Black mould on driveways and paths in Cairns: why it grows and how to remove it safely

Falls are common in later life. Healthdirect, the Australian Government's health information service, states that one in four people aged 65 and over have at least one fall each year. Its prevention advice for the home includes a line that reads like a job list for a Cairns yard: remove moss, slime or fallen leaves from outdoor paths.

The growth on concrete matters most where people walk every day in all weather. When you look at a parent's home, or your own, check these spots first:

• The path from the driveway or carport to the front door, which gets walked in the rain with shopping in hand.
• Back steps and landings, which are often shaded and rarely in full sun.
• The path to the clothesline and the bins, used several times a week.
• Any area under trees or beside a fence, where leaves hold moisture against the concrete.

A clean path is one of the quickest safety fixes around an older person's home, and one of the easiest to arrange for a parent who is finding the yard harder to manage. For the funded and private options, see our guide to lawn and garden help for older Cairns residents.

What most people call black mould on concrete is living growth, not dirt. Concrete is porous, so it holds water after rain, and anything that stays damp and shaded gives that growth a place to settle. Cairns supplies the damp in large amounts.

The Bureau of Meteorology's long-term figures for Cairns Aero show a mean annual rainfall of 2,025.5 mm, most of it between December and April: 404.8 mm in January, 437.5 mm in February and 422.1 mm in March, with around 16 rain days in each of those three months. Mean 9am relative humidity sits between 75 and 78 per cent from January to May, and the January mean maximum is 31.5 degrees. Warm, wet and humid for months at a time is the ideal setting for growth on any surface that does not dry out between showers.

That is why the problem is patchy. A sunny, open driveway may stay reasonably clean while the shaded strip beside the house, the south side of a fence or the path under a mango tree turns black. It is also why cleaning in Cairns is maintenance rather than a single fix: the conditions that grew the film return every wet season.

A stiff broom, water and patience will lift light growth from small areas, and a household pressure washer can do more. Before reaching for stronger products, know the three mistakes that cause the most trouble.

1. 1Mixing products. Healthdirect warns not to mix cleaning chemicals, because some create dangerous gases when combined, and the Victorian Government's Better Health Channel says not to mix bleach with ammonia, acids or other cleaners, which can release hazardous chlorine or chloramine fumes. Read and follow the label on whatever you use.
2. 2Letting the runoff go down the drain. Queensland's Environmental Protection Regulation 2019 lists chemicals, including biocides such as fungicides and herbicides, and waste water from outdoor cleaning as prescribed water contaminants. Cairns Regional Council enforces this and says substantial penalties apply for unlawfully contaminating waters.
3. 3Holding the wand too close. A narrow jet held hard against old or soft concrete can leave marks. Start with the nozzle well back, test a hidden corner, and work in even passes.

Cairns Regional Council asks residents to avoid harmful or toxic chemicals when pressure cleaning and to collect wastewater rather than let it reach the gutter; its tips for car washing suggest letting wash water soak into lawn or garden. On a sloping Cairns driveway that means thinking about where the water will run before you start, not after.

Cairns Regional Council states that permanent water conservation measures apply to all Cairns residents. Its restriction sheet calls these Level 0, and for cleaning paved areas it says cleaning can be done using high pressure, low volume cleaners or a hand-held hose with a trigger nozzle at any time.

That changes if restrictions rise. At Levels 1 to 3 on the same sheet, hosing of paved areas is prohibited unless the cleaning is required because of an accident, fire, health hazard or other emergency, although a bucket filled directly from a tap is still allowed. At Level 4, water must not be used to clean paved areas at all outside those same exceptions. The sheet notes that failing to comply may incur penalties under section 43 of the Water Supply (Safety and Reliability) Act 2008.

Restriction levels can change, so check Council's water use and restrictions page before any big clean. The runoff rule does not change with the level: whatever comes off the concrete, whether chemical or simply dirty wash water, should not be sent into the street gutter or a stormwater pit. Our high pressure cleaning page covers how we work within the current rules.

A DIY scrub suits a small patch. A contractor makes more sense when the area is large, the growth is thick, the surface is a safety risk for someone in the house, or the job is tied to a deadline such as a listing or an exit inspection.

Vendors in particular tend to leave hard surfaces too late. In our pre-listing yard timeline the driveway and front path are washed about two weeks before photos, so the concrete has settled by the shoot. If the whole yard needs presenting, pre-sale garden presentation brings the washing, mowing and garden work together.

We wash driveways, paths and patios across Cairns, including the southern suburbs such as Mount Sheridan, Bentley Park and White Rock, and because pressure washing sits alongside mowing and edging for us, the lawn edges can be done on the same visit.

There is no single interval that suits every property. A shaded path beside the house regrows faster than an open driveway in full sun, and a wet year regrows faster than a dry one. The practical approach is to watch the surfaces rather than the calendar.

• After the wet: rainfall at Cairns Aero drops sharply from April (201.8 mm) to May (92.0 mm). Late autumn is a sensible time to look at every path and decide what needs washing.
• Before the wet: rainfall climbs again from November (87.9 mm) to December (193.8 mm). A clean in the dry months means paths start the wet season clear.
• Any time a path feels slick underfoot or shows a dark film where people walk, treat it as a safety job rather than waiting for the next round.

Between cleans, sweeping leaves off paths and trimming back plants that shade them both help the concrete dry faster after rain.

We do not publish a set price because driveways and paths vary so much, and every job is quoted before it starts. The quote mainly depends on:

• The area to be cleaned, and whether it is one path or the whole property.
• How heavy the growth is and how long since the last clean.
• Access for equipment, including steps, slopes and narrow side paths.
• Where the wash water can safely go, which affects how the job is set up.
• Whether other work, such as mowing or edging, is done on the same visit.

Quoting the specific surfaces you want done, rather than the whole property by default, is often the simplest way to keep the cost where you expect it.