The word “mulch” became a running gag in the comic book Groo the Wanderer.

If I had a dollar for every time I’ve heard “mulch” called “compost” I’d be a wealthy blogger. Let’s set the record straight. The Oxford English Dictionary defines mulch as,

Partly rotted plant material, etc.; (Hort.) loose material consisting of straw, decaying leaves, shredded cuttings and bark, etc., spread on soil or around or over a plant to provide insulation, protect from desiccation, and deter weeds. Also: textile or other artificial material used for the same purpose.

I’d argue that compost, properly defined, is fully not partially rotted organic material (or textiles or plastic, though I don’t like plastic mulch). The distinction is important. If you integrate mulch, i.e. partially rotted material, like wood chips or straw into soil you’ll cause a temporary nitrogen deficiency. Mulch should be used as a top-dressing that both saves water and, over time, contributes soil nutrients as it breaks down. The confusion stems from the fact that you can use compost as mulch and from the fact that a pile of mulch will eventually break down and become compost. I’d also argue that there’s a problem with the dictionary’s definition. I consider fresh (not even “partly”) rotted materials like wood chips and straw as “mulch” too. It’s easy to see why “mulch” and “compost” are often mistakenly used interchangeably.

Interestingly, the earliest known use of the word mulch comes from Samuel Purchas 1657 book about bees, A theatre of politicall flying-insects wherein especially the nature, the vvorth, the vvork, the wonder, and the manner of right-ordering of the bee, is discovered and described : together with discourses, historical, and observations physical concerning them : and in a second part are annexed meditations, and observations theological and moral, in three centuries upon that subject (As an aside, I think we need to bring back really long book titles and creative spelling). In this book Purchas uses the word “mulch” as a suggested material for a beekeeper’s smoker, “Then make a smoak of mulch and wet straw.” Later uses of the word mulch are also about the use of half rotten straw as a mulch for top-dressing plants.

As for why you should mulch, especially here in California, see our blob post Yet More Reasons to Mulch.