The depth rule
Ask how much mulch you need and the first answer is a depth. As a rule, two to three inches of settled mulch over the soil is enough to hold moisture, keep the soil temperature even and shade out most weed seed.
Less than two and the weeds come through. More than three or four and the mulch starts working against the plants: water from a light rain soaks into the mulch and never reaches the roots, and the layer underneath stays wet, sours and mats.
The mistake is treating mulch as an annual top-up. A bed that gets three fresh inches every April for five years is not a three-inch bed. Hardwood mulch breaks down slowly, maybe an inch or so a year in a sunny bed, less in shade, so most of last year’s layer is still there. Push a trowel into the bed before ordering. If you hit an inch of old mulch, you need one or two more, not three.
The volume math, in plain words
Mulch is sold by the cubic yard, and a cubic yard is 27 cubic feet. Spread one inch deep, a yard covers 324 square feet. That is where the formula comes from: measure the bed in feet, multiply length by width, multiply by the depth you want in inches, and divide by 324. The answer is cubic yards. For bags, convert the other way: multiply cubic yards by 27 to get cubic feet.
So one yard covers about 108 square feet at three inches, or about 162 square feet at two inches. A bed 30 feet long and 6 feet wide is 180 square feet; at two inches that is 180 times 2 divided by 324, a little over one yard.
Curved and irregular beds can be broken into rough rectangles and added up. Round up a little, because mulch settles after the first rain and the edges always take more than you think. Bagged mulch is usually two or three cubic feet per bag, so a yard is roughly nine to fourteen bags.
Length times width times depth in inches, divided by 324. Round up a little for settling and edges.
| Bed size | Square feet | At 2 inches | At 3 inches |
|---|---|---|---|
| 30 × 6 ft | 180 | 1.1 cu yd | 1.7 cu yd |
| 20 × 10 ft | 200 | 1.2 cu yd | 1.9 cu yd |
| 40 × 4 ft | 160 | 1.0 cu yd | 1.5 cu yd |
| 10 × 10 ft | 100 | 0.6 cu yd | 0.9 cu yd |
Mulching a bed properly, in order
Clean the bed
Pull weeds by the root and rake out leaves and debris so the new layer is not sitting on next year’s problems.
Check what is already there
Push a trowel down to the soil and measure the old layer. Rake out matted mulch and pull built-up beds back to grade.
Cut the edge
Spade a clean vertical edge a few inches deep along the lawn and throw the soil back into the bed.
Measure and order
Length times width in feet, times the inches you need to bring the total to two or three, divided by 324, or use the calculator above. Round up slightly.
Spread evenly
Lay the mulch to an even depth across the bed, slightly lower at the edge cut so it sits in the trench rather than spilling onto the lawn.
Pull it back
Clear a ring of bare soil around every trunk and stem so the root flare shows, and keep a gap below the siding.
Water it in
A light watering settles the layer and keeps fine mulch from blowing before the first rain.
Keep it off trunks, stems and siding
A mulch volcano is the cone piled up against the base of a tree. It looks tidy and it slowly kills the tree. Bark is not built to stay wet. Mulch held against the trunk keeps the bark damp, invites rot and boring insects, gives voles a sheltered place to chew, and encourages roots to grow up into the mulch and circle the trunk.
Pull mulch back so the root flare, where the trunk widens into the roots, is visible, and leave a few inches of bare soil around it. The same applies to shrub stems and perennial crowns.
Against the house, keep mulch a few inches below the bottom of the siding and away from any wood sill. Wet mulch against siding holds moisture where you do not want it and gives termites and ants cover to reach the structure without crossing open ground. The soil and mulch should slope away from the foundation, not toward it. Beds that have been topped up for years often have lost that slope, and water now runs back toward the wall.
When to remove old mulch instead of adding
Remove old mulch before you add any when:
- The layer is already three inches or more.
- The old mulch has matted into a crust that sheds water.
- There is white fungal growth through it.
- The beds have risen so much that the soil line is creeping up trunks and siding.
Older Main Line beds that have been mulched every spring for decades often sit several inches higher than they were built, and the shrubs in them show it: shallow roots, thin growth, plants that wilt in a dry week.
Rake out what is matted, fluff or work in what is broken down, and pull the beds back to grade where they have built up. Old, well-rotted mulch is good organic matter and can be worked into the soil rather than hauled away. Then add only what brings the total back to two to three inches, and keep the new layer thinner near trunks, stems and the foundation.
Spring or fall, and the edge first
Spring is the usual time, once the soil has warmed and dried a little, typically April in southeastern Pennsylvania. Mulching too early, over cold wet soil, holds the cold in and slows the start of root growth. A spring layer is in place before the weed seed germinates and before summer dries the bed out.
Fall mulching works too, after the leaves are down and cleaned out, and it protects the crowns of newly planted perennials and shrubs through freeze-thaw. What does not work is mulching over a bed full of leaves and weeds: you are just feeding the next layer of problems.
Cut the edge before any mulch goes down. A spade edge is a clean vertical cut a few inches deep between lawn and bed, with the soil thrown back into the bed so the edge forms a shallow trench. It keeps grass from creeping in, holds the mulch where it belongs and gives the bed a line. Re-cutting it each spring is most of what makes a bed look finished.
Which mulch, and what goes under it
The common mulches behave differently:
- Double-shredded hardwood is the standard here for good reason: it knits together, stays put on a slope, breaks down at a useful rate and feeds the soil as it goes.
- Dyed mulch is often made from recycled wood, holds its color for a season and adds less to the soil; if you use it, ask what wood it is made from.
- Pine bark nuggets last longer and look clean but float and wash out on slopes and in heavy rain.
- Leaf compost, or leaf mulch, is the best thing for the soil and the shortest-lived on top of it, which suits perennial beds better than shrub borders.
Weed barrier fabric under mulch usually backfires in planted beds. Mulch breaks down on top of the fabric into a thin layer of soil, weed seed blows in and roots right into it, and the weeds then grow through the fabric and anchor in both layers, which makes them harder to pull than if the fabric were not there.
Fabric also blocks the organic matter from reaching the soil and gets in the way every time a plant goes in or comes out. Fabric has a place under stone and gravel paths.