Why bare soil bleeds water
Bare soil is a radiator. Sun heats the surface, wind strips dry air across it, and capillary action pulls water up from deeper layers.
On a hot windy day, bare ground loses 6 to 10 litres per square metre. Over a summer week, that is 40 to 70 litres gone. None of it reaches a root.
Mulch breaks all three mechanisms at once.
How it works
A 10 to 15 cm layer of organic mulch drops surface temperature by 10 to 20 degrees Celsius. It insulates the soil, kills the wind at ground level, and traps a still air buffer above the crumb.
Field trials across climates show the same number. Evaporation falls 50 to 70 percent under good mulch. In arid trials with thick wood chips, the drop has hit 80 percent.
The practical payoff is fast. A garden that drinks 200 litres a week bare may only need 60 to 100 under mulch. Pair that with drip lines under the mulch and total demand falls 70 to 80 percent against bare, sprinkler-watered ground. That doubles the runtime of your rainwater tank.
What to use
Arborist wood chips. The gold standard for perennials, orchards, and food forests. A mixed load of wood, bark, and leaf shreds, 10 to 15 cm deep, suppresses weeds, holds moisture, and feeds the soil food web for 12 to 18 months. Tree crews often drop them free.
Straw. Light, easy to spread, perfect for annual beds. Apply 10 to 15 cm. Get straw, not hay. Hay carries seed and will reseed your bed into a weed nursery. Lucerne hay is the exception: nitrogen-rich, expensive, loved by no-dig gardeners.
Leaf litter. Free in autumn. Run the mower over the pile first. Whole leaves mat and shed rain. Shredded leaves break down in 6 to 12 months into crumbly leaf mould that lifts soil water storage for years.
Living mulch. A low ground cover under the main crop. Clover or other nitrogen fixers shade soil, feed it, and hold moisture year-round. Works best under perennials.
Get the depth right
Thin mulch is worse than none. A scatter that leaves soil showing heats up and re-radiates, baking the surface. Minimum useful depth is 7 to 8 cm. Aim for 10 to 15. Past 20 cm you waste material.
Mulch wet soil only. Lay it over dry ground and you lock the dryness in. Light rain will not penetrate. Water deeply first, or mulch the day after a real downpour, then spread thick over the damp surface.
Keep a 5 to 10 cm gap around every trunk and stem. The volcano mulch piled against bark holds wet against the cambium, invites fungal rot, and shelters borers. It kills trees slowly. Ring of bare soil at the crown. Mulch wide, not high.
For seedlings, hold the mulch back until the plant is tall enough to clear it, then draw it in.
What to skip
Sawdust and shredded paper compact into a mat that sheds water and ties up nitrogen as it rots. If you have a pile, compost it first or blend it with coarser material.
Rubber mulch is shredded tyres. It leaches zinc, adds no organic matter, and has no place in a food garden. Dyed mulch is usually pallet and construction scrap. Avoid both.
Gravel suppresses weeds but radiates heat into the root zone. Fine for cactus beds in true desert. Wrong choice for vegetables, fruit trees, or anything you plan to harvest.
See also
- Drip Irrigation for lines run under the mulch
- Soil Water Storage on building the tank in the ground
- No-Dig Gardening for the cultivation system mulch was built for
- Rainwater Harvesting Basics to pair tank capacity with reduced demand
- Watering Timing for when to apply the water mulch protects