deep brown. Exposure to harsh winds, hard freezes or direct winter sun causes foliage discoloration in boxwoods. Water your boxwood deeply before the first expected frost in your area to reduce drought stress that also makes the plant susceptible to injury. If possible, wrap the main trunk with burlap and mulch heavily before the winter, or provide a shade cloth to reduce exposure to winter sun.
  
  

Pests

• Boxwood leaf miners, aphids and spider mites are pests that damage boxwood foliage, causing parts of the plant to appear yellow and wilted. Leaf miners cause the foliage to appear water-soaked and feature blisters underneath that provide a safe haven for larvae during the winter. Resembling tiny maggots, adult leaf miners stunt plant growth and cause yellow foliage. Aphids and spider mites sap essential juices from the foliage, causing infested leaves to appear yellow and wilt. Treat leaf miners with a registered fungicide, and use neem oil or insecticidal soapy solution to deter aphids and mites.
  
  

Disease

• Leaf diseases cause yellowing foliage on boxwoods. While leaf burn causes the tips to turn yellow before browning and falling off, symptoms of leaf spot include yellowing foliage with visible black fruiting bodies upon inspection. Because boxwoods weakened by winter injury are vulnerable to leaf diseases that causes yellowing foliage, protect the plant from strong or drying winds and salt runoff during the winter.
  
  

Insufficient Nitrogen

• Lack of nitrogen causes boxwood leaves to appear stressed and turn yellow. Because boxwood is a foliage plant, it requires plenty of nitrogen to keep its foliage lush green and healthy. Early symptoms of nitrogen deficiency include yellowing of the foliage at the base of the plant. For best results test the soil beforehand to confirm deficiency, and feed the plant an application of a high-nitrogen fertilizer. Space fertilizer granules 6 inches away from the stem and irrigate deeply to release nutrients in the soil.