40 things you shouldn’t put in compost and WHY?

Composting is a great way to reduce waste and create nutrient-rich soil for your garden. However, not everything belongs in a compost pile. Some materials can be harmful to the composting process or end up creating more problems when added to your garden’s soil. Knowing what not to put in your compost can help you maintain a healthy system and get the most out of this sustainable gardening practice.

Meat, Fish, and Dairy Products

It may seem obvious, but any type of meat, fish, eggs, or dairy products like cheese should be avoided in compost piles. These items decompose much more slowly than fruits and vegetables. As they decompose, they release odors that can attract pests to your compost. Meat and dairy also have higher fat contents that break down into smaller particles. This can coat other compost materials and prevent proper aeration. The fatty particles persist as contaminants in your finished compost.

Cooked Foods and Oils

Cooked foods like pasta, rice, or baked goods can also cause issues in compost. Like meat and dairy, cooked foods break down slowly and release odors. Food particles from greasy, oily, or buttery foods remain unchanged through the composting process. When added to soil, these undigested particles attract rodents and provide little nutritional value for your plants.

Pet Waste

It’s best to avoid adding any type of pet waste, like dog or cat feces, to compost piles. Pet waste often contains harmful pathogens that can survive the composting process. Parasites like roundworms and giardia can contaminate compost and infect humans if we consume vegetables grown in waste-tainted soil. Pet waste can also give off an ammonia odor as it decomposes, which attracts flies and other pest insects.

Weeds

As tempting as it might seem, weeds and invasive plant cuttings are not good compost candidates. Some weed seeds and roots can re-sprout after being chopped up in the compost pile. Composting does not generate enough sustained heat to destroy tougher weed seeds or persistent root segments. These can reestablish weed plants throughout your garden when introduced along with compost. Avoid adding bacterial or fungal disease-ridden plant materials as well.

Diseased Plants

To avoid spreading infections, any plant matter that shows signs of fungal, bacterial, or viral plant diseases should stay out of the compost pile. Compost rarely reaches hot enough temperatures throughout to destroy plant pathogens. Diseases like blights, mildews, and mosaic viruses can easily survive and infect healthy plants later on. Remove and dispose of diseased plant matter separately from compost materials.

Shiny Paper

Paper makes a great addition to compost when used in moderation. But glossy paper printed with inks and dyes, like magazines, catalogs, flyers, and junk mail, should not go into compost. Inks, dyes, and other chemicals from shiny paper do not break down well during composting. When introduced to soil, residual chemicals from inks and dyes can be absorbed by plant roots, flowers, fruits, and vegetables. Plain newsprint paper, cardboard, paper towels, and napkins are better paper options.

Glossy Cardboard

Grease-stained pizza boxes and glossy cardboard materials pose problems similar to shiny paper waste. The inks, dyes, and waxy coatings used in water-resistant cardboard contain chemicals that persist through composting. It’s best to remove tape, staples, or glossy sections before adding plain cardboard to compost piles.

Charcoal Ash

While charcoal ash may seem like a natural compost addition, it is too alkaline and throws off the ideal neutral pH balance sought in compost systems. Charcoal ash can introduce compounds that are harmful when taken up by plant roots. Use it only sparingly and in very small amounts if you want to add it to an outdoor compost pile. Even small amounts of ash can overwhelm confined or indoor compost bins.

Sawdust from Treated Wood

Sawdust may help provide carbon content, but avoid using sawdust from pressure-treated lumber. Chemical preservatives from treated wood, like chromated copper arsenate, can be toxic. While composting temperatures may reduce levels of these contaminants, toxins often persist. These can be absorbed by plants along with nutrients once introduced to the soil through compost.

Coal or Wood ash

Misconceptions about adding coal ash or wood ash containing lime persist about their use as compost amendments. However, their alkaline properties can create an imbalance with acidic compost components. The lime can react with nitrogen, causing it to dissipate as ammonia gas. Trace metals and sulfur from coal ash contaminate compost over time. An excess of wood ash also raises pH levels too high for successful composting.

