Susan Fleming times the peak of fall foliage every year to her birthday, Oct. 14, when the hickory, maple and sweet gum trees on the property of her rural Fayette County bed-and-breakfast should be ablaze in color.

	Bob Donaldson, Post-Gazette Hay bales glisten in the afternoon sunshine underneath changing fall leaves along Pa. Route 228 in Middlesex Township, Butler County. More photos. Click photo for larger image.

But here it is, more than two weeks later, and the woods around Chalk Hill, where she operates the Cottages at Fayette Springs, are still sprinkled with green. Splashes of yellow and orange and small bursts of red only recently arrived.

"It's been very strange," Ms. Fleming said. "This is not normal weather."

Some Pennsylvanians, including tree experts, suggest the leaf-changing/falling pattern in 2005 is later than in any autumn in decades, as late as some people can remember in their lifetimes.

People talk about how trick-or-treaters usually crunch through rustling leaves, a sound barely heard yesterday because so many leaves were still clinging to branches. City parks and country roads reveal as many green leaves as not.

Foresters and meteorologists hold the unusually warm evenings of September and much of October accountable. It usually takes a string of days in which temperatures drop into the 30s to reduce the production of chlorophyll.

Chlorophyll is the substance that gives leaves a green color from photosynthesis. Once it disappears, the other colors it dominates in leaves for so much of the year stand out instead.

Fall foliage typically peaks by mid-October in much of Pennsylvania, but temperatures in the first half of the month averaged 5.8 degrees above normal, which is considered substantial. That came after a warmer-than-normal September and unusually dry summer for much of Pennsylvania, which also slows autumn's color transformation.

"Particularly with early-changing species such as sugar maples, it was just remarkable how long they stayed green this year," running up to four weeks behind schedule, said Marc Abrams, a Penn State University professor of forest ecology and physiology who hasn't identified such a late pattern in 20 years of studies.

Trees change colors on different schedules, with many maples at the forefront and oaks at the tail end of that sequence among the most common Pennsylvania trees. When temperature changes are more typical, with lots of cool October nights, the transitions come close enough to one another to produce some of the spectacular scenery common in fall in area forests.

This year, Dr. Abrams said, not only is the color change later, it's over a more extended period of weeks as different trees react in their distinct ways to the unusual rhythms. The result is pleasant but unspectacular, a bit like a new orchestra with sections of strings, brass and woodwinds that are talented but don't know how to work together to maximum effect.

"We don't have a peak of color this year, so much as a plateau," agreed Ed Dix, a botanist for the Pennsylvania Bureau of Forestry. "Right now, we're probably right in the middle of that plateau. ... I would say that on average, we're one to two weeks behind schedule across the state."

He noted that appraising the levels of fall beauty can be a subjective thing.

"It's not like we have some meter to go around and scan the state," Mr. Dix said.

The late season here, while unusual historically, is not peculiar to Pennsylvania. The delay of color has been noted in newspapers from Lafayette, Ind., to Bangor, Maine. The Midwest and Northeast have been dryer and warmer than usual for months, so that even New England's trees haven't all turned.

The much-anticipated color display arrived two to three weeks late, said Alice DeSouza, director of New Hampshire travel and tourism development. She said it benefits local businesses that thrive on the spending of people drawn to view the foliage.

"We're thrilled because not only does it extend the season for our guests, but it's wonderful for those of us who live here," Ms. DeSouza gushed.

In Western Pennsylvania, the schedule change meant the fall foliage festivals held in many counties weeks ago took place under a canopy of green, instead of gold and magenta. And in early November, people accustomed to looking at bare or brown trees with a touch of sadness can receive a prolonged lift this year.

"I think the best is yet to come, the way forecasters this whole week are predicting temperatures in the 50s and 60s," said Julie Donovan, a Laurel Highlands Visitors Bureau official involved in planning an open house tour of 16 bed-and-breakfasts for Sunday. "When we coordinated this, I never imagined there would be this many leaves on the trees."

The potential downside, as many residents in the state's higher elevations realized last week, is that an early-season snowfall, when leaves are still aplenty, can become so heavy as to break branches or topple trees. Those can fall on power lines and knock out electrical service, as occurred last week to Accu-Weather Inc. meteorologist Henry Margusity from a 4-inch snowfall in State College.

He said there's no snow in the current forecast, and predicted Saturday could be one of the most pleasant weekend days to be outside all autumn -- with a temperature of 70 possible both that day and Friday -- before a cold front arrives Sunday.