September 22 was the equinox. Here in the Northern Hemisphere, days will get slightly shorter as nights will get slightly longer. The intensity of the sunlight will diminish as the sun moves farther to the south. The weather will of course get cooler. In this particular climate, the rainy season will begin. Deciduous plants, as well as some evergreen plants, will respond accordingly to the changes.
Deciduous plants defoliate as they go dormant for part of the year. A few defoliate through hot and dry summer weather. Otherwise, almost all deciduous plants defoliate for dormancy through cool and stormy winter weather. They are more resilient without foliage that is sensitive to both frost and wind. While days are short and sunlight is diminished, foliage would not be very active anyway.
The foliage of many deciduous plants gets remarkably colorful during defoliation. Sweetgum, pistache and ginkgo are famously colorful. However, many deciduous plants, such as valley oak and silver maple, are not impressively colorful. Regardless, all deciduous plants that defoliate for winter will eventually start to do so, or have started already. Several evergreen plants shed debris too.
Gravity is a force that is more reliable than weather. It pulls foliar debris downward onto roofs, roadways, decks, patios, lawns and whatever happens to be below deciduous trees.
The timing of defoliation is ironic. Gutters and downspouts that were empty all summer now collect debris as the rainy season begins. They may need cleaning more than once if deciduous trees above defoliate slowly. While patios and decks get less use, they need more raking to avoid staining. Fallen leaves promote rot in turf grass and ground covers, and inhibit penetration of sunlight.
At this time of year, it is difficult to believe that evergreen trees are generally messier than deciduous trees. Deciduous trees tend to defoliate only once annually, although some drop bloom or fruit separately. Evergreen trees drop about as much debris, but do so for a longer season or continuously throughout the year.
Tony Tomeo 
Just love Autumn coloured leaves.
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This is one of my favorite topics to write about! Not many natives of California appreciate autumn foliar color as much as I do. Actually, I can think of none. It is more popular farther north and in the East.
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We get quite a show here in the forest…but it is mainly the different Oaks that give the colour, Red Oak is particularly wonderful. Liquid Amber is another favorite in France…not so many Acers though.
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The color that New England is famous for is maples; but much of North America has more oaks than maples. However, except for the red oak and related scarlet oak, not many oaks are as colorful as the maples. While in Oklahoma, I noticed the preponderance of blackjack oaks, which were very interesting, but no more colorful than a brown paper bag. The valley oaks and other oaks here are even less colorful. In river valleys of the Rocky Mountains and the Sierra Nevada, various cottonwoods provide color, but are only yellow. It is amusing to see what is colorful in other regions. Australia mostly lacks natural foliar color in autumn, so relies on exotic species,such as sweetgum or Liquidambar. Everyone seems to know that species!
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