Mulch does a lot of quiet work in a garden: it holds moisture, suppresses weeds, moderates soil temperature and slowly feeds the soil as it breaks down. But it only works well at the right depth. Too thin and weeds push through; too thick and roots can suffocate or the bark can rot. Here are sensible depths for common situations, and how to turn them into the volume you need to buy.
Recommended depths
- Flower and vegetable beds: 2 to 3 inches of a fine material such as shredded bark or compost.
- Shrubs and hedges: about 3 inches, kept a little clear of the stems.
- Trees: 2 to 4 inches spread in a wide ring, with a gap around the trunk. Piling mulch against the trunk, sometimes called a mulch volcano, invites rot and pests.
- Paths: 3 to 4 inches of a coarser chip, topped up yearly as it breaks down.
- Around acid-loving plants: pine bark or needles at 2 to 3 inches.
If a material is very fine, like compost, apply it thinner. If it is coarse, go a little deeper, because large pieces leave gaps.
Turn depth into volume
Volume in cubic feet equals area in square feet times depth in feet. Depth is usually given in inches, so divide by 12. A bed that is 20 feet long and 4 feet wide is 80 square feet. At 3 inches (0.25 feet), it needs 80 × 0.25 = 20 cubic feet.
Rings and circles
For a circular ring around a tree, calculate the area of the outer circle and subtract the circle you leave clear at the trunk. A ring with an outer radius of 3 feet and a clear radius of 0.5 feet has an area of π × (9 − 0.25) ≈ 27.5 square feet. At 3 inches deep, that is about 6.9 cubic feet.
Bags and bulk
Bagged mulch is commonly sold in bags of 2 cubic feet, though sizes vary, so read the label. Twenty cubic feet would need ten 2-cubic-foot bags. For large areas, bulk delivery by the cubic yard (27 cubic feet) is often cheaper. A 1,000-square-foot area at 3 inches deep needs 250 cubic feet, which is a little over 9 cubic yards.
Allow for settling
Fresh mulch settles and thins as it is watered and rained on. Expect to top up by an inch each year. For a first application, it is wise to order about 10 percent over to cover uneven ground and compaction.
Prepare the bed first
Pull weeds and water the soil before you spread. Lay cardboard over very weedy areas to smother them. Avoid plastic sheeting under mulch in beds with plants, because it prevents air and water reaching the roots. Edge the bed so mulch stays where you put it.
Choosing material
Shredded hardwood lasts longer and knits together well on slopes. Pine bark nuggets look tidy but can float away in heavy rain. Compost feeds the soil but breaks down quickly. Wood chips from a tree surgeon are often cheap or free, although they may include leaves and small branches. Avoid dyed mulch of unknown origin near edible crops.
Keep it from the house
Keep mulch a few inches away from foundations and wooden siding to discourage termites and damp.
Quick calculator use
When the bed shape is awkward, sketch it, split it into rectangles and circles and add the volumes. An online mulch cubic feet calculator can turn area and depth into cubic feet and bag counts, which is handy when comparing bulk and bagged prices.
Summary
Pick the depth that suits the plants, convert it to feet, multiply by the area, add a small allowance and keep mulch away from trunks and walls. The right depth keeps the benefits and avoids the problems.