Hemlocks are very susceptible to drought stress, and will grow poorly when planted in a location that is excessively dry or shaded by other trees or structures. Hemlocks thrive in a location that is moist but well-drained, with bright unobstructed sunlight. Poor growth of hemlock may be avoided by careful selection of the planting site. Insect Problems Hemlock rust mites , Nalepella tsugifoliae. This pest is an eriophyid mite that feeds openly on the hemlock foliage.
Foliage turns from the normal dark green to yellow particularly in the spring.
They differ from spider mites in that they are extremely small, their size can be measured in micrometers, cigar-shaped and slow moving. Rust mites overwinter as adults in bark cracks near foliage. Thorough coverage with ultrafine horticultural oil or insecticidal soap, which are among the compounds registered for control of this pest in Connecticut, will reduce the population. Hemlock scales , Fiorinia externa and Nuculaspis tsugae. Two armored scales introduced from Japan are pests of hemlock in forests and ornamental plantings in the eastern United States.