What Not to Put into the Compost Bin

With the active composting season beginning in many climates, it may be worthwhile going over what you should not put in the compost bin.

By Larry Hodgson

Just a quick reminder that composting means taking a waste product, often from the garden or the kitchen, and recycling it into a useful soil amendment: compost. Every gardener should be doing it. But some things just don’t belong in the compost bin, like the following:

Synthetic and Animal Products

Obviously, you would never put glass, metal, non-biodegradable plastic or other synthetic materials in a composter . . . and certainly not chemicals! They simply don’t break down! But there are also some organic products it’s best to avoid, at least when it comes to home composting.

That would include animal products (fat, meat, pet excrement [including cat litter], disposable diapers, etc.). That’s because of the undesirable microbes that can develop. Of course, compost that heats up thoroughly should be able to destroy even the most harmful microbes, but home composters don’t have the reputation of generating much heat. Also, the smell of rotting meat, on top of being undesirable, tends to attract vermin. For these reasons, it is best not to compost animal products, although pet brushings are fine if used in moderation.

Weedy Plants? Mostly No, But. . .

Also avoid composting seed-bearing weeds because, again, the seeds might not be destroyed by heat and then the compost you produce could spread weeds when you use it. So, compost their leaves and stems, but not their seed heads.

The same goes for weed roots. You can put the roots of annual weeds and plants in the compost, of course. They die on their own at the end of the season . . . and even more quickly when you pull them out! But not perennial weed roots.

Various perennial weeds with roots.
Don’t put perennial weeds in the compost while their roots are still alive. Photo:

Roots of perennial weeds, especially those that have invasive root systems (horsetail, goutweed, Japanese knotweed, quackgrass, etc.), are a different story. True enough, even living roots and rhizomes often do decompose in an average home compost bin. They’re actually done in by a long period without any light as much as by actual rotting! Even so, it doesn’t always happen. And why take unnecessary risks?

Of course, you can get around that by killing the rhizomes beforehand. You could, for example, soak them in water until they start to rot or thoroughly dry them in the sun until there is no life left in them. Dead roots and rhizomes? Of course you can compost those!

Diseased Plants: Opinions Diverge!

Powdery mildew on zucchini leaves.
Powdery mildew on zucchini leaves. Should they go in the compost bin or not? Not everyone agrees. Photo: Paul Maguire, depositphotos

How about diseased plants or leaves: ones infested with fungi, bacteria or viruses? Surely they shouldn’t go into the compost bin? But it’s not quite that simple.

Authorities will tell you not to compost diseased leaves. They find it safer to generalize and I understand that.

However, I’m not “an authority.” I’m a home gardener and the “no diseased plants in the compost bin” is one rule that many gardeners willingly break. After all, Mother Nature is an expert at decontaminating dead plant parts. It’s what she does best! And isn’t she always right? Diseased leaves actually decompose faster than healthy ones! They’re often halfway gone by the time they hit the ground. I almost always put diseased leaves in the compost bin. Then I try to figure out and correct the factor caused my plant to become ill to start with.

Perhaps the article: Should You Compost Diseased Leaves? could help you get a better idea of what to do in your specific situation.

Poisonous Plants

That’s pretty straightforward. Add them straight into the compost pile! (Yeah, I know. Surprising, isn’t it?) Don’t even give it a second thought! Plant poisons break down very quickly and are soon turned into minerals plants can use for their growth. In fact, some, like nettles (admittedly, nettles are not really poisonous plants, but they sure are irritating!) actually stimulate faster composting!

Poison ivy.
One of the few poisonous plants that has no place in the compost bin is poison ivy. Photo: robeo123, depositphotos

There is one major exception: poison ivy (Toxicodendron radicans) and related plants. Not that they don’t decompose into non-toxic products, but they do so s-l-o-w-l-y. And remain a possible source of harm for the person doing the composting. Never burn poison ivy either. So, bag and bury it. Other methods are suggested in the article Poison Ivy: Hands Off!

Products That Are Slow to Decompose

Beware too of organic products that are very slow to decompose. Branches, logs and lumber take years—often decades!—to decompose thoroughly, for example, unless you pass them through a chipper shredder. And corn cobs and fruit stones are not much faster. You might want to save wood for a campfire and lumber for a construction project and put the others out with the trash.

Machine shredding oak leaves.
You have to chop oak leaves really finely if you want to see them decompose in a single season. Photo: fiskness, depositphotos

Even certain types of leaves take a long time to decompose. It may be wise to thoroughly shred oak leaves and conifer needles to hasten their decomposition. And to leave them out in the rain for a while so their tannins—the part that makes them so slow to decompose—mostly wash away.

Crushed eggshells.
Eggshells need to be ground up well, much better than this, to be of much use in compost. Photo: Cala Mitysyl, depositphotos

Many people put eggshells and oyster shells into the compost bin. And that’s not bad in itself. However, if you can, do at least reduce them into a fine powder first. Otherwise, they make take decades to decompose. Of course, even if they don’t decompose to any degree, at least you’ll be recycling them. Nevertheless, it’s still best to grind eggshells up thoroughly if you expect them to contribute much to the quality of your compost.

Finally, animal bones left over from the table should not be composted for two reasons: risk of contamination and their extremely slow rate of decomposition.

To learn more about composting, don’t hesitate to visit the site of the Compost Council of Canada.

Text based on an article originally published in this blog on April 17, 2016.

Garden writer and blogger, author of 65 gardening books, lecturer and communicator, the Laidback Gardener, Larry Hodgson, passed away in October 2022. Known for his great generosity, his thoroughness and his sense of humor, he reached several generations of amateur and professional gardeners over his 40-year career. Thanks to his son, Mathieu Hodgson, and a team of contributors, will continue its mission of demystifying gardening and making it more accessible to all.

8 comments on “What Not to Put into the Compost Bin

  1. Thanks for all this info! I have a worm farm in the basement and I never know whether they would eat weeds/houseplants/cut flowers, etc.

  2. Tree services that I work for recycle their chips. Yeas ago, there were places to dump such chips on the highways, from where those who worked on the highways could disperse the chips. Nowadays, chips go to a recyclery, where they are shredded into a finer texture, and composted. However, palm, yucca and New Zealand flax are not allowed. Such debris must be chipped and disposed of separately.

    • Great article, thank you! I never realized i need to keep weed seeds and roots out of the compost pile!

      • Oh, but this is not my article. (You replied to my comment instead of to the article; but it is no problem.)

  3. I am happy to put logs at the bottom of my compost bin, and below large raised beds. They eventually break down beautifully and produce heat while they do.

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