Potential problems using yard waste as landscape mulch

The disposal of yard waste is an environmental issue of increasing national concern. Yard waste accounts for up to 25% of the volume of material placed in municipal landfills (Jackson, 1993). In 1986, the Environmental Protection Agency predicted that one-third of existing landfills would be closed by 1992 (Repa and Sheets, 1992) and all current landfills in 14 or more states are predicted to reach full capacity by 1994 (Wilkinson, 1989). One way to diminish the problem is to use yard waste as landscape mulch. These studies using yard waste as landscape mulch were designed to examine potential problems such as allelopathic activity, mulch toxicity on plant growth, and changes in soil pH.

Yard waste was collected from a municipal composting facility in Knoxville, Tennessee on 10 dates during 1993. The allelopathic effect of the 10 yard waste collections was tested on germination of Ipomoea purpurea (morning-glory), Tagetes filifolia (marigold), Celosia crestata (cockscomb), and Lolium multiflorum (annual ryegrass) at Knoxville in a field research plot. Fresh yard waste collection date 4-22 limited marigold germination to 13% with the control allowing 80% germination. Collection dates 3-2, 3-19, and 5-7 limited cockscomb germination to 0% with the control giving 73% germination. Collection date 3-19 limited morning-glory germination to 28% with the control allowing 73% germination. Collection dates 3-2, 3-19, 4-7, 4- 22, and 5-7 limited annual ryegrass germination to 0% with the control giving 75% germination. The most promising collection in terms of fresh yard waste inhibiting the germination of four species of seeds was collection 5-7 with only 15 % mean germination. Composted yard waste collection date 4-22 limited marigold germination to 43 % with the control allowing 93% germination. Collection date 3-2 limited cockscomb germination to 52% with the control allowing 88% germination. Collection date 3-2 limited morning-glory germination to 75% with the control allowing 95% germination. Collection date 4-22 limited annual ryegrass germination to 56% with the control allowing 95% germination. The most promising collection in terms of inhibiting the germination of four species of seeds was collection 4-22 with only 58% mean germination.

Air-dried uncomposted yard waste was tested for its influence on plant growth of Euonymus kiautschovicus 'Manhattan' (Manhattan Euonymus), Hedera helix (English Ivy), and Forsythia x intermedia (Border Forsythia). None of the 10 collection dates inhibited plant growth of Euonymus or Forsythia and there tended to be a stimulation of Forsythia. No statistical analysis was made of the English Ivy plants because all plants were killed due to cold winter temperatures.

Fresh and composted yard waste was tested for its influence on soil pH. All 10 collection dates of fresh and composted yard waste (except 7-9 for composted yard waste) produced a significant increase in soil pH over a six month period of time when compared to the control.

In these studies fresh and composted yard waste exhibited some beneficial allelopathic responses, was not detrimental to the growth of plants, and raised the pH level of soils. It is expected that yard waste can serve the environment better by being used in landscapes rather than being deposited in landfills.