When hemlocks are planted on very wet sites, a brown, oozing canker may form at the base of the plant. Cankers are unsightly, and will allow entry of other pathogens and insects that may be harmful. This malady is prevented by planting only on well-drained sites and avoiding excessive irrigation. Drought stress, shading, and low light conditions. 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.