In the springtime, Housman thought it was the cherry.

In the fall, I think it is the ginkgo.

Gingko tree

That time of year when the lovely ginkgo
Paves our streets with fans of gold,
And we like royalty lightly tread upon them.

Gingko leavesAnd this from Herb:

It rained this afternoon,
and before I got home from work,
It was dark and cold.

Passing absent mindedly beneath a streetlamp
I found myself
on a pavement of gold.

Ginkgo leaves—
only a carpet of fallen ginkgo leaves,
but too much like the streets foretold.