Organic Versus Inorganic
Because organic mulches decompose, they need to be replaced. Depending on the type of mulch used, replacement intervals vary from one to four years.
These break down gradually to release nutrients into the soil and help improve its structure. Layers will need replacing when the material has fully rotted down. Among the best materials are leaf mould, garden compost, spent mushroom compost, wood chippings, processed conifer bark, well rotted manure, straw (for strawberries), spent hops (poisonous if eaten by dogs) and seaweed.
Replenish mulch when there’s 1 inch or less of it. How often you need to replace it depends on how fast the material decomposes, exposure to the sun, temperature, the amount of rainfall, and the length of your growing season. Generally, I top off my mulch once a year. To keep from disturbing the soil, simply add another layer to the existing mulch.
Recycled rubber mulch is an attractive, durable alternative to traditional wood mulch, and can save money over time. Replacing mulch every year becomes time consuming and expensive. Compared with the expense of annually applying wood mulch, the up-front purchase of rubber mulch actually costs up to 65 percent less over a nominal five-year period.
Organic mulches require annual replacement because they break down and decompose, eventually adding organic matter and nutrients into the soil. Bark and wood nuggets must be maintained at a depth of 2 inches to provide the best benefits, so fresh mulch must be added on top at least once a year to maintain this depth.
When to replace mulch in the garden depends a great deal upon the type of mulch and what you want to accomplish. Winter mulches should be removed in spring, after the danger of frost has passed, while summer and spring organic mulches are generally replaced annually. Many gardeners wait until after they prune to replace the mulch so that pruning cleanup and mulch replacement is combined into one task.
Maintain mulch. Replace mulch as needed during the growing season to maintain the 2- to 4-inch depth. Rake up and replace organic mulch in the spring, especially around roses and fruit trees.