Rhododendrons, other broad-leaved evergreens, and conifers should be thoroughly watered during the late fall and never be allowed to freeze for the winter in a dry condition. This is more often the cause of winter-killing than is severe cold. The reason why these plants are more susceptible to injury on this account is because they hold their leaves during the winter months and thus transpiration continues to some degree, and if the plant is not well filled with moisture, a drying-out process occurs and the plant is devitalized. Because of the mass of fine, hairlike roots which find their way through the earth near the surface of the ground, rhododendrons cannot endure any considerable dry period or any cultivation of the surface of the ground, in the same manner as that adopted in caring for deciduous shrubs and the deeper-rooted evergreens.
Rhododendrons and other ericaceous plants should not be fertilized with manure until well established. In the late fall after the rhododendrons have been established one year they may be fertilized by the application of a two-inch layer of well-rotted stable manure over the top of the mulch. This should be repeated each year. Chemical fertilizers are seldom or never used on ericaceous plants. Bone meal should never be used as its bone phosphate of lime is sure to react upon the soil acidity so essential to the successful culture of rhododendrons and allied plants. The use of bone meal has been known to be fatal to these plants. Epsom salt has been recommended as a fertilizer for rhododendrons, but this is probably a mistake and in any event a doubtful procedure because of the chemical nature of this material. As epsom salt is magnesium sulphate and has been used in England to top-dress clover as a substitute for lime, it probably would react in the soil similar to lime and would be injurious to all ericaceous plants.