Mulching: How Much and What Kind

Mulch is the cheapest thing you can do for a newly planted tree and the easiest thing to get wrong. Too thin and it does nothing. Too thick and it starves the roots of air. Piled against the trunk, it rots the bark and kills a tree that cost you a season of growth, and it does it slowly enough that nobody connects cause to effect.

The number that matters: 75 to 100mm

For coarse organic mulch over the root zone of a tree or shrub, 75 to 100mm is the working depth. Below about 50mm you get patchy weed suppression and the soil surface still dries out. Above about 100mm you start restricting gas exchange, and roots need oxygen just as much as they need water.

Fine materials need less, not more. Sugar cane, lucerne and finely shredded mulch should sit at 50 to 75mm because they compact and knit into a mat. Coarse arborist chip or chunky composted hardwood can run to the full 100mm because the air spaces stay open.

Measure it. Push a stick in and check. Almost every garden we visit that says it mulches annually is carrying 200mm or more in places, because nobody ever removes the old layer before adding the new one.

Why mulch against the trunk kills trees

Bark at the base of a trunk is built for air, not for a permanent damp compost jacket. Bury it and the bark stays wet, softens, and becomes an open door for collar rot. At the same time the tree responds to being buried by pushing adventitious roots out into the mulch layer, which then circle the trunk and eventually strangle it. The tree looks fine for three or four years and then declines with no obvious cause.

The fix takes ten seconds. Keep mulch 100 to 150mm clear of the trunk on a young tree and 200mm clear on a large advanced specimen. You should be able to see the root flare, the point where the trunk widens and the first structural roots leave it. If you cannot see the flare, the tree is planted too deep or mulched too high, and both are worth correcting now rather than later. Our guide on planting a tree properly covers getting that flare at the right level in the first place.

How wide the mulch ring needs to be

Width does more good than depth. Feeder roots sit in the top 200 to 300mm of soil and spread well beyond the canopy edge, so the bigger the mulched area, the more of the root system is protected from heat, drying and mower damage.

• For a newly planted advanced tree, mulch a ring at least 1.5m across, and 2m is better.
• Extend the ring out towards the dripline as the canopy grows.
• On a hedge or a garden bed, mulch the whole bed as one continuous area rather than making rings around individual plants.
• A mulched ring also keeps the whipper snipper away from the trunk, which is a leading cause of death in young street and garden trees.

Grass competing at the base of a young tree measurably slows it down. Killing or removing turf across the mulch ring rather than just mulching over it makes a visible difference in the first two years.

Which mulch for which job

• Composted hardwood chip. The default for trees. Chunky, stays put on a slope, breaks down over 18 to 24 months and feeds the soil as it goes. Buy composted, not fresh, so it has already passed its heating stage.
• Arborist woodchip. Mixed leaf, wood and bark straight from a chipper. Excellent value, superb soil biology, but variable. Let it sit and heat for a few weeks before it goes near plants, and keep it out of formal front gardens where the look matters.
• Pine bark. Neat, slow to break down, slightly acidic. Good under camellias, gardenias, magnolias and blueberries. The fine grade floats in heavy rain, so use 10 to 20mm nuggets on any slope.
• Sugar cane and lucerne. Fast breakdown, high nutrient, excellent for vegetable beds and for kick starting poor soil. Around trees they disappear in about three to six months and need constant topping up, so treat them as a soil conditioner rather than a mulch.
• Tea tree and forest blend mulches. Attractive, mid rate breakdown, good all rounders for garden beds where appearance matters.
• Pebble, gravel and scoria. Permanent and clean looking, and the right call around cycads, agaves, succulents and architectural plantings that hate a damp collar. It adds no organic matter, and it stores and re radiates heat, so keep it away from anything that suffers in reflected warmth.

Whichever you choose, buy from a supplier who screens for contamination. Mulch made from treated timber or from unscreened green waste can carry herbicide residue that stunts everything you own.

Nitrogen drawdown: the real version

The old warning that mulch robs the soil of nitrogen is half true. Microbes decomposing raw carbon do draw nitrogen, but that happens in the thin zone where mulch meets soil, not through the root zone. Coarse mulch on the surface has almost no effect on an established tree.

It does matter in two situations. Fresh, fine, uncomposted material dug into the soil will lock up nitrogen badly, so never incorporate mulch into a planting hole. And on shallow rooted annuals and new small plants, a fresh sawdust heavy mulch can cause visible yellowing. A light application of a balanced fertiliser before mulching removes the issue entirely.

Things to leave out of the design

Weed mat under mulch is a long term problem sold as a short term fix. Within two or three years the mulch above it has broken down into a perfect weed seedbed, roots have grown into the fabric, and the whole lot has to be cut out by hand. Skip it and mulch a little deeper instead.

Plastic sheeting is worse, because it stops water and air reaching the root zone. Old carpet leaches dyes and glues. Volcano shaped mulch cones around trunks, the look copied from bad commercial landscaping, are simply the trunk burial problem in a decorative shape.

Two practical cautions. Keep organic mulch at least 300mm clear of house walls and subfloor vents, because it is a bridge for termites and a fuel load close to the building. In bushfire prone areas, use gravel or a low flammability mulch within the first few metres of the house and save the woodchip for further out.

Topping up without burying the tree

Organic mulch is meant to disappear, that is the point. Expect to top up once a year, usually in early spring before the soil heats, or in autumn if your summers are brutal. Before you order more, rake back what is there and measure. If you are already sitting at 100mm, do nothing but pull it back off the trunks.

If the mulch has crusted and water is running off rather than soaking in, break the crust with a rake and water through a wetting agent. Hydrophobic mulch over dry soil is one of the most common reasons a well watered tree still wilts. Our guide to watering a newly planted advanced tree goes into how to check whether the water is actually reaching the rootball.

Getting it right from day one

The best time to mulch correctly is the day the tree goes in, while the soil is loose, the level of the root flare is obvious and there is no established turf in the way. Every tree we supply is delivered and planted by our team, and the mulch ring is formed as part of that work, at the right depth and pulled back off the trunk. You can read what that day looks like on our supplied and installed page.

Mulch matters most on the plants with the most to lose, which is any advanced specimen carrying real size at planting. Browse what that looks like in our tree range.

Set the depth at 75 to 100mm, pull it back off the trunk so you can see the flare, make the ring as wide as you can stand, and check it once a year with a stick rather than a guess. That is the whole job.