We should be covering vacant lots with mulch instead of gravel to prevent weed growth. Other benefits of mulching are:

1) Mulch is made from organic waste materials, which means it is greener.
2) Because mulch is a waste product, its production is more economical than gravel, which must be mined.
3) Because mulch is lighter than gravel, it is cheaper and greener to deliver where needed.
4) Gravel adds to the heat island effect; mulch does not.
5) Mulch helps hold in soil moisture.
6) Mulching is a proven means of dust control.
7) Mulch restores organic nutrients to the topsoil.
8) If the city started a mulching/composting program, this may even be a net revenue generator rather than cost center.

Additional benefits of mulch are described in the provided link.