Sewage sludge, pharmaceutical fermentation residues, cranberry wastes, and food flavoring wastes that had been composted by in-vessel techniques were tested as substitutes for Canadian sphagnum peat in a Cornell peat-lite mix-A. Marigolds (Tagetes erecta L. ‘Lemondrop’) were grown in a medium containing 50% (by volume) vermiculite, and 0%, 10%, 20%, 30%, 40%, or 50% compost, with the remainder comprised of Canadian peat. Marigold growth was improved when any or all of the peat was replaced with composted sewage sludge. Except for media containing 40% and 50% composted food flavoring waste, plant growth in nonliquid fertilized media containing the other composts was equal or superior to conventional Cornell peat-lite mix. Except for media containing 50% pharmaceutical, 50% cranberry, or 40% or 50% food flavoring compost, plant growth was improved by supplemental liquid fertilizer. Improved growth was related to increased levels of plant nutrients, while decreased growth, at the highest proportions of compost, resulted from excessive NH4N, pH, or soluble salts. Differences in aeration, water holding capacity, and other physical media properties were small. We conclude that many types of organic wastes, composted by in-vessel techniques, can be used as a substitute for part or all of the peat in a conventional peat-lite potting media.