Several factors contribute to weed seed mortality during composting. In compost systems assembled an...

1. Why is an increase in temperature important in making a
compost that will not spread weeds and pathogens?
Several factors contribute to weed seed mortality during composting. In compost systems assembled
and managed in accordance with requirements for organic certification, the most important factors
are the interaction between weed species, temperature, time, and moisture (Eggley, 1990;
Shiralipour and Mcconnell, 1991; Eghball and Lesoing, 2000; Larney and Blackshaw, 2003;
Dahlquist et al., 2007). In general, the higher the temperature to which weed seeds are exposed
during the active phase of composting, the higher the weed seed mortality. Similarly, the longer the
duration of high-temperature exposure, the higher the weed seed mortality. Thus, Dahlquist et al.
(2007) estimated that three of the six weed species they examined under controlled laboratory
conditions were unaffected by temperatures of 108 °F, but 90% of the seeds of all six species were
killed after less than three hours at 140 °F (Table 1). Furthermore, all six species suffered 100%
mortality after less than an hour at 158 °F. Similarly, in Texas, Weise et al. (1998) found that, in
composting manure at 35% moisture, barnyardgrass, pigweeds, and kochia seeds were killed after
three days at 120 °F; Johnsongrass seed was killed with three or more days of exposure at 160 °F;
but field bindweed seeds were killed only after seven days at 180 °F.
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