Composting feels like magic. Throw your wilted lettuce, onion skins, fallen leaves, and other garden waste in a bin. Let it sit. Flip the pile with a shovel a few times, or give the tumbler a spin. TA-DA! Dark, crumbly, rich garden goodness is yours, ready to spread in your veggie bed or topdress your perennials.
It feels almost too good to be true. After scooping out that first batch of finished compost, you may start to wonder, “What else can I throw in there?”
Not everything belongs in the compost pile. Adding the wrong materials can attract pests, brew unpleasant odors, and even contaminate your compost with harmful substances. To ensure your compost remains safe and effective, it’s important to know what to keep out. Dig in, and let’s talk about what shouldn’t go in the compost pile.
Diseased Plants
Did you battle blight in your tomatoes this season? Maybe you fought fusarium wilt on your peppers? Well, guess what: Many viruses and fungal diseases persist happily in the soil or dead plant material for long periods of time.
Some plant pathogens and diseases even survive long winters. Tossing diseased plant material into your backyard bin is a surefire way to guarantee you’ll be seeing those same diseases next year after you spread your amendments back in the garden.
Many pathogens die at high temperatures. However, most backyard composting systems won’t reach and sustain the 130-150 degrees Fahrenheit necessary to eradicate disease.
What To Do Instead
Place diseased or infected plant material in a trash bag and dispose of it with trash headed for the landfill. Burying the infected plant debris outside the garden is also an option.
Burying will reduce the chance of spreading disease from debris to plants currently in the garden or to plants that will be in the garden the next year. Be sure to thoroughly disinfect any shovels, trowels, or pruners afterward.
Meat Scraps
Let’s start by stating that meat and bones are biodegradable and will decompose. Eventually. Large chunks of protein-rich meat and bone can significantly slow down the decomposition process.
In the meantime, they smell quite unpleasant and can attract a ton of nuisance wildlife over to your compost bin. Rotten meat can also harbor harmful bacteria like E. coli and Salmonella. If these pathogens aren’t nuked properly, they are a food safety risk in your garden.
What To Do Instead
Meat scraps are best fermented in a Bokashi composter. Then, add the resulting Bokashi product to your backyard pile. Alternatively, dispose of meat in a heavy-duty trash bag with other household trash.
Dairy, Oil, and Grease
Like meat scraps, cooking oils, grease, and dairy products can also attract flies, rodents, and other wildlife pests to your yard. They smell pretty bad when they decompose, and dairy products such as cheese, milk, and yogurt are particularly difficult to break down.
These foods have high fat content and a lot of moisture. They’ll need loads of dry, carbon-rich material (“browns” in compost-speak) like shredded cardboard and wood shavings to offset the large amount of nitrogen and moisture they add to the pile.
What To Do Instead
Dairy products can be composted in a hot composting system, ideally one that maintains an internal temperature of at least 130°F (54°C). At this temperature, most harmful bacteria and other pathogens are killed. However, these temperatures are fairly difficult to achieve in a backyard composting system.
Your best bet is to contact your municipal or other large-scale yard waste facility; most of these industrial facilities are capable of safely processing dairy, grease, and other fatty animal byproducts.
Treated Wood
Wood is an excellent, carbon-rich ingredient. However, only clean wood, like branches, logs, and raw lumber that has never been painted or stained, should be added to the compost bin. Unless you cut it down yourself, it can be difficult to know the origin of many wood products.
Composting is based on the premise of decomposition, and pressure-treated lumber, plywood, and strand board are all treated with chemical compounds that actively prevent their decay. These chemicals can contain arsenic, chromium, and other toxic components that are dangerous to both human health and the environment.
You wouldn’t want these toxins to be added to your veggie bed next season, nor would you want them to leach into the ground below your pile. Keep these items out of the garden in general.
What To Do Instead
Check with your state’s environmental protection department before burning, as exact regulations may vary from state to state. Burning treated wood creates toxic fumes and is generally prohibited. Dispose of treated wood at a landfill.
