Boxwood leaves are normally medium to dark green, however they may turn orange, yellow or red in the winter months. This is known as bronzing, and it isn't usually anything to worry about. However, if bronzing is accompanied by massive leaf loss, it could be a sign of infestation.
Causes of Bronzing
Bronzing is a natural occurrence. It is the plant's reaction to freezing temperatures, and it is not harmful. Older plants are more likely to turn orange or red during this time. Older leaves may even turn yellow and drop off the shrubs. This is also not harmful.
Occasionally, bronzing may be caused by nematodes. Nematodes are roundworms. Some varieties are beneficial, but others are parasitic to plants. These parasitic nematodes can cause stunted growth, dying foliage, leaf dropping and bronzing.
Preventing Winter Bronzing
Winter bronzing cannot typically be prevented, but it can be reduced by shading plants from harsh winds using wind breaks or covers. Keep the soil healthy, because plants that are weak due to poor soil quality or lack of moisture are more likely to bronze during the winter.
Use lime to bring soil pH to approximately 7.0. Fertilize plants well, and keep them watered often. Healthy plants are less susceptible to nematode damage.
Twospotted Spider Mites
A less common cause of bronzing is infestation by twospotted spider mites. These mites are common in the southern United States. They damage plants by using their mouth parts to cut into leaves and suck out sap. This causes surrounding leaf tissue to collapse. If you notice small spots on leaves, followed by yellowing or bronzing in the summer months, it is quite possibly an infestation by these pests. Treat plants with a strong insecticide meant for killing spider mites to eradicate them.