Effect of Heat Treatment and Short-Duration Composting on Germination of Weeds in Green Waste

The use of minimally processed green (or yard) waste as a mulch for tree crops and vines has the potential to spread weeds. In a laboratory study, the germination of cootamundra wattle (Acacia baileyana) and tomato seeds in green waste was determined at four temperatures (35°C, 45°C, 55°C, and 65°C) and three moisture contents (40%, 50%, and 60% w w-1 w.b.). Moisture content had no significant effect on tomato seed germination at 35°C or 45°C. At 55°C, seed germination was generally greater at 40% or 50% than at 60% moisture content, but the differences were not significant after the first hour of exposure. Cootamundra wattle seeds survived more than 5 h exposure to 55°C and 65°C with no decrease in germination and were not affected by increasing moisture content. In field composting conditions conducted in windrows, 8% of cootamundra wattle seeds germinated after 136 h exposure to temperatures above 55°C. Bermudagrass did not survive much longer than eight days on the outside edge of a windrow (3.3% of stem fragments germinated after eight days), but the bulbils of watsonia (Watsonia meriana) were able to germinate after more than 5 days buried 1 m deep in a windrow (3.3% germination) where temperatures reached over 60°C. This study indicates that in order to control heat-resistant weeds like cootamundra wattle, green waste feedstock should be adequately prepared for composting and managed with careful process control, especially when the duration of composting is short.