Compost clinic: Weeds

“Hi,

Congratulations on becoming a Master Composter (do you get letters after your name?)

I’m a keen composter, but have a problem with weeds growing in it. Last year I tried sieving it, but that was back breaking and didn’t achieve anything. Any advice for either getting rid of the current crop of weeds or preventing new ones?

Thanks
Richard”

It turns out that Richard has two weed problems with his compost – weeds growing through it and weed seeds germinating in it after he’s used the compost on the garden. I can sympathise, since I’ve had both problems myself.

I used to have a serious bindweed problem in the garden, and the only way I could stop it growing up through the compost bins was to put weed control fabric over the soil before I put the compost bin down. It takes several years to kill bindweed this way (by starving it of light), but it does work in the end. You can also dig over the soil underneath the bin and remove any perennial weed roots before you start filling the bin.

The easiest way not to have any weed seeds in your finished compost is not to put them in the compost in the first place. Consign seeding weeds to the dustbin (or send them for community composting, if you have a local collection or can take them to the recycling centre). The same goes for diseased plant material. A lid or cover on the heap will stop weed seeds from blowing in.

The only guaranteed way to kill weed seeds once they’re in the compost is to build a hot heap. To build a hot heap you need to collect equal amounts of ‘greens’ and ‘browns’, then build the heap in one go – alternating layers of both types of material, around 6 inches thick. Under these ideal conditions, the compost will heat up – but you’ll need to turn it every few days to keep enough air in the pile and to move material from the outside to the middle. After several weeks of this treatment, with a much reduced compost volume, you leave the remaining compost to mature.

However, the latest edition of the US magazine Organic Gardening reports on some recent research that suggests time, and not temperature, is the key to eliminating weed seeds from your compost. So rather than rush to use your compost as soon as it looks ready, let it rot for at least 6 months to reduce your weed problems.

And, in answer to your first question, no I don’t get letters after my name. But I do get a name badge, a snazzy uniform and a certificate when I’ve done 30 hours of volunteer work in my local community :)

Got a compost question? Leave me a comment or send me send me an email and I’ll cover it in a future Compost Clinic.

Posted Apr 30, 09:56 AM.   Posted in .

3 Comments for Compost clinic: Weeds

  1. I take a very passive attitude toward composting, which results in nice (fairly) seedless stuff about 3-4 years old. I just pile the stuff up, and let it rot. One needs the room for this though. The pile can get big. And it migrates…as I harvest compost from one end, and continue to pile at the other.

    RIGreening · May 1, 10:40 AM

  2. Emma,

    Thanks for the reply.

    My wife is now banned from putting weeds in the compost bin and they are now taken to the local recycling centre
    Hopefully, over time, this will reduce the problem.

    Interesting that it may be time rather heat which kills the seeds. My goal is to have a compost bin like the one at Wakehurst Place:
    www.kew.org/places/wakehurst/compostcorner.html

    Although my wife may object.

    Richard.

    Richard · May 3, 09:04 AM

  3. A big compost heap is certainly the thing dreams are made of :)

    Emma · May 3, 09:33 AM

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