Roughly 75 millimetres once it has settled is the working depth most established garden beds are mulched to — deep enough to suppress weeds and even out soil moisture, without smothering the soil or the plants growing in it. Mulch on a maintained garden gets redone on a cycle for the same reason it works in the first place: it is meant to break down. A bed that never needs topping up usually means the mulch was never doing much.
Why the same bed needs mulch again
Organic mulch is not inert. As it breaks down it feeds soil organisms and adds structure to the soil underneath, which is most of the reason it is used on a planted bed rather than gravel or pebbles. That breakdown is also why the layer thins out over time and needs replacing — a mulch that lasted forever would not be improving the soil beneath it. How often depends heavily on what it is made from: softer materials such as sugar cane, pea straw or lucerne mulch typically need topping up every six to twelve months, wood chip generally holds for two to three years, and coarser bark chip can last three to five. Inorganic mulch such as pebbles or gravel needs almost no replacing, but it also does none of the soil-building work organic mulch does, which is why the two are usually chosen for different parts of a garden rather than treated as interchangeable.
A bed that has thinned back down to bare soil, or where weeds have started coming back through, is the practical sign a top-up is due — not a fixed date on the calendar. Redoing mulch is usually a top-up back to the working depth rather than stripping the bed and starting again, since the partially broken-down material underneath is still doing useful work as it finishes decomposing. On a bed that has been established for years, mulch is usually only part of what needs revisiting — see looking after mature planting in Somerville and Baxter for the soil and pruning side of the same long-term maintenance question.
Placement matters more than depth
Depth gets most of the attention, but where the mulch sits is what actually protects the plants. Mulch piled up against a stem or trunk — the “volcano” shape often seen where mulch has simply been tipped and spread outward — traps moisture against the bark rather than around the root zone. That trapped moisture is what leads to collar rot and other fungal problems, not the mulch itself. A gap of roughly 50 to 100 millimetres kept clear around the base of each plant, so the mulch forms a flat layer rather than a mound against the stem, avoids the problem without giving up any of the benefit further out.
The same logic applies at the edges of a bed. Mulch that regularly washes or gets kicked onto a path, lawn or driveway is usually a sign the bed needs a defined edge rather than a bed that simply needs less mulch — an edge holds the material where it is meant to be and cuts down how often it needs sweeping or topping up along that boundary. Mulch also needs to stay clear of air vents, weep holes and irrigation risers, all of which are easy to bury without noticing during a routine top-up.
Mulch and termites
Mulch does not draw termites in from open ground the way it is sometimes assumed to. What it does is make the ground underneath more comfortable for termites that are already active nearby, by keeping the soil moist and sheltered from the sun. Keeping a clear, mulch-free strip along the house’s foundation line — rather than running garden mulch right up against the wall — is a sensible precaution for that reason, separate from the collar-rot argument for keeping it off plant stems.
Mulch, bushfire risk and where it sits near the house
For gardens in bushfire-prone parts of the service area, organic mulch is a fuel source, and the CFA’s guidance on landscaping for bushfire recommends keeping it away from windows, decks and eaves rather than banked up against the house. Non-combustible alternatives such as pebbles or gravel are the safer choice in that zone immediately around a dwelling, with organic mulch kept for beds set further out. This matters more on the larger, semi-rural blocks common through Tyabb and Pearcedale than on a small suburban courtyard bed, but it is worth checking against a property’s own bushfire rating rather than assumed either way.
Organic or inorganic, and what changes the choice
Organic mulch — bark, wood chip, sugar cane, pea straw — suits most planted beds because it improves the soil as it breaks down and supports the plants growing in it. Inorganic mulch such as pebbles or decorative gravel lasts far longer and needs almost no topping up, which makes it a reasonable choice for paths, slopes, driveway edges or drought-tolerant native plantings where soil improvement is not the priority. Salt-laden coastal wind around Somers and Balnarring can dry organic mulch out faster and disperse lighter material, which is one of the practical reasons a heavier bark or a partly inorganic edge sometimes suits an exposed bed better than a fine, light mulch.
Neither option is inherently correct — the right choice depends on the bed, the exposure and how much ongoing soil improvement the planting actually needs. That is part of what planting and mulching work is scoped around on a first visit, rather than a single standard mulch being applied everywhere on a property regardless of what each bed is doing.
What to include in a quote request
A useful mulch quote covers the approximate area of the beds involved, whether they currently have mulch that just needs topping up or are starting from bare soil, and whether there is a preference for organic or inorganic material. Mentioning any beds close to the house, near a fence line shared with a neighbour, or in a part of the property with known bushfire exposure helps scope the job properly from the outset. Photos help with an initial conversation, though a site visit is still the most reliable way to judge existing depth, drainage and how far material has spread from where it was originally placed. Quotes are free, and a site visit is required for every new customer.
Get in touch through the contact page with those details, and Brian can talk through what depth, material and placement suit the beds on the property.
Quick answers
How deep should mulch be on an established garden bed?
Roughly 75 millimetres once settled is a workable standard for most garden beds, deep enough to suppress weeds and hold moisture without smothering the soil beneath it. Fresh mulch compacts over the following weeks, so a slightly deeper initial layer accounts for that settling.
How close can mulch sit to a plant stem or tree trunk?
Not against it. A clear gap of roughly 50 to 100 millimetres around the base of a stem or trunk, rather than mulch piled up against it, avoids trapping the moisture that leads to collar rot and other fungal problems.
How often does mulch on a maintained garden need redoing?
It depends on the mulch type. Softer organic mulches such as sugar cane or pea straw typically need topping up every six to twelve months, wood chip generally lasts two to three years, and coarser bark can last three to five. An established bed rarely needs the full depth replaced at once — usually a top-up back to the working depth is enough.
Does mulch attract termites?
Mulch does not draw termites in from a distance, but it can make conditions more comfortable for termites already active nearby by keeping the ground moist and sheltered. Keeping mulch clear of the house's foundation line reduces that risk rather than removing it entirely.
Is organic or inorganic mulch better for a garden bed?
Organic mulch such as bark or wood chip breaks down and improves the soil as it goes, which suits most planted garden beds, while inorganic mulch such as pebbles lasts far longer and suits paths, slopes or drought-tolerant plantings where soil improvement matters less.