SEPTEMBER 2018 RESEARCH UPDATE
Hot is Healthy
Everybody knows 55 C x 5 x 3. A compost pile has to reach 55º for 5 turns with a total of 3 days at temperature between turns. Those are the requirements to reach pathogen kill to make sure that the compost is Class A and is safe for general use. In our world that is pretty much the ‘apple a day’ equivalent. Recently Tania Gheseger from Metro Vancouver asked me for papers that show that that same 55 x 5 x 3 kills weed seeds too. My first reaction was to tell her that ‘everybody knows that’, thinking that it was part of the general wisdom rather than a conclusion from peer reviewed studies. Then I suggested that she ask Andy Bary at WSU, one of the compost experts who I’ve come to count on. Finally, I did a bit of a literature search myself. And there are in fact papers that document weed seed kill during the composting process. Those papers are the subject of this library. It turns out that everybody has their favorite compost feedstocks and their favorite (or most despised) weeds. You know my favorite compost feedstocks- food scraps and biosolids. My arch nemesis weeds include knap weed, a plague in my garden in Cle Elum with a tap root that is about 8’ long, and horse nettle- a thorny beast from my grad school days. Red rooted pigweed comes in third but doesn’t rate a picture.
It turns out that most of the papers also have their feedstock and weeds of choice. The first paper is from 2003. Here the authors attempted not to show bias in their weeds- they tested 5 weed species for the first go around and 13 for the second. Bags containing the seeds were placed at different depths of an animal manure compost windrow. The authors also did not follow the 5 x 3 rule. The pile was turned after 14 days, then weekly for two additional weeks and then occasionally up to day 99 for the first trial. For the second trial turning started on day 7 and was done weekly until day 29 and then intermittently until day 91. Temperatures stayed pretty high in the pile at all depths for the first go around and were consistently over 55 only at the middle depth for the 2nd go around. All weeds except stinkweed bit the dust (or the compost) for the first go-around with stinkweed seed viability decreasing from 82% in the control to just under 8% at the top of the compost pile. For the second trial, while there were a small percentage of survivors at day 42, all the weeds were toast by day 91.
The authors of the second paper have a favorite fairy tale in addition to favorite weed species. They put 5 types of weed seeds in compost piles and kept the piles at 35, 45, and 55 C. Unlike in Goldilocks, it turns out here that the hot pile was just right; the most effective with 100% weed seeds not viable. The middle temperature and cool temperature were sufficient to destroy most, but not all weeds, with variability also seen by type of plant. Solanum nigrum aka black nightshade was the hardiest of all with 30% viability at 45 C.
The third paper forgets about the compost pile completely and just focuses on temperature. Weed seeds were kept at different temperatures ranging from 39-70 C for different times. The time and temperature required for full kill are reported. You can kill weeds really quickly at 70 C, in less than 1 hour. Cool that down to 39 and it will take you 672 hours to kill the annual sowthistle with all the others tested germinating just fine. Fifty degrees was the sweet spot with 100% mortality in 4 hours (annual sowthistle) to 113 hours (tumble pigweed). To make the 5 x 3 translation, 113 hours is the same as 4.7 days.
The fourth paper tests the ability of composting done according to the National Organics standards (55-77 C for 15 days and at least 5 turns) to kill both plant pathogens and weed seeds. Here the pathogen was early blight and the weed was giant crabgrass. Compost was made with manures and different high C materials. The temperatures killed the blight and the crabgrass across all carbon sources. It seems like this 5 x 3 at 55 has some benefits besides pathogen kill. The final papers test those numbers to see if they mean that you can take noxious weeds/invasive species into your compost facility and not stay up worrying at night. They tested both common buckthorn and garlic mustard. The temperatures killed them both.
So that long-held rule-of-thumb of 55 C x 5 x 3 appears to knock the weeds on their behinds as well as it does the pathogens. As we focus more and more on producing quality soil amendments, with quality here meaning both low concentrations of contaminants and products that make your gardens shine, the value of time and temperature stays true. Thanks again to Tania for forcing me to expand my horizons here.