Weeds
Ok, this one isn’t a strict no. As long as the roots and seeds are effectively killed, having some in your compost won’t be detrimental to your garden. Weeds are just “unwanted” plants – they’re still full of nitrogen and other nutrients valuable to compost. The exceptions are invasive plants, like Japanese knotweed or bamboo, which should not be added to a home compost pile. Read more on that below.
What To Do Instead
If you are able to regularly turn and manage your pile to achieve high temps, you may be able to successfully compost weeds. However, to be on the safe side (and save yourself a lot of weeding hours in the future), trim the seed heads before tossing them in. Fermenting small amounts of weeds in a Bokashi composter is another option.
Invasive Plants
I’m currently battling goutweed in a portion of our yard and can vouch for this invasive plant’s tenacity: even a tiny fragment is enough to sprout a whole new individual. Don’t toss invasive plants in the compost! One improperly decomposed bit of plant can kick off a new infestation in your garden.
As with diseased plant material, high temperatures are needed to effectively kill persistent invasive plant material. Small backyard compost piles rarely get hot enough to do so.
What To Do Instead
The best strategy in managing invasive plants is to halt their spread (and stop purchasing them from garden centers in the first place!). While you won’t want the invasive plant material in your home compost pile, it’s also incredibly important that the material doesn’t just get moved elsewhere. Tossing it in the nearby woods will just exacerbate the problem.
Kill the plant material onsite if possible, either by desiccation or solarization. Dead plant material can be buried, but it needs to be several feet deep in the soil, a potentially daunting task. If the material cannot be managed onsite, double bag it, including the roots, shoots, and seed heads, and dispose of it with your household trash.
Pet Waste and Litter
Pet waste can be full of parasites and harmful bacteria that can find their way into your garden bed if not destroyed properly. Most backyard composting systems do not reach the high temperature necessary to kill off these pathogens.
Clean, unused cat litter can be composted, provided it is plant-based, like those made of wood or paper pulp. Scented litter and those containing sand or clay cannot be composted.
What To Do Instead
Scoop the poop! Pet waste left to accumulate on lawns can leach pollutants into streams and lakes. Scoop, bag and dispose of pet waste in the trash headed for the landfill.
Wrappers and Packaging
This one seems like a no-brainer, but speaking as someone who has pulled my fair share of banana stickers out of our bin, I can attest that food packaging can show up in surprisingly sneaky ways.
Nonbiodegradable items like produce labels, staples from tea bags, and wax-coated cardboard containers should not go in the compost pile. Plain, unmarked coffee filters and tea bags can be composted but they should be shredded first.
What To Do Instead
Paper, glass, and certain plastics can be recycled. Be sure to rinse food cartons thoroughly, and check your municipal recycling guidelines if you are unsure about what they’ll accept. Many states and municipalities will have universal recycling programs that accept all items together, however some areas require paper, glass, and plastic to be separated.
“Compostable” Tableware and Bags
In a case of flawed advertising, plates, cutlery, and bags listed as “compostable” are typically not compostable in a backyard compost pile. High temperatures are needed to break down many of these materials.
Most residential systems, and even some municipal composting facilities, just cannot guarantee complete degradation of these products. One study revealed that a particular type of “biodegradable” bag failed to decompose even after being buried in soil for 3 years. An exception? Utensils or plates made of completely untreated wood.
What To Do Instead
Look for products verified by the Biodegradable Products Institute (BPI), which provides technically and scientifically credible certifications for materials that actually biodegrade in the environment. Consider using reusable tableware and cutlery, which can simply be washed and reused.
Final Thoughts
This might feel like a long list of items not suitable for the compost bin but don’t worry, there are plenty of suitable materials to feed your compost pile. Ultimately, you just need to exclude items that are particularly attractive to wildlife or pose a hazard to human or environmental health. By being aware of these exclusions, you can create a fantastic, nutrient-rich amendment that supports a thriving and sustainable garden.
