Mulch should be applied to new transplants once the soil has warmed in mid-June. A thick layer of mulch applied in April or May will keep the soil cold and slow rooting and top growth of new transplants. For most perennials, a 1 to 2 inch thick layer of a coarse textured compost is recommended. Pine needles are also a suitable mulch. For established planting, mulch should be applied in early summer to conserve soil moisture during summer dry spells.
Organic mulches decompose and are absorbed by the soil over the growing season.
Re-apply to established perennials in the fall as needed. This insures that the roots and crown are protected from the winter cold and keeps plants dormant until later in the spring. Gravel mulch keeps the crowns of the plants dry during the spring thaw, holds heat around the plants in the summer and encourages many rock garden and native plant species to re-seed and spread. Planting season for mountain gardeners begins as the snow melts and the soils thaw and dry out somewhat. Fall planting can be somewhat hit or miss.