What Can You Compost?

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.

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.

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.

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!