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I usually discard weeds but it occurred to me that they would probably be good for compost.

Are there any steps I can take to use them in compost without having them grow again when I use the compost? Do different types of weeds need different treatment prior to composting?

The main weeds I have are dandelions and "farmers friend".

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If the weeds have gone to seed, any seeds in the compost mix will likely germinate when you come to use the compost. These, you have to dispose of differently.

Otherwise:

  1. Put the pulled weeds into a light-blocking sack for a week to make sure they don't have any energy to grow in the compost.
  2. Cut them up into small pieces to break their roots and leaves and encourage faster composting. A chipper is great for this if you have one, but I just use secateurs.
  3. Put the shredded weeds into a covered composter. You can bury them if the compost material if you don't have a cover.

After this kind of treatment, even the hardiest of dandelions should give up and compost properly. Provided you break up the roots, they shouldn't give you any problems.

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    If it's just a mulch pile and not a proper compost pile, then yes, they can go to seed. But, if you're composting properly, your compost pile will reach a temperature that will kill the seeds and prevent germination.
    – Daniel Bingham
    Jun 9, 2011 at 22:32
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    ^^^ what Daniel Bingham said. A robust, full composting will kill seeds. However, how many or what percentage of people do that correctly is an open question.
    – PoloHoleSet
    Mar 29, 2017 at 16:08
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Generally, they just need to be buried in the compost heap. If the heap gets hot enough, it will kill off pretty much every seed imaginable (except for tomatoes - I remember visiting sewage treatment plants and tomatoes still managed to germinate even after everything else was processed/sterilized).

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    If the compost isn't getting hot enough, you can "cook" it prior to using it. I do this before by putting the composted soil I've separated from the pile into a wheelbarrow and covering it with a dark piece of plastic. Leave the covered compost in the sun for several days to a week and it should get hot enough to kill any viable seeds remaining in your composted soil.
    – Shane
    Jun 9, 2011 at 16:12
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    @Shane, +1 excellent tip! Though please allow me to throw in one word of warning (applies to a compost pile as well), heating compost to temperatures above 160°F (71°F) isn't recommended (or necessary). Optimum compost "cooking" temperature is between 135°F (57°C) & 150°F (65°C).
    – Mike Perry
    Jul 15, 2011 at 20:15
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    How on earth do you get the compost that hot?! I'm lucky if mine gets much above the ambient air temperature (I'd guess around 30-40°C at most).
    – Mal Ross
    Jul 23, 2011 at 23:29
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    @Mal, you need high-nitrogen input e.g., green grass clippings or kitchen waste, mixed with leaves or whatever, roughly 50-50, kept moist. It will smell like a dairy barn or silo. If it gets farther and smells more like a pig farm, you've got too much nitrogen! Not only will it kill weed seeds - it will be ready to use much faster (less than a season) too.
    – Ed Staub
    Sep 1, 2011 at 0:38
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    Haha, I wish I knew what different kinds of farm smelt like. Currently, my compost's stone cold. Suspect it's got too much nitrogen.
    – Mal Ross
    Sep 1, 2011 at 9:01
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Since some perennial weeds can regenerate from even tiny root fragments, they are not considered safe for composting unless you use the Berkeley method of hot composting, and measure the temperature to ensure that your compost temperature is within bounds as prescribed by that method.

Another way to treat perennial weeds is to dry them out to kill the roots and stems as some can regenerate from stem cuttings (eg. Tradescantia). I put them on a plastic tarpaulin and leave them in full sun until I'm sure that they are desiccated and dead.

If the weed has flowers eg. Scottish thistle, I remove the flowers and place in a bath of water and drown them for a month or so.

Dried weeds can then be used as a mulch to compost in-situ, or, added to the compost pile.

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My suggestion would be to check it the weed is flowering. If it has any flowers remove them before throwing it in your compost bin. If done right, your compost bin will reach high enough temperatures to kill the seeds and plant but I generally remove the flowers before putting it in the compost bin

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