Blue Princess, sometimes shortened to Princess, is a cultivar of Ilex x merservae, a hybrid plant. This attractive holly grows to a maximum height of 4.5 m (15 feet) and width of 3 m (10 feet) and is desirable for its bluish-green leaves, purple stems and cold-hardiness. Although bred to be tough, I. x merservae "Princess" suffer from problems common to all hollies causing the leaves to turn yellow and fall off the tree.
Environmental stresses cause an otherwise healthy Princess holly to shed its leaves. Although they are cold hardy, severe freezes cause the tree to drop its leaves. Nutrient deficiencies in the soil also cause leaves to drop. This is called chlorosis and stems from high soil pH. The leaves turn yellow before they drop. Rapid temperature changes during the winter and open exposure to drying winds can cause leaf scorch, which if severe enough will also cause defoliation of the tree.
Fungal diseases also cause leaves to drop from holly plants. Although some fungi only affect the tree in a minor, aesthetic ways, others are more serious. Cylindrocladium is a fungus causing tiny, yellow spots that later grows larger and turns purplish-brown to appear on the leaves. Holly trees are so sensitive to this fungus that just one spot may cause the tree to shed that leaf. Anthracnose is caused by a number of different fungi. This fungal disease causes brown patches to appear on the leaves, which eventually drop from the tree. Web blight is a fungus that thrives in warm, wet weather and kills leaves from the bottom of the plant upward.
Sometimes, you can treat the cause of falling leaves after symptoms appear. Other times, the very fact that the leaves are dropping means that it is already too late. Damage from hard freezes cannot be treated. Treat chlorosis by amending the soil with acid-rich fertiliser. In the case of fungal diseases, prune infected branches to prevent the spread of the diseases, and spray the tree with a fungicide. In minor cases, that may be enough to stop the disease from getting worse.
Prevention is usually better than the cure when it comes to foliage problems in Princess holly trees. A 7.5 to 10 cm (3 to 4 inch) layer of mulch around your holly helps protect it from very cold frosts. Shading a holly tree from direct afternoon sunlight helps prevent sun scorch. Fungicide applied in the spring as a preventive measure cuts down on the incidences of fungal infections. Ensure your plants have plenty of room around them for air to circulate, and rake up and dispose of any infected leaves left on the soil under the plant, as fungi overwinter in the ground.