Large Bones

Large, thick bones from meat scraps should not go directly into compost piles. Dense bones do not break down readily or provide much nutritional value. As bones decompose very slowly, they take up space for long periods that other organic materials could occupy. If including bones, crush them into small fragments first, or substitute bone meal fertilizer instead.

Manure from Meat-Eating Animals

Herbivores grazing on grasses and plants provide manure safe for compost piles. However, manure from meat-eating animals like dogs, cats, or humans can contain pathogens that are dangerous if accidentally ingested. Compost piles do not sustain consistently high enough temperatures to eliminate these hazardous pathogens. Manure with possible harmful pathogens requires a specific composting setup to reach lethal temperatures throughout.

Invasive Weeds

Weeds like bindweed, Japanese knotweed, or kudzu are extremely hardy and fast-growing. Tiny root fragments left in compost can sprout wherever they are introduced. Seeds from weeds also tend to persist through the composting process. Avoid adding clippings or debris from aggressive or invasive weeds, and prevent them from infiltrating your garden spaces later on.

Large Branches and Twigs

While woody plant materials provide important browns in compost when chopped finely, adding long branches or large twigs complicates the process. Branches and twigs resist breakdown and take far longer than other organic materials to decompose. Chop or grind them into smaller pieces first before adding woody plant debris to your compost pile.

Walnut Tree Leaves or Twigs

Certain tree components, especially those from black walnut trees, release natural compounds that inhibit plant growth. Called “juglone”, the substance can damage some vegetables and other plants when released as walnut twigs and leaves decompose. Prevent this unwelcome outcome by never adding any part of a black walnut tree to compost piles or directly to garden soil.

Pine Needles and Cones

Pine needles and cones are also risky compost ingredients, though not toxic. As they are slow to break down, needles and cones can take up space for extended time periods before decomposing. Pine leaves and cones may also lower the pH level of compost into undesirable acidic ranges for proper decomposition. Chopped finely, pine needles can be added sparingly to outdoor compost piles. But avoid them in indoor or small compost bins.

Dog or Cat Poop

Never add dog or cat feces to compost piles, as pet waste can contain harmful pathogens. Parasites like roundworms, hookworms, and tapeworms can survive composting and infect humans later through contact with waste-contaminated soil or produce. While pet waste may seem natural, the risks it presents are simply not worth it.

Litter Box Waste

Both cat litter and its waste should stay out of compost piles altogether. Cat feces carry pathogens, while conventional clay litter contains bentonite clay that does not break down. Some cat litter brands also have chemical perfumes, clumping agents, or antimicrobials that persist and contaminate compost. Even biodegradable cat litter brands are not designed to compost. Avoid using any type of cat litter as an ingredient in compost piles or bins.

Liquids

Too much liquid causes compost piles to become overly saturated. Liquids like milk, cooking oil, or juice make compost into a dense, airless sludge over time. Never include liquid in large batches in compost all at once; it cuts off oxygen, increases odor, and halts the aerobic decomposition process. If adding small batches of liquid, mix it thoroughly into dry, bulky compost materials to absorb the moisture.

Vermicomposting Castings

Castings from worm compost bins still contain active microbiota even after passing through worms. If added to normal outdoor compost piles, these beneficial microbes will die off under hotter, more oxygenated conditions. Either use vermicompost castings directly in the garden or add them later to finished compost once temperatures have cooled and stabilized.

Dryer Lint

Lint easily passes through laundry dryer vents as tiny fibers and dust particles. But many common detergents and dryer sheet chemicals leave residues on lint. Composting does not sufficiently breakdown these synthetic additives. Chemical residues then end up contaminating soil and edible plants. Stick to lint-free natural cloth drying methods and avoid adding dryer lint to compost piles or bins.

Processed Foods

Heavily processed foods like chips, candy, baked goods, and junk food are never a good match for compost piles. Highly processed food waste breaks down much slower than raw fruits, veggies, grains, and plant trimmings. Processed foods also contain chemical preservatives, artificial coloring, sweeteners, and flavorings that persist through composting. These can throw off nutritional balance later when compost is added to the soil.

