I commonly heard people saying oily or meat products should not be added to their compost, but I've rarely heard anything about plants in this regard. In particular, I would like to know the rules about small-scale (I have lid-covered bins of 70cm (27in) side and 80cm (31in) height) and cold composting.

Do fresh pruning waste carry seeds of weeds so that they should not be used in cold composting? For instance, I have a bag of Duranta repens prunings (leaves and branches) collected last week. Is it suitable for cold composting (I heard it is toxic as well as a weed)? How about general grass clippings?

Also, if anyone also knows about Duranta repens C/N ratio, please also tell me. I don't want to start a new thread for such a small question.

1 Answer 1


There's a bunch of questions there, so I'll take them one by one. First, you should not add meat or fish, cooked or otherwise, to a compost system, unless its a Bokashi method, and oil of any kind should be excluded. Second, if your composting is anaerobic and cold, then you should not add certain pernicious weeds such as the bindweeds or dandelion roots, which will grow from a tiny piece of stem or root; other weeds are fine so long as you remove flowers and seedheads such as berries or pods first, just add the leaves and stems.

In regard to poisonous or toxic plants, yes, you can add the green parts to your heap, whether its cold or hot composting - the toxic substances will be broken down over time and will not make the resulting compost toxic - the only exception to this might be Yew, which I wouldn't recommend adding - the needles take too long to compost down anyway.

With regard to C:N ratio, the easiest ratio to aim for is roughly two thirds carbon, one third nitrogen, or three quarters carbon to a quarter nitrogen. The official, recommended ratio is 25 or 30 parts carbon to 1 part nitrogen, but that does not take account of moisture ratio, so for scientific purists, its actually a bit more complicated than the official ratio suggests. Fresh, green materials count as nitrogen, but dead leaves and woody stems and branches count as carbon. Woody material will take an awfully long time to break down in a cold composting system, so its probably best not added at all. However, if you're short of browns (carbon) then chop the wood into the tiniest pieces possible, which will help it to break down more quickly. You can add grass clippings; if they are green, they're a useful source of nitrogen, but they should be sparingly added, a thin layer at a time - if you add a lot at once in a thick layer, they will become a smelly, slimy mess.

With regard to Datura repens, you should protect your skin when handling the plant if you want to strip off the leaves for composting - all parts are toxic and may cause a skin reaction, and remember not to touch your face with any gloves you may be wearing for protection, both from the thorns and the plant material.

For basic guidelines, more reading here http://www.gardenmyths.com/how-to-compost-browns-greens/

  • Really, thanks so much for the information!!!
    – y chung
    Oct 16, 2016 at 17:22
  • Hi, I have one question to add. My bins actually have air holes on the sides and I also created some air holes within the compost using rolled chicken wire and so it is not anaerobic. It is cold because currently the entire ingredients of the compost is dead leaves. Does that make a difference?
    – y chung
    Oct 17, 2016 at 2:11
  • 1
    Read this regarding holes gardenorganic.org.uk/composting-myths-exposed. Note also that masses of dead leaves are best composted separately, (because they rot down differently from fresh garden waste) in black plastic bags, stuffed to the top, if they're dry, wet the contents, tie the bags shut, a few holes poked in the bottom and left somewhere out of the way for 1/2 years; resulting product looks like black soil and is called leafmould. Best not stood on a paved surface, can cause staining.
    – Bamboo
    Oct 17, 2016 at 12:02

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