My leaves of choice are oak and elm because they are dark colored and contain lots of tannins and other complex materials which are substrates for humus formation, also the dark leaves look nice in the alleys. I use the smaller elm leaves in more open areas as they are less susceptible to being blown around. As well as putting leaves in the alleys in the fall, I keep about 25 bags for use during the summer as light mulches around plants or to tidy up alleys.
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