By SF Gate Contributor Updated May 20, 2021
Mulch holds moisture in the soil while suppressing weed growth, but too much or too little mulch often has negative effects. Excess mulch can hold too much moisture, making the soil waterlogged, and encourage harmful diseases in the plants. Too little mulch isn’t effective in preventing weeds. Learning to apply the appropriate amount of mulch keeps your plants healthy and makes planting beds aesthetically pleasing.
While there is no set thickness of mulch in a planting bed, a depth of 3 inches is generally a safe choice.
General Mulching Recommendations
The depth of mulch generally ranges from less than 1 inch to 4 inches deep, depending on the type of mulching material used, according to the Missouri Botanical Garden. The general recommendation is about 3 inches of mulch in a planting bed. Many materials like wood chips settle so add an extra 1/2 to 1 inch of the material to account for the decrease in volume. For mulch made of grass clippings, decrease the depth to under 1 inch. When using hay or straw, use a layer 4 to 6 inches deep to account for the loose nature of the material, which compacts easily.
Adding More Mulch
Over time, organic mulching materials decompose, leaving you with a thinner layer of mulch, reports the University of Florida IFAS Gardening Solutions. Most materials settle even if they don’t break down into the soil. If you need to add more mulch to the planting bed, dig down to determine the thickness of the existing mulch. Add enough new mulching material to get back to the recommended depth.
In some cases, you may need to remove old mulch first. For example, mulch sometimes harbors disease pathogens, so removing the old material reduces the risk of future plant damage. Treat these situations as you did the initial application. Mulch the bed to the recommended depth once the old mulch is removed.
A Layer of Protective Mulch
In areas with cold winters or some freezing, cover plants and bulbs with a layer of protective mulch. The winter mulch also protects the soil from rain in areas that don’t freeze to prevent lost nutrients. A layer of mulch about 3 inches deep to cover the plants is ideal for seasonal protective mulching. Seasonal straw mulch can be up to 6 inches deep to provide the most protection. In the spring, remove the protective mulch layer about two weeks before you plant .
Warnings to Heed
Deep mulch protects the soil, but it can damage the stems or trunks of plants and trees growing in the planting bed. Thin the layer of mulch in the areas immediately surrounding plants or trees. Avoid letting the mulch actually touch the stems or trunks. A mulching material piled up against a stem or trunk can cause rot, which seriously harms or even kills the plant. The border around the planting bed needs to stand taller than the soil line to accommodate the desired thickness of mulch. If the mulch extends above the border, it will wash or blow away. [external_footer]