The first 50 Baltimore residents to participate in the city's hazardous waste material drop-off day Saturday will receive a free tree as part of the city's effort to enlarge its canopy, officials said yesterday. Hazardous materials will be collected from 9 a.m. to 3 p.m. at Patterson High School, 100 Kane St.; 11 a.m. to 4 p.m. Sunday, at Patterson; and 9 a.m. to 3 p.m. May 4 at Baltimore Polytechnic Institute. Proof of city residency will be required to drop off material.