Fall foliage will be at its best in the Philadelphia region this week.
In the southeastern part of Pennsylvania, leaves will be at their peak color over the next seven days, according to the state Department of Conservation and Natural Resources' weekly foliage report.
Higher elevations in south and southeastern Pennsylvania have already lost their peak, but lower altitudes still have plenty of color.
Most of the color we're seeing now is from trees in the oak family, the report says. Some maple and blackgum trees are also in peak color, but leaves on many of those trees have already fallen.
"Red oak is producing orange to yellow foliage while scarlet oak is deep red," it says. "Some tree species such as scarlet oak will hold their leaves for many more months or until spring when the new buds begin to break."
The report says Michaux State Forest -- about 150 miles west of Philadelphia -- and drives along Route 233, Interstate 81 and U.S. Route 30 should all provide abundant views of the foliage.
Some parks best for seeing the changing leaves are Delaware Canal, Nockamixon, Ralph Stover and Tyler state parks in Bucks County; Marsh Creek State Park and White Clay Creek Preserve in Chester County; Ridley Creek State Park in Delaware County; Evansburg State Park, Norristown Farm Park and Fort Washington State Park in Montgomery County; and Benjamin Rush State Park in Philadelphia.
The increasingly long nights are the chief factor prompting autumn color. The National Arboretum explains the process like this:
Like most plants, deciduous trees and shrubs are rather sensitive to length of the dark period each day. When nights reach a threshold value and are long enough, the cells near the juncture of the leaf and the stem divide rapidly, but they do not expand. This abscission layer is a corky layer of cells that slowly begins to block transport of materials such as carbohydrates from the leaf to the branch. It also blocks the flow of minerals from the roots into the leaves. Because the starting time of the whole process is dependent on night length, fall colors appear at about the same time each year in a given location, whether temperatures are cooler or warmer than normal.
That process means production of chlorophyll in the leaves stops, revealing the fall colors that the pigment normally blocks, the arboretum says.
Factors like temperature, soil moisture and sunlight also affect fall foliage quality.
The best displays, according to the Pennsylvania conservation department, follow a "succession of warm, sunny days and cool, crisp but not freezing nights."
The Philadelphia region holds the last glimpses of fall foliage in the state: Except for a small pocket of warmer areas in southwestern Pennsylvania, leaves are past their peak color in the rest of the state.