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We have moderately sized (under or around 0.5 m3) backyard composter. It is of simple cubic form with hatches at the bottom where the ready stuff is being taken out. Mostly food scraps go in there, but we also add garden waste, whether brown or green. Last spring I took first batch of the compost out, as it looked ready — nice, and dark.

However, as I used it in the plant containers as a top soil, some weed started to grow. I don't know what species it was, but it was some sort with a tiny light-blue flowers, looked like high-desert wild plant variety. That is, not a noxious ugly beast, but a weed nonetheless. It was uniformly distributed in the compost, and showed up nearly in each container I put it in.

Apparently the pile was contaminated by some plant that have gone to seed. So, the questions are: If I leave the pile to work longer, would it help to get rid of the seeds? Can the seeds even be decomposed, or they are too well protected? It's not that big of a hustle to weed them out in the containers, but I assume it would be a bigger problem if I spread it in the yard.

Highly Irregular
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theUg
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2 Answers2

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In theory, if you get your compost hot enough, it will kill the seeds. In practice, it can be hard to get a pile as small as yours to the temps needed to reliably kill all of the seeds. Also, only material in the center of the pile will heat up all the way; anything that stays on the cooler edges will not be killed.

Instead of trying to get it hot, you could just avoid putting seedy material into your compost. I don't put leftovers from canning tomatoes or raspberry/blackberry jam into the main compost pile because of the crazy amount of volunteers they will produce.

Or if you aren't ending up with too many weeds, just pull 'em out when they sprout and don't worry too much about it. I end up with a fair amount of seedy hay in my compost/manure pile, and I've basically resigned myself to the fact that I'm going to have to hoe those weeds out when I use the compost in the garden.

bstpierre
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    there are some seeds that have evolved to survive intense heat especially wildfires. –  Feb 14 '13 at 22:26
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    Are those the kind of seeds that you tend to find in your compost pile? – bstpierre Feb 14 '13 at 23:37
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    you would be surprised where weeds pop up. Thats why they are called weeds instead of unplanned foliage –  Feb 15 '13 at 04:16
  • I would also think it could be issue with high-desert/alpine flora where wildfires are common, and the controlled variety is even welcome in forest management. – theUg Feb 16 '13 at 17:30
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It’s a bit of a pain, but you can cook the compost in a big solar cooker. It's how I make my starting mix. It kills all the seeds. It also kills the good organisms too, but a little worm tea or a handful of other good soil will grow the good stuff back. It’d be a pain to do as a regular thing, but for a one-off deal, it would get the job done.

theUg
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StuntGirl
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