Round Robin: Using Mulch and Compost

Denver, Colorado—During the question-and-answer period following one of my recent lectures, a woman asked me what I used for mulch. My reply, “I don’t mulch,” was greeted with as much outrage as if I’d announced that I perform the ritual sacrifice of kittens. “But you have to mulch!” she cried.

“Why?” I asked.

“Because,” she sputtered, “it’s ... it’s good for your plants.”

“Don’t my plants look healthy?” I countered.

The woman looked as if she suspected that I was in league with the devil. I went on to explain why I don’t mulch. Organic mulches such as bark nuggets or wood chips are gradually decomposed by soil microorganisms that take nitrogen from the soil to fuel the process—at the expense of surrounding plants. I once saw a garden that was mulched with 6 inches of wood shavings. Its owner couldn’t understand why the plants hadn’t grown an inch in three years. I suggested that he remove the mulch and apply fertilizer to his nitrogen-starved perennials. The results were dramatic.

I’m also staunchly opposed to using weed-barrier fabric or, worse, black plastic covered with big hunks of bark. Within a few years, these barriers are full of weeds that drift in with dust on top of the fabric or plastic.

Mulch is routinely recommended for shading the soil, conserving moisture, and keeping weeds from sprouting. In my thickly planted borders, however, plants stand shoulder to shoulder. Every square inch of soil is covered, keeping the earth shaded and cool. There’s not much room for weed seeds to sprout.

Some forms of mulch make sense under certain conditions. In a rock or dryland garden, a layer of loose pebbles several inches deep can help to prevent evaporation of soil moisture and keep plant crowns—the basal area that’s most vulnerable to rotting—high and dry. It also might prove useful in an extremely windy area. Succulents such as aloes and agaves and Mediterranean herbs such as santolina and lavender might thrive in the reflected heat from a gravel mulch. After all, the stony soil in which they grow in the wild is nature’s own version of a gravel mulch (and it doesn’t include black plastic).