Mulch, how often to add
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Mulch slowly decomposes over time, and new mulch is needed to fill in gaps and build the mulch layer back up to about 2 to 4 inches. It is generally recommended that fresh mulch be added on about a yearly basis.
When adding new mulch, it is best to leave the existing mulch in place and add the new mulch to the top. Mulch decomposes over time and adds to the fertility of the soil.