Mulch is one of those jobs that is easy to put off, and then suddenly it is 35 degrees, the soil has gone hard and crusty, and everything looks tired. Knowing when to mulch garden beds is most of the battle, and getting the timing right makes the whole summer easier.
Here is the short answer: mulch before the weather changes, not after. Mulch works by holding conditions steady, so it does its best work when it goes down ahead of the heat rather than in the middle of it.
The best time to mulch: spring and early summer
If you only mulch once a year, do it now.
Through spring the soil is warm, there is usually still some moisture in it, and your plants are putting on new growth. A layer of mulch at this point locks that moisture in and keeps the soil cool underneath, so your plants are not fighting the heat when it arrives.
Leave it until January and you are mulching soil that is already dry and already hot. Mulch will hold whatever it finds underneath it, so it is much better to trap a cool, damp soil than a baked one.
For most gardens in Queensland and northern New South Wales, October and November is the window. Further south you have a little more time, but the principle is the same: get it down before the hot weather, not during it.
Mulching again in autumn
Autumn is the second good window, and it does a different job.
Mulch laid in autumn keeps winter weeds from getting started, because weed seeds need light to germinate and a decent layer of mulch blocks it. It also protects soil life through the cooler months and keeps beds looking tidy.
If you mulch twice a year, spring and autumn is the rhythm to aim for. If you only manage once, make it spring.
How thick should mulch be?
This is where most people go wrong, usually by being too careful with it.
- Lay it 5 to 7cm deep. That sounds like a lot when you are spreading it, and it will settle down to about half that within a few weeks.
- Thinner than 5cm and weeds will still find the light. A skinny layer looks neat but does not actually do the job you bought it for.
- Keep it clear of stems and trunks. Leave a gap of about a hand’s width around the base of each plant. Mulch piled against a trunk holds moisture right where you do not want it.
- Water the bed before you mulch, and again afterwards. Dry soil under fresh mulch stays dry, and a good soak afterwards settles everything into place.
A simple way to check you have enough: push your fingers into it. If you hit soil straight away, add more.
Which mulch for which job
Not all mulch is doing the same job, and matching it to the spot makes a real difference to how long it lasts and how it looks.
| Where you are mulching | What works best | Why |
|---|---|---|
| Vegie patch and herb beds | Sugar Cane Mulch or Pea Straw Mulch | Light, easy to spread around small plants, and breaks down quickly to feed the soil |
| Flower beds and around shrubs | Sugar Cane Mulch or Softscape Mulch | Soft, natural look that settles in well around established planting |
| Native gardens and ornamental beds | Tea Tree Mulch | Coarser and longer lasting, sits well in a natural style garden |
| Trees, shrubs and larger beds | Hardwood Mulch | Chunky and slow to break down, so you are not topping it up every season |
| Pathways and high traffic spots | Rustic Bark Nuggets or Landscape Mulch | Holds its shape underfoot and stays put |
| Feature beds and front gardens | Red Chip or Black Chip Mulch | Adds colour and definition, and keeps its look for a long time |
| Beds you are trying to reclaim from weeds | Weed Block Mulch | Designed to smother what is already there and stop new weeds coming through |
A lot of gardens end up using two: something light through the vegie patch and something chunkier out the front.
Will mulch take food away from my plants?
This one comes up a lot, usually as a worry about sugar cane mulch.
The short version is no, not in any way that matters to your plants. When mulch sits on top of the soil, the breaking down happens right at the surface where mulch meets soil, not down in the root zone where your plants are feeding. As it breaks down it is actually adding organic matter back in, which is why beds that get mulched every year end up with better soil than beds that do not.
If you want the longer explanation, we have written about what nitrogen drawdown actually is and when it is worth thinking about.
Worth knowing too: not all sugar cane mulch is the same when it comes to weed seeds. Ours goes through a dust extraction process under cover before it is bagged, which keeps it cleaner. Here is how that works.
What about mulching in a wet summer?
If you garden somewhere that gets a proper wet season, it can feel counterintuitive to add mulch. You are not short of water, so why bother?
Mulch is just as useful in the wet, for different reasons. It cushions the soil against heavy rain, which stops the surface compacting and washing away, and it slows runoff so more of that rain actually soaks in. More on why mulching matters in wet climates.
Get the kids on it
Mulching is one of the few garden jobs small people are genuinely good at. Bales of sugar cane mulch break apart easily by hand, it is light, and there is nothing to damage. Give them a bucket each and a bed to cover, and show them how to leave a gap around the stems.
It is a Saturday morning job, not a whole weekend. And it is the one thing you can do this month that your garden will thank you for right through until March.
