Western hemlock thrives in humid areas of the Pacific Coast and northern Rocky Mountains. Growth is best in mild, humid climates where frequent fog and precipitation occur during the growing season. The best stands occur in the humid coastal regions. The frost-free period within the coastal range of western hemlock averages less than 100 to more than 280 days. It grows well on sedimentary, metamorphic, and igneous materials.
Western hemlock is found on most soil textures.
Height growth, however, decreases with an increase in clay content or soil bulk density. This is attributed to inadequate soil aeration or the inability of roots to penetrate compact soils. The pH under stands containing western hemlock ranges from less than 3.0 to nearly 6.0 in the organic horizons. The pH in the surface mineral horizons ranges from 4.0 to 6.3 and that of the C horizons from 4.8 to 6.2. The optimum range of pH for seedlings is 4.5 to 5.0.