Extreme rainfall may be causing tree leaves in some parts of Ontario to change colour earlier than normal, and weather factors in the coming weeks could affect the timing or duration of the annual fall spectacle, experts say.

The Greater Toronto Area and other parts of the province experienced record-breaking downpours this summer. While some summer rain is beneficial for the colour-changing process, experts say trees in places that experienced flooding may have received too much water.

“It’s likely that the trees in wetter locations will kind of be more stressed from having too much water,” said Sean Thomas, a professor of environmental change at the University of Toronto.

“Stressed trees will tend to have earlier leaf senescence.”

Generally, the leaves start changing in mid-September, signalling the coming of fall. An earlier leaf senescence — the final stage of leaf development for deciduous trees — means leaves turn from green to yellow, orange and red and then fall off earlier in the season, Thomas said

He said trees that are changing colour already may be stressed, although a warm spring may also be a factor.

About 10 per cent of leaves have already started to change colour in a handful of Ontario provincial parks, according to an Ontario Parks tracking map.

“There are spectacular viewing opportunities throughout Algonquin Provincial Park along the Highway 60 corridor and other access points, such as the park’s trails and day use areas,” the Ontario Parks website says.

Too wet or too dry a factor in length of fall spectacle

While several experts agree a fall of brilliant colours is almost guaranteed, they say unique climate conditions, such as the record rainfall and the weather forecast, create uncertainty about the colour timeline.

The weather to come will help determine the amount of time the fall colours will last before leaves fall, says professor Amory Ngan of the Institute of Forestry and Conservation at the University of Toronto.

“Leaves don’t last very long if we have a lot of rain,” said Ngan.

If significant wind and rain comes to pass this fall, that would mean a shorter fall leaf display, he said.

Ideal fall weather conditions for a long, brilliant autumn are a gradual decrease in temperature from summer highs, and not too much rain or wind, says Ngan.

Weather conditions in the fall will determine the timing and duration of the fall season, experts say. This photo was taken in early fall in 2020 in the Ontario municipality of Markham. (Craig Chivers/CBC)

“It’s the exact opposite of the weather conditions that we’re beginning to experience with climate change,” he said.

Conversely, drought conditions in the late summer and early fall could also mean a short fall, said Trisha Tan, a spokesperson for the Toronto Regional Conservation Authority.

“We might see an early shutdown of the trees as they prepare for winter then and that would be what would cause the leaves to fall early from trees without being able to reach their full colour potential,” Tan said.

Tan says the duration of the fall spectacle depends on the weather balance being just right.

Getting out to enjoy the leaves

The province has many parks and conservation areas with impressive tree canopies for visitors to enjoy the fall colours, however long they last this year.

If visiting a provincial park, the fall leaf tracker allows visitors to choose a visit date when leaves are at their peak fall colours.

Those who work for conservation authorities are encouraging Ontarians to get out to see the colour show this fall season. These were the fall colours are seen in Etienne Brulé Park in Toronto in October 2023. (Evan Mitsui/CBC)

Many conservation areas also have tree-canopied trails to check out.

Those who work for conservation authorities say they hope people come out this season.

“People have been commenting on it to me that things are further ahead,” said Adrian Bryan, co-ordinator of forestry operations for Conservation Halton.

“Get out and enjoy the colours while they’re with us,” he said. “I think that it will probably be short, but it will probably be beautiful.”