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Yard Waste Compost May Make Good Greenhouse Potting Mix

A study at two Wisconsin greenhouses has found that compost mixes, including one from urban leaf waste, can produce acceptable bedding plants under commercial conditions.

Under Wisconsin law, cities and towns must compost yard waste rather than sending it to a landfill. To recover composting expenses, city officials would like to sell the compost local residents don”t use to customers who can purchase the material in bulk. The study shows that commercial greenhouses, a $50-million industry in Wisconsin, might find the compost useful.

Commercial growers and scientists have known for some time that certain properly aged composts can suppress fungal diseases that attack plants, according to Rick Voland, a horticulturist at the University of Wisconsin-Madison. Voland began the study both to evaluate the suitability of compost as an alternative to commercially available chemical fungicides and to test a possible market for recycled wastes.

The study evaluated the growth of begonias, impatiens and torenias in four potting mixes. Two of the potting mixes were the commercial mixes used by the owners of Jim”s Greenhouse in Fall River, Wis. and America”s Best Flowers in Cottage Grove, Wis. The other mixes were made in part with compost of urban leaves from Glendale, Wis. or compost of cranberry pulp and duck manure from the Pheasant Run Recycling Center in Bristol, Wis.

“Under some conditions, the plants in the compost mixes grew more slowly than those in the commercial mixes,” Voland says. “But it was difficult to detect this because the variability in growth among plants in each of the mixes generally overshadowed this difference between the mixes. Problems with nutrients or pests, such as attracting fungus gnats into a greenhouse, were no more common with the compost mixes than with the commercial mixes.”

Voland says more research is needed before greenhouse owners will adopt compost-based mixes as alternatives to commercial mixes, most of which include peat, perlite or similar materials.

“It”s important to choose good composts, properly matured, and containing low levels of salts. These two Wisconsin composts met those criteria and produced beautiful crops of three species of bedding plants under commercial greenhouse conditions,” Voland says.

The study was supported by a grant from the Wisconsin Recycling Market Development Board to the University of Wisconsin System Solid Waste Recovery Research Program.