The Washington DC Department of Public Works collects leaves, branches, Christmas trees and yard waste throughout the year and combines them into a rough mulch. This rough mulch / organic matter / compost is available free to DC residents. You can take up to five 32-gallon bags (or cans) per trip. You must bring your own bags and shovel it yourself. This mulch is available "as is" which may include non-organic matter such as some trash or plastic bits; consider sifting the compost before using it in your garden plot.
Pickup is available weekdays, 1 to 5pm, and Saturdays 8am to 3pm, at the Fort Totten Transfer Station, 4900 John F. McCormack Drive, NE.