What Can You Compost?

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Robert Pavlis

Composting is the process of degrading organic matter. So any organic matter should be good for the compost bin—right? Maybe. You will find lots of lists showing you what you can and can’t compost. Are the lists correct? Why can’t you not compost everything that is organic? Let’s look at this closer.

Composting meat and cheese
Composting meat and cheese

What Can You Compost?

Most organic matter can be added to your compost pile and it will be turned into good compost.  In this post I will look at the items that some people claim you should not compost.

Composting Meat, Cheese and Fat

Meat, cheese and fat are organic, and microbes have no problem turning them into good compost. However, the composting of these occurs best when the compost pile heats up to high temperatures. In cold compost piles the material takes too long to compost. So if your compost pile is heating up well, there is no problem adding these to the pile.

There is another potential problem with meat, cheese and fat. They attract animals to the pile. This may or may not be a problem for you. You might consider their digging efforts as cheap labor to turn the compost pile? Animal issues also depend on the type of container used for making compost. Traditional open bins provide easy access to animals, but new closed home composters don’t.

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Meat does add a lot of nitrogen, and as discussed in How to Compost: Browns & Greens, extra nitrogen is generally needed since most people have a lot of browns.

Composting Paper

Paper is mostly cellulose and lignin, with some smaller amounts of clay, calcium carbonate and other filler material. Cellulose and lignin are both organic materials derived from plants. Cellulose is a long chain of sugar (glucose) molecules and is easily digested by microbes. Lignin is a complex organic molecule that is not easily decomposed.

Paper will compost, but the lignin in paper will only compost well at high temperatures. Even then, paper composts slower than most other forms of organic matter, except maybe wood chips and twigs.

The inks used on paper are mostly organic inks and are not a significant toxicological concern. Cardboard uses very little if any glue, and the glue is easily decomposed. Bottom line, paper and paper products can be added to the compost bin. If your compost is like mine, which does not get very hot, it does take a long time to compost.

There are also some good reasons not to compost paper. It is easily recycled and that may be a better environmental way to deal with it. Paper is also not very nutritious—it adds almost no nutrients to the garden although it does add some humus in the long run. Paper has a C:N ratio of 200-400:1 which means that it requires a lot of nitrogen to decompose. See How to Compost: Browns & Greens for more details on the importance of the C:N Ratio.

Composting Weeds

Should you compost weeds? To answer the question you have to look at different weed issues. In general weeds are just plants and there is no reason not to add them to the compost pile. The following may be exceptions.

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Some weeds may have already gone to seed, or they might be like the dandelion which goes to seed even after you pick it. If your compost pile gets really hot, the heat will kill the seeds and therefore they are fine to add to the compost pile. If the compost pile does not get hot enough, the seed may be viable in the finished compost. You are then spreading weed seeds throughout your garden.

Are seeds in the compost really a problem? Is your garden so clean that you don’t already have millions of weed seeds? Will adding a few more really make a difference? I doubt it. I tend to compost weeds, but I try to do the weeding before they go to seed. I don’t worry about a few seeds getting through the composting process, and my piles don’t heat up very much.

If I know a weed has very invasive seeds—I would not add it to the compost pile.

A second kind of weed spreads through runners. Quackgrass and Bindweed are good example of this. Don’t add these kind of weeds to the compost pile. I will usually leave these lying in the sun for a week or two to make sure they are really dead. Then I put them into the garbage.

Composting Diseased Plants

The common advice is that you should not compost any plant material that contains diseases. This seems to make a lot of sense. Why add diseases to the compost, that might in the future infect more plants.

If a compost pile gets really hot, most diseases will be killed off. I don’t believe most back yard composts get hot enough for this to happen—mine certainly doesn’t. But I still compost diseased plants.

Let’s think about this in more detail. Lets say you have a plant with mildew. Where did the mildew come from? It came from spores that float in the air. These spores are all over your garden, and your neighbors garden—they are everywhere. If you take a few leaves off your plant and dispose of them in the garbage, you will still have mildew spores everywhere. You might as well compost the leaves and get some value out of them. See Powdery Mildew Treatments for more on Mildew.

A couple of years ago we suddenly had late blight on tomatoes. This is a serious fungal disease which kills plants in a matter of days. So it seems to make sense that you should clean up the infected plants and not compost them. But consider this, you will never pick up every piece of infected plant material. Some infection remains. And even if you do a thorough job, late blight spores travel 20 km by wind. So if someone in your town does not do a thorough job cleaning their garden, there is a good chance late blight will be back next year.

There is one other thing to consider. Virtually every piece of plant material in your garden is infected with some kind of disease. Most are just small infections you will not notice unless you look closely. If you decide not to compost any diseased plant material, you will have nothing to compost.

It probably makes sense not to compost heavily invested plant parts—it might make a difference. You certainly should never compost a plant infected with a virus. Composting minor infections of other diseases is OK.

Composting Grass Clippings

Grass clippings can be composted, but there are a few things you should be aware of.

First of all you should not have any grass clippings from your lawn for composting. It is much better for your lawn, and less work for you to just leave them on the grass. Use a mulching mower, mow regularly, and just leave them on the grass. They break down quickly and feed the grass. They do not cause thatch—that is a myth.

Some people suggest that you should not compost grass clippings because they make the pile smell. That is only true if you don’t do it correctly. Grass contains a lot of nitrogen and moisture which is the reason grass soon turns into a soggy, smelly mess. Add small amounts of grass at a time to the compost bin, or mix it will high carbon material (see How to Compost: Browns & Greens) such as leaves and straw and you won’t have an odor problem.

