How deep to lay mulch, and the mistakes that undo it

The depth is the decision

Mulch does its work by covering soil, so almost everything that goes wrong with it is a question of how much went on and where it ended up. Get the depth right and it holds moisture, keeps weed seed in the dark, feeds the soil as it breaks down and keeps the surface temperature steady. Get it wrong and it does the opposite of two of those.

The number for most organic mulches is 5cm to 7cm, settled, not fluffed up out of the bale.

Too thin does nothing

Under about 3cm, light still reaches the soil, so weed seed germinates through it and the moisture benefit is small. A thin scatter looks tidy on the day and that's about all it does.

This is the commonest fault by a mile, because a bale looks like a lot of material until you've spread it and discovered it covers about three square metres properly. It's worth measuring the bed and buying enough to do it once rather than spreading what you have over twice the area.

Too thick works against you

Past about 10cm of a fine mulch you get the fault nobody warns about. The layer mats down, dries out and becomes water repellent, so light rain and a short water both land on top, wet the mulch and evaporate without ever reaching soil. The bed underneath gets drier while looking beautifully mulched.

A thick layer also keeps the soil surface too wet in a wet winter, which is how roots rot in heavy ground, and it can stop the soil warming in spring so everything is slow to get going.

If you already have a matted layer, break it up with a fork and rake it back to depth rather than adding more on top. Adding more is the instinct and it's the wrong move.

Keep it off the stems

Mulch piled against a trunk or a stem holds moisture against bark that isn't built to stay wet, and collar rot and borers both follow. It also gives slugs and snails somewhere to live within reach of exactly the tissue they want.

Leave a clear ring of about 10cm around annuals and more like 15cm around a tree or a shrub. Mulch the root zone, not the plant.

Water the soil first

Mulch holds the situation it finds. Lay it over dry soil and you've sealed dry soil in, and the first few waterings will struggle to get through.

Give the bed a proper soak first, lay the mulch, then water again over the top to settle it. If the soil is genuinely water repellent already, a soil wetter before mulching is worth the extra step.

The nitrogen question, and where it's real

Coarse woody mulch, so bark and chip, needs nitrogen to break down, and the microbes doing that work will take it from the soil surface if it isn't in the mulch. That's real, and it's why a woody mulch can leave plants looking pale and hungry.

It's also overstated. It only happens in the thin zone where mulch meets soil, so it matters for shallow rooted annuals and hardly at all for an established shrub. The two ways round it are to keep woody mulch for ornamental beds and trees, and to use a straw or a sugar cane in the veggie patch where the roots are shallow and the mulch breaks down faster anyway. A feed when you lay it covers the rest.

Which mulch, in one line each

• Sugar cane. Light, breaks down quickly, ideal for veggie beds and anything shallow rooted.
• Pea straw. Similar, with more nitrogen in it, so it feeds as it goes. Can carry some seed.
• Lucerne. The richest of the straws and the best soil builder, and priced accordingly.
• Pine bark and chip. Long lasting and tidy, for shrubs, trees and paths rather than the veggie patch.
• Composted mulches. A middle road that breaks down at a moderate rate and is easy to spread.

Top it up rather than replace it

Organic mulch is meant to disappear, because that's it turning into soil. Straw mulches want topping up once or twice a year, bark holds for longer. Rake back what's left, check the depth, add enough to get back to 5cm to 7cm, and keep it off the stems while you're in there.

Shop related products

• Martins Organic Sugar Cane Bale 11kg. The default for a veggie bed.
• Neutrog Whoflungdung Mulch 20kg. A composted mulch that feeds as it breaks down.
• Easy Wetta Garden Mulch 50L. Bagged, for when a bale is more than the job needs.
• Brunnings Garden Compost with Seaweed 25L. To go under the mulch rather than on top of it.

Did you know we also keep pea straw, lucerne, pine bark and composted mulches in the shop by the bale and the bag, and those don't all make it onto the website. Give us a ring on (08) 8536 4333 to check what's on the shelf, and the rest of the garden centre range is online.

Read next

• Which mulch to use in a vegetable patch. Choosing the material, where this one is about laying it.
• How to check soil moisture before you water. How to tell whether the mulch is doing its job.
• The role of organic matter in soil health. Where mulch ends up once it has broken down.

I always buy one bale fewer than the bed needs, every single time, and then talk myself into spreading it thin.

Cheers,
Candeece

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