Field studies were conducted near Oxnard, CA, to assess propagule survivability of two annual and two perennial weed species in 7.6 m3 static piles of fresh or aged yard waste mulch over a 56 d period. Mulch temperatures > 60 C generated at depths > 30 cm in fresh mulch caused all weed propagule mortality, whereas sublethal temperatures at shallower depths in fresh, and at all depths in aged piles allowed propagule survival. A controlled environment laboratory experiment suggested ≥ 1 d exposure provides 100% mortality of seed of little mallow at 72 C, California burclover seed (without burs) at 85 C, rhizomes of bermudagrass at 69 C, and nutlets of yellow nutsedge at 70 C. These studies indicate that reinfested, aged mulch does not produce sufficient heat to destroy weed propagules and, therefore, may become a weed-carrying substrate. Fresh mulch should be mixed to expose surviving weed propagules at shallow depths to lethal temperatures found at 30 cm or deeper in fresh mulch piles.
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