The plume of smoke curling up from Kings Park from midday on Thursday will follow three decades of restraint by the park's authority.
Fire trucks from the Department of Parks and Wildlife and Department of Fire and Emergency Services are on the scene to support the controlled burning of about six hectares in the southwest corner of the park near Poole Avenue.
Eight fire trucks, career firefighters and three Department of Fire and Emergency Services district officers are on the scene to help Botanic Gardens & Parks Authority and Department of Parks and Wildlife staff manage the fire, expected to last until about 4pm.
Roads in Kings Park including Lovekin Drive, Poole Avenue and a section of May Drive are closed.
Bushland pathways in the area concerned will be closed to pedestrians and cyclists for up to three days.
BGPA chief executive Mark Webb said the burn would be the first controlled burn allowed in 30 years.
He said that before the period of abstinence, burns both planned and unplanned had been regular at the park but had created an environment plagued by rampant weeds.
Species such as the South African veld grass thrived. Not only did the weed's flammable qualities mean it helped fires spread, but fires reciprocated by promoting the species' quick regrowth and seed production.
"The more fire we had, the more weeds we got, and then the more fire we got," Mr Webb said.
"The board of the day decided to try to break that cycle and so we stopped planned burns and focused on controlling the grass weeds – mainly veld grass – by other means."
Mr Webb said the board's moratorium had remained until Thursday's cautious reintroduction.
"Now we think we've got the weeds largely under control – we can look again at how to use fire as a management tool," he said.
He said unlike usual government-led prescribed burns, Thursday's burn was primarily about contributing to a larger research project.
The project was investigating how to reduce fuel loads in urban bushlands while balancing the other priorities – protecting biodiversity and preventing weed spread.
He said research done in country areas did not apply to urban bushlands because of the unique conditions city surroundings created.
"If you're burning karri forest, you don't have the same weed load," he said.
"We know that some weed species such as veld grass, and even some native plant species, such as sheoak, respond well to regular fire. Other species do not.
"Ultimately, we want an ecosystem that is resilient and represents the biodiversity that you would expect to find on these Swan Coastal Plain heathlands and banksia woodlands."
Other techniques, such as physically removing fuel loads rather than burning, are also being investigated.
Mr Webb said the authority would also experiment with burns in Floreat's Bold Park and then eventually other urban bushlands on the Swan Coastal Plain, but they would be done one at a time because of the sensitivity of their settings and the need to prioritise public safety.
"You have to consider the assets that surround Kings Park, such as hospitals," he said.
The fires will be slow burning and low intensity, meaning animals and mature trees should escape unscathed.
They will be repeated in six and 12 years' time, though timeframes could change depending on results.
DFES has warned residents only to call triple-0 to report a fire where emergency services are not in attendance, or if the situation is life-threatening.
Those affected by smoke should call their GP or 1800 022 222.
f the smoke is particularly thick they should close windows and doors and turn off airconditioners.
DFES has warned motorists to drive carefully through the area and use headlights if necessary.
Further information is available from Main Roads WA by calling 138 138 or from BGPA at www.bgpa.wa.gov.au.
