How much mulch you need, and when to spread it

Mulch is the cheapest improvement you can make to a garden bed. It holds moisture, keeps weeds down, protects roots from heat and cold and tidies up the whole yard. It is also easy to get wrong. These are the numbers and habits we use on our own jobs.

How deep should it be?

Aim for 5 to 7 cm. Less than that and weeds push through. More than that and water struggles to get through to the roots, and the bed can stay soggy and smell sour. If you already have old mulch, rake it up to loosen it and top up only to the right depth rather than adding a fresh thick layer on top.

Working out how much to buy

Mulch is sold by the cubic metre by landscape suppliers, and in bags at garden centres. A quick rule: one cubic metre covers about 14 square metres at 7 cm deep, or about 20 square metres at 5 cm.

Measure the length and width of each bed, multiply to get square metres, and divide by 14 for a 7 cm layer. Round up, because it always settles.

Crew tip

measure beds in sections if they are curved. Treat each curve as a rough rectangle and add them up. It is close enough.

Which type to choose

Shredded hardwood or cedar bark is the usual choice for beds around the house. It knits together, so it stays put on a slope and in a heavy rain, and it breaks down slowly. Fine compost is better under vegetables and annuals because it feeds the soil.

Avoid dyed mulch around plants you care about, and skip landscape fabric under mulch in planting beds. It looks neat for a year or two, then the mulch breaks down on top of it and weeds root in the new layer, which makes them harder to pull.

When to spread it

There are two good windows.

• Late spring, once the soil has warmed up and the perennials are up. Mulching too early keeps the soil cold and slows growth.
• Late fall, after the first hard frost, to protect roots through winter and to hold in moisture.

If you only do one, pick spring. It does the most for weed control through summer.

“Mulch should look like a donut, not a volcano.”

The two mistakes we see most

Volcano mulching. Piling mulch against the trunk of a tree holds moisture against the bark, which invites rot and pests. Keep mulch 10 cm back from the trunk and spread it wide, out to the edge of the branches if you can.

Layering on top of layers. Adding a fresh 5 cm every spring without raking the old layer builds up a thick mat that sheds water. Rake and loosen what is there first.

Prepare the bed first

Mulch on top of weeds just gives the weeds a head start. Pull or cut them out first, edge the bed with a spade so the mulch stays in place, and water the soil if it is dry. Then spread the mulch and leave the crowns of perennials uncovered.

Beds that need extra care

Brampton clay can stay wet, so use a slightly thinner layer and keep it loose. Hot, open Mississauga front yards benefit from a full 7 cm to hold moisture. Shady Toronto lots with big trees need less mulch but more attention to the root zones, where you should avoid piling anything deep.

Keeping it fresh through the summer

Mulch fades and thins as the season goes on. In midsummer, fluff it with a rake to break up any crust and let water through. If bare spots show, top them up with a thin layer rather than starting again. Pull weeds when they are small, ideally after rain when the roots slip out easily.

A word on soil

Mulch only does its best work over healthy soil. If your beds are packed hard, fork them over before you spread anything, and work in a couple of centimetres of compost. The mulch then keeps that good soil moist and cool through the summer instead of just covering a problem.

Doing it yourself, or not

A single cubic metre of mulch is heavier than it looks, and spreading it is mostly wheelbarrow work. If you have more than a few beds, we are happy to do it. Our Estate care plan includes bed weeding and mulching, and our landscape design team can add new beds or reshape tired ones. Send us a note on the contact page and we will arrange a visit.