JeffCo Master Gardeners: Planning for Spring: What Kind of Mulch Should I Use? by Donna Duffy

Gravel mulch at Kendrick Lake Gardens in Lakewood

[This blog was originally posted on March 18, 2014 and has been updated with new research-based information.]

The benefits of mulch are so well known that the question no longer is “Should I mulch?” but “What mulch is best for my landscape?” For die-hard gardeners, mulching is one garden task you can do any time of year – even in the winter.

A mulch is any material that provides protection and improves the soil when applied to the soil surface. Mulches can:

reduce surface evaporation
improve water penetration and air movement
control soil temperature fluctuations
protect shallow-rooted plants from freeze damage
improve soil structure and nutrient availability

Small wood chip mulch around roses

Your first decision will be deciding between organic and inorganic mulch. According to PlantTalk Colorado, the most common organic mulches in Colorado include wood chips, chunk bark, pole peelings, pine needles, lawn clippings and straw. Organic mulches gradually break down and add nutrients to the soil. Decomposition of fresh wood mulches can create nitrogen deficiencies, so you’ll need to be prepared to supplement the area with a fertilizer and replenish the mulch occasionally.

Gravel mulch at Kendrick Lake Gardens in Lakewood

Inorganic mulches are stone-based and include rock, cobblestone, pea gravel, lava rock and crushed rock. They store and radiate heat, so avoid putting large areas of unshaded rock next to your house.

Regardless which type of mulch you choose, resist the urge to install plastic sheeting or landscape fabric underneath the mulch. Air and moisture don't penetrate plastic or fabric, so plant roots won’t develop but weeds will still come through.

The selection of a mulch depends on its intended use. If appearance is the main goal, inorganic or inert mulches may be the best choice. If the main objective is soil improvement, consider an organic mulch that gradually breaks down. If the area is used primarily for annual flowers, it often is more practical to use a temporary organic mulch, such as composted leaves or grass clippings that can be turned under each fall.

Mulches used to enhance appearance and control weeds may be applied at any time. Apply most mulches to a depth of 3 to 4 inches. Straw, dried leaves and similar materials can be applied to a depth of 4 to 6 inches. Around woody plants, push the mulch away so it isn’t touching the woody bark.