Mulch is the cheapest thing you can do for a garden before a Melbourne summer. A good layer means less watering, keeps clay from baking hard and smothers most weed seed. It is also easy to get wrong.
How deep, and how much to order
Aim for 50 to 75 millimetres of coarse mulch. Thinner than that and light reaches the weeds. Deeper than 100 millimetres and light rain never reaches the soil. To work out an order, measure the bed in square metres and multiply by the depth in metres. A bed 10 metres long and 2 metres wide at 75 millimetres needs 1.5 cubic metres. As a rule of thumb, one cubic metre covers about 13 square metres at that depth. Bulk mulch delivered loose usually costs $70 to $110 a cubic metre, which is far cheaper than bags once you need more than half a metre.
Which kind
- Coarse eucalyptus or arborist chip: our pick for native and shrub beds, lasts two to three years
- Pea straw or lucerne: for roses and vegetables, feeds the soil, gone in a season
- Fine composted mulch: looks neat, but can crust and shed water if laid thick
- Pebbles or gravel: for succulents and narrow strips beside paths
Fresh chip from a tree crew is fine on top of the soil. Do not dig it in, because it borrows nitrogen while it rots.
The mistakes we see most
Volcano mulching is the big one: mulch heaped against a trunk like a cone. It keeps the bark wet and invites collar rot. Pull mulch back so there is a clear ring of 50 to 100 millimetres around every trunk and stem.
The second is mulch piled against the house. Keep it below the damp-proof course and clear of the weep holes in the brickwork, and leave a gap along the wall so a termite inspector can see the slab edge.
The third is mulching dry ground. Soak the bed first, or wait until after decent rain. Otherwise you are sealing the dryness in. And skip the weed mat under organic mulch. Roots grow into it, water runs off it, and in three years it is a mess to pull out.
When to top up
October and November are the months for it here, after the soil has warmed and while it is still moist. Rake the old layer to break any crust, then add enough to bring it back to depth. If you would prefer not to shovel two cubic metres off the nature strip, we do it as part of a maintenance visit.