12 Things You Should Never Compost, According to Gardening Experts

Meat and Bones

While meat composts surprisingly well, it tends to attract a lot of pests. Lorin Nielsen, head horticulturist at Epic Gardening, explains that flies,
local wildlife, and even pets can be drawn to the odor of decomposing meat. "Rotting meat can accumulate a significant amount of bacteria during the decomposition process, which can pose health risks to humans unless it is properly hot-composted," she says. Additionally, bones can take years to break down. Due to these issues, it is best to avoid composting meat or bones in a home compost system.

Dairy Products

Dairy products like cheese, milk, yogurt, and sour cream omit a rotten smell as they decompose, so you may want to skip adding them to your compost pile. “They can also attract pests and become a health risk for humans,” says Nielsen. That said, egg shells are a great, calcium-rich addition to compost.

Oils and Fats

Oils and fats do not break down like other materials and can remain in your compost for potentially years, says Nielsen. While these substances have an organic origin and can technically be composted, the risks often outweigh the benefits.

Animal Waste

Animal waste should never be added to your compost pile. Parasites such as roundworms, hookworms, and harmful bacteria can survive in your pet's waste, posing a serious hazard to human health.

Processed Grain Products

Processed grain products, such as bread and pasta, release a significant amount of nitrogen. While nitrogen is essential for decomposition, excessive nitrogen can disrupt the balance of your compost pile. Additionally, adding too many grains can lead to mold growth. While a small amount of bread or noodles won't harm your compost, avoid composting large quantities of these ingredients, Nielsen says.

Receipts

Paper can be beneficial for composting because it adds carbon, which helps balance the nitrogen found in food scraps. However, you should avoid composting receipt paper, also known as thermal paper. This type of paper contains harmful chemicals like BPA and its derivatives. Thermal paper is also used for shipping labels, so avoid composting those as well, says Noah Dillard, a product manager for Fedco Seeds.

Glossy, Colorful Cardboard

Brown corrugated cardboard is excellent for composting, but Dillard warns against adding glossy and colorful cardboard to your compost pile. The high-color gloss often contains heavy metals, which could be detrimental to your garden.

Diseased Plant Material

To effectively kill fungi and bacteria found on diseased plant material, a compost pile must reach temperatures of 141 to 145 degrees Fahrenheit for several days. Achieving this temperature is often not feasible in a home composting system. As a result, pathogens and pests may survive, posing a threat to your garden or farm. 

Charcoal Ash

In small quantities, you can add ashes from your wood-burning stove to your compost pile. However, you should avoid composting ashes from charcoal or coal. These materials contain high levels of sulfur, which can be harmful to your garden plants. Additionally, charcoal briquettes are often infused with chemicals that can damage your plants.

Compostable Plastic

Although this type of plastic is called compostable, it will not break down in a home compost pile in a reasonable amount of time. “These plastics require high-heat compost piles, which generally isn’t achieved unless you are dealing with industrial-scale composting," says Dillard. Unless an item is specifically marked as being able to be composted at home, it needs to go to a commercial composter.

Avocado Peel

Composting avocado peels is a waste of compost space (and time). “Many peels and rinds have evolved to be a defensive, protective layer that won’t break down easily,” says Dillard. If you insist on composting your avocado peels, speed up the decomposition process by chopping the peel into smaller parts first.

Citrus

Citrus is safe to add to either a static compost pile or a hot compost pile, as long as you turn it regularly to keep it aerated and hot. However, in a worm composting bin, there is a risk of acidity building up, which can harm the worms. "A few peels here and there won’t hurt a worm bin, but an abundance of them can acidify the food and bedding the worms live in," says Nielsen.

That said, you should avoid composting citrus peels even in a static or hot compost pile, as they take a long time to break down. Like avocado peels, you can speed up the decomposition process by cutting the peels into smaller pieces.