There is also a concern about using grass that was treated with herbicides. For the most part these chemicals break down quicker in a compost pile than in soil, and most of the herbicide will be gone before you add the finished compost to the garden. If you want to be cautious, don’t add the compost to an area where you will grow seedlings, which are more sensitive to herbicides.

Forty years ago I paid the neighborhood kids a few pennies to bring their dad’s bags of grass clippings to my house. I then composted the material along with my own clippings. A win-win-win situation for everyone. Twenty years ago I started using a mulching mower and have left my grass clippings on the lawn ever since. Bagging grass clippings is just not a smart thing to do!

References:

1) Meat in Compost: http://blogs.extension.org/gardenprofessors/2011/12/01/meat-in-compost/

2) Biodegradation of lignin in compost environment – a Review: http://www.sciencedirect.com/science/article/pii/S0960852499001042

3) Using Paper Wisely in the Compost: http://www.gardensalive.com/product/using-cardboard-and-paper-wisely-in-the-compost-and-the-garden/

4) Composting Grass Clippings: http://extension.missouri.edu/p/g6958

5) Photo Source: Charles Hope

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

19 thoughts on “What Can You Compost?”

  1. While discussing about using diseased leaves for composting, you said that “You certainly should never compost a plant infected with a virus” To my knowledge viral particles are very delicate and do not live / cannot propagate once the host tissue is dead. They are not stable outside their host tissue and perish in short time.

    Reply
  2. Great information, thanks a lot for that. I need more compost than my little pile provides, and though I have lots of leaves piled up, I have no way of shredding them, so they’ll take longer to compost than the few months between now and planting time. So I’m considering ordering bulk compost. What do you think of city compost which is composed of the contents of people’s mandatory green bins (Nova Scotia)? I’m a bit nervous about pharmaceutical drug residues. Would that break down? Also, they say that there is .01% of plastic which they can’t remove. Would that present a problem for food crops?

    Reply
    • Why would municipal green bin compost contains drug residues? I think you are thinking about compost from waste treatment plants.

      The plastic won’t do any harm. It is usually good compost.

      Reply
  3. PS my mom planted potatoes by putting them o hard ground and covering them with hay. I am going to try the same with pine needles. What do you think?

    Reply
    • You can grow a lot of things in weird ways. But i doubt that using either hay or pine needles is a good way to grow good potatoes. T he roots will have a hard time getting nutrients and water. Without a good root system you don’t get good potato growth.

      Reply
    • pine needles contain a lot of chemicals that degrade slowly, so they decompose slowly. However, over time they do decompose, and you could use any of the methods I have described. However, it might be best to use them as a mulch. Then you don’t care if they decompose slowly.

      Reply
  4. Great article, Robert! What are your thoughts on composting tea bags? Apparently, many of them contain polypropylene. Thanks!

    Reply
    • Tea and coffee contain hundreds of carcinogenic chemicals. We still drink it. Why? It all depends on concentration. Everyone is so concerned with the names of chemicals but they don’t understand that it is the amount that is important. Any one of the hundreds of chemicals in tea and coffee, and all plant material could kill you if you eat enough of it. Lucky for us the amounts exist in small quantities. for more on this see https://www.gardenmyths.com/chemicals-coffee/#more-108

      I assume you have tea bags because you drink tea – so you have already had a cup containing polypropylene. I’d be less worried about it in soil, than in a hot cup of tea.

      Reply
  5. Hi Robert, I am new to your blog and quiet enjoying it. I bump into it when looking for verifications on the benefits of Bokashi, as I had my doubts which you confirmed with a well written piece.
    In the entry above you keep mentioning the compost’s hear as a factor. In relations to grass clippings, I must admit that I am quiet hooked on the stuff and can sense, hear and smell a lawn mower going on somewhere. I add the clippibgs to my heap but mix it in, never throw it on on top as a chocking layer.the mixing heats up the heap dramatically and decrease sing the compost time. The result is a nice balanced freshly smelled compost with plenty of nutrients.

    Keep up with the good work,

    Boaz

    Reply
  6. My gardening friend who is a skilled gardener mixes his mowings fifty fifty with his Autumn leaves (ok there is an issue of timing!) uses the decayed mixture to make up his seed and potting compost after adding a little fertiliser
    I bury all my newspaper in great wodges and value their water retention. It stays buried for years and does not decay – I don’t want it to!

    Reply
    • Grass and leaves work well together – the timing issue is just too much work for me to manage. I leave most of my leaves on the grass or garden as well. I mow a bit more in fall to cut up the leaves. The picture of a really lazy gardener 🙂 Even though I have lots of sugar maples, I do virtually no raking.

      Reply
  7. Hi Bob,
    When you say very high temperature, what is the range. I live in India and in summer (approximately 4 months) temperature goes beyond 45 degree centigrade, which makes pile temperature more than 75 degree centigrade. Is this considered a very high temp.?

    Thanks

    Reply
    • The temperature is not so important for making compost, but a temperature of 150F or 65C is recommended to kill pathogens. Reaching 180F (82C) is too high and beneficial microbes will also be killed.

      Reply
  8. What are you thoughts on egg shells in compost? They don’t decompose, though maybe they yield calcium. It seems easier to just crush them and toss directly into the garden.

    Reply
    • I have to do a section on egg shells. The reality is that they are very tough chemically and don’t decompose in compost or in the garden. I don’t know for sure, but suspect they do very little for our garden.

      Reply

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