Mulch Calculator

How much mulch do I need?

Mulch is sold by volume, so you need the area of your beds and the depth of the layer. Multiply area by depth, keeping the units consistent: 200 square feet at 2 inches (1/6 of a foot) deep is 33.3 cubic feet. Bulk mulch is usually priced by the cubic yard, which is 27 cubic feet, so that bed needs about 1.23 cubic yards; bagged mulch commonly comes in 2- or 3-cubic-foot bags, so you would need 17 of the smaller bags. A handy shortcut: one cubic yard covers about 324 square feet at 1 inch deep, 162 at 2 inches and 108 at 3 inches.

For irregular beds, split the area into rectangles and enter each one, or estimate a circle's area as 3.14 × radius². A 2- to 3-inch layer is right for most beds; use less over fine-textured mulch or heavy clay soil and up to 4 inches of coarse material on paths. If old mulch is still in place, only top it up to the target depth.

The benefits of mulch

Mulch is any layer of material spread over the soil surface. In nature, fallen leaves and needles do the same job on a forest floor. In a garden, a good layer of mulch saves water, cuts down on weeding, protects plant roots and gives beds a tidy, finished look. Organic mulches also feed the soil as they break down.

Mulch regulates soil temperature and protects against weather

A layer of mulch insulates the ground. In summer it shades the soil and keeps roots cooler, and in winter it slows freezing and thawing, which can heave young plants and perennials out of the ground. Mulch also breaks the force of heavy rain, reducing soil crusting and erosion on slopes, and keeps soil from splashing onto leaves.

Apply winter protection after the ground has frozen in cold climates, and pull mulch back a little from crowns in spring so the soil can warm up.

Mulch can increase moisture retention

Bare soil loses water quickly to sun and wind. Mulch slows evaporation, so the soil stays evenly moist for longer and plants need watering less often, an important saving in dry climates and during summer restrictions. Water soaks through organic mulch into the soil while the surface stays drier, which also discourages some weeds. Water the soil well before mulching, because a thick layer can also keep light rain from reaching dry ground.

Mulch reduces weed growth and the possibility of plant diseases

Most weed seeds need light to germinate, and a mulch layer of 2 to 3 inches blocks it. Weeds that do sprout in loose mulch are easy to pull. Mulch also keeps soil, and the fungal spores in it, from splashing onto lower leaves during rain or watering, which helps reduce diseases such as early blight on tomatoes.

Avoid piling mulch against tree trunks and plant stems. These "mulch volcanoes" hold moisture against the bark, inviting rot, insects and rodents, and can lead to girdling roots. Leave a few inches of clear space around stems and trunks, and keep the layer no deeper than about 3 to 4 inches so that roots still get air.

Types of mulch

Mulches fall into two broad groups. Organic mulches come from plant material and decompose over time, improving soil structure and adding nutrients but needing to be renewed. Inorganic, or synthetic, mulches such as stone, rubber and plastic last much longer but do nothing for the soil. The best choice depends on the plants, the look you want, your budget and how often you are willing to replenish it.

Organic mulches

Shredded hardwood and bark are the most common landscape mulches. They knit together, stay in place on slopes and break down slowly over one to two years. Bark nuggets last longer but can float away in heavy rain. Wood chips, often free from tree services, are excellent around trees and shrubs and on paths. As fresh wood decomposes on the surface it can tie up a little nitrogen at the soil surface, but research shows this rarely harms established plants, so there is no need to avoid it except right next to seedlings.

Pine straw (fallen pine needles) is light, does not wash away easily and is popular in the southern US. Despite its reputation, it does not make soil noticeably more acidic. Shredded leaves are free each autumn and make a fine, nutrient-rich mulch; shred them first so they do not mat. Compost improves the soil quickly but breaks down fast and can carry weed seeds unless well made. Straw is a classic for vegetable gardens and strawberries, but choose straw rather than hay, which is full of seeds. Grass clippings work in thin layers once dry; thick fresh layers turn slimy and smell.

Dyed mulches are usually made from recycled wood and colored with iron oxide or carbon-based dyes, which are generally considered safe, but check the source to avoid treated or painted lumber. Cocoa hulls smell pleasant but contain theobromine and can be toxic to dogs that eat them.

Synthetic mulches

Gravel, river rock and crushed stone never decompose and suit dry gardens, succulents, paths and areas near buildings where fire risk or termites are a concern. They absorb and radiate heat, which can stress plants in hot climates, and they are heavy and hard to remove later. Lava rock is lighter and porous. Rubber mulch, made from recycled tires, is long lasting and cushions falls, so it is common under playground equipment, but it adds nothing to the soil and burns readily.

Black plastic sheeting warms soil in spring and smothers weeds in vegetable beds, though it stops water and air unless it is perforated. Landscape fabric lets water through and is often laid beneath stone; under organic mulch it tends to fail as weeds root in the decomposing layer on top. Biodegradable films made from plant starches are an alternative for annual crops.

Frequently asked questions