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.
Western hemlock is highly productive on soils with a high range of available nutrients.
Western hemlock is very shade tolerant. On drier upland slopes in Glacier National Park, western hemlock often achieves dominance over western redcedar. The reproductive cycle of western hemlock occurs over 15 to 16 months from the time of cone initiation in early summer, until seeds are shed in the fall of the following year. Fertilization and seed development occur in the second year. Fertilization occurs in coastal western hemlock about mid-May.