Postharvest plant residues from Kentucky bluegrass (Poa pratensis L.) seed production may be useful as an organic amendment in container mixes. Postharvest residues of `South Dakota' bluegrass were composted with and without an N amendment, such as cattle manure (M) or alfalfa seed screenings (As). After composting, all residues were ground to 4- to 6-mm segments and mixed with sphagnum peat and perlite (2:1:1 by volume). Media that contained amended bluegrass residues had higher electrical conductivity (EC) values and lower C: N ratios (<19:1) than media made with other bluegrass residues. Tomato (Lycopersion esculentum Mill. `Laura') seedlings grown in residue composted with As had at least 3.5- and 4-fold more shoot dry weight and leaf area, respectively, than plants grown in any other composted medium. In addition, seedlings grown in composted bluegrass residues amended with As had 34% and 41% more shoot dry weight and leaf area than plants grown in a 75% peatmoss–25% perlite medium. Composted residue amended with alfalfa seed screenings appears to be suitable as a peatmoss extender in container media.