Mulch And What It Actually Does

Mulch is often described as something you add for appearance or as a slow fertilizer. Its main effects are physical, and they follow from the simple fact that bare soil is an unnatural state.

Evaporation slows dramatically under cover

Water leaves soil from the surface. Sun and wind reaching bare ground pull moisture out of the top few inches faster than roots use it.

A layer of coarse material blocks direct sun and slows air movement at the surface, so the soil beneath stays damp for considerably longer after rain or watering.

In a hot summer this is the difference between watering a bed every other day and watering it once a week.

Soil temperature swings less

Bare soil heats hard in afternoon sun and loses that heat quickly overnight. Roots in the top layer experience the full swing.

Mulch insulates in both directions, keeping summer soil cooler than the air above it and delaying the freeze-thaw cycles that heave shallow-rooted plants out of the ground in winter.

Those freeze-thaw cycles are a common and invisible cause of losses in beds that looked fine in fall.

Weed suppression is about light, not barriers

Most small weed seeds need light to germinate and have very little stored energy. A seed that sprouts under two inches of mulch exhausts itself before reaching the surface.

Mulch does not stop established perennial weeds with deep roots, and it does not stop seeds that blow in and land on top of it.

Which is why mulched beds still need weeding, but the weeding is pulling a few surface seedlings rather than clearing ground.

Depth and placement decide whether it helps or harms

Too thin a layer lets light through and does little. Too thick a layer can keep water from reaching the soil at all in light rain.

Mulch piled against stems and trunks holds moisture against bark that is not adapted to it, which invites rot and gives rodents cover to chew.

Pulling the layer back a few inches from every stem solves that, and costs nothing at the time of spreading.

Organic mulches change the soil as they break down

Bark, leaves and wood chips decompose from the bottom, adding organic matter to the surface layer and feeding the organisms that build soil structure.

That is why organic mulch has to be topped up periodically while gravel does not, and why gravel gives none of the soil benefit.

Shredded leaves gathered in fall do the same job as bought bark and cost nothing, which makes them the most underused material available to a home gardener.