Mulch Calculator

Mulch is a layer of material applied to the surface of the soil. Mulches can be organic or synthetic, and they are used both for looks and for what they do for the soil: regulating its temperature, helping it hold moisture, and slowing erosion.

How much mulch do I need?

The amount depends on the area to cover and the depth of the layer. Generally, a 2 to 4 inches (5 to 10 cm) layer gives the benefits of mulch without being too thick. A layer spread too thinly will not suppress weeds or protect the soil against heat and erosion very well, while a layer spread too thickly can prevent water from reaching the soil and deprive plants of moisture.

The benefits of mulch

As an insulating layer, mulch keeps soil temperatures more moderate, which keeps roots healthier, and it shields the soil from sun and heavy rain that would otherwise dry it out or wash away topsoil. The same insulation slows evaporation, so the soil retains moisture longer and plants need watering less often. A mulch layer also blocks the sunlight that weed seeds need to sprout, and seeds blown in by the wind are kept from reaching the soil at all. For diseases that live in the soil and infect plants through the roots, mulch acts as a protective barrier.

There are limits: a layer that is too thick, or a material that absorbs too much water, can keep light rain from ever reaching the soil and reduce the oxygen below it. The right type and amount for the situation matter.

Types of mulch

Organic mulches decompose over time, feeding the soil with nutrients as they break down and attracting beneficial insects and worms. Shredded leaves are easy to collect and protect soil through the growing season, though they can mat if not shredded enough. Wood mulches - bark chips, wood chips, hardwood, pine, shavings - decompose at different rates and suit many different looks. Dried grass clippings return relatively high levels of nitrates to the soil, but fresh clippings are too wet and can mat and mold. Because organic mulches decompose, they need replenishing more often than synthetic ones, and some can attract pests or carry weed seeds.

Synthetic mulches such as plastic sheets, rubber, and dyed mulches do not biodegrade, so they last longer and need less maintenance - but they add no nutrients to the soil. Rubber mulch, made from recycled rubber, lets water pass straight through and dehydrates weed seeds before they reach the soil, though it can contaminate soil and is a fire hazard. Plastic mulch prevents compacted soil and holds heat and water well, but it can overheat soil in warm climates and can trap too much moisture. Dyed mulches are organic mulches colored with synthetic additives for appearance; early concerns about toxic substances have largely been addressed by later studies, but checking how a mulch was dyed is still worthwhile.