Biohazardous Medical Waste

Needles, syringes, blood-stained bandages, or other medical waste can introduce pathogens into compost that few systems generate enough heat to destroy. Even trace pharmaceutical residues on disposable gloves, masks, or tubing accumulate over time. Backyard compost piles are not equipped to neutralize hazardous medical waste and byproducts. Never place any biohazardous medical products into compost meant for food gardens.

Plastics and Synthetic Fibers

No type of plastic belongs in backyard compost piles, no matter how “biodegradable” or “compostable” it claims to be. Plastics contain synthetic polymers and petroleum byproducts that are impossible for microbes to break down. At best, plastics fragment into small bits that persist through composting. These microplastics later end up contaminating soils, absorbing toxins, and being ingested by wildlife. Keep all plastics, rubber, synthetics, and poly materials out of compost.

Pressure-treated wood

Wood products treated with copper chromated arsenate (CCA) contain toxic metals and pesticide residues. While composting can potentially reduce levels of these contaminants, it rarely eliminates them entirely. Avoid using sawdust, chips, or mulch from pressure-treated lumber anywhere near vegetable gardens or compost piles. Viruses, bacteria, mold, and fungi can also thrive in the moist environment of untreated wood scraps.

Waterlogged Wood Mulch

Wood mulch provides organic matter as it decomposes in garden beds over time. But when waterlogged, wood breaks down anaerobically and often develops persistent fungal or mold growth. Waterlogged wood, especially cedar and cypress mulches, also takes far longer to decompose. Let excessively wet wood dry out completely before adding it to compost piles to prevent density and aeration issues.

Treated Paper Products

Disposable paper plates, cups, towels, and napkins often contain added dyes, chlorine bleach, and other chemical residues from production. Unless specifically designed for commercial composting facilities, these treated paper items and leftovers will persist through backyard composting. Avoid adding bits of towel, plate, or cup that held and absorbed meat, oils, or other food scraps as well.

Old Compost Itself

Finished compost contains stabilized organic matter and fewer active microbes, so adding it back to piles is ineffective. Fresh compost doesn’t provide significant nutrition to decompose other wastes. The composting process works by carefully balancing common materials like fresh trimmings, leaves, manure, and food scraps. Feedstock for effective compost relies on these basic ingredients.

Bread, Grains, or Pasta

Starchy breads, cereal, grains, and dry pasta break down very slowly in compost compared to fruit and vegetable scraps. They can quickly accumulate and compact into piles to form dense masses with little airflow. The anaerobic environment encourages the growth of odor-causing molds and yeasts. If adding these items, balance them by mixing thoroughly with bulky plant debris to prevent densification.

Grease, Fat, or Oil

High-fat foods like lard, greasy bacon drippings, salad dressings, or vegetable oils break down minimally during composting. Fat particles coat other materials and the walls of compost bins, preventing access to oxygen. Rancid fats also produce foul odors. Small amounts of oils can be mixed into dry ingredients like sawdust, but avoid adding bulk grease, fats, or oils to compost piles.

Citrus Fruits and Onions

Citrus fruit peels, onions, and members of the allium family of plants contain acids and sulfur compounds. Though beneficial in moderation, large amounts lower the pH of the compost considerably. Too many onion peels or citrus wastes prevent other organic materials from properly decomposing. If adding these acidic items, balance them by mixing in a larger quantity of drier, neutral plant debris.

Hot Peppers

Chili peppers, habaneros, and any hot pepper varieties can be risky to work into compost piles. Capsaicin oils and seeds survive the composting process. When compost containing hot peppers is later handled or applied as mulch, the peppers can still burn skin on contact. They may also inhibit delicate plantings if the compost mix is too concentrated with residual oils. Avoid using hot pepper waste to prevent these unwanted effects.

Weed Seeds and Seed Pods

Immature seeds and seed pods rarely get hot enough during backyard composting to destroy their potential viability. Seeds from weedy plants and vegetables can sprout unwanted volunteers later on. Avoid adding seed heads, pods, or flowers gone to seed from herb plants. Whenever possible, sterilize compost that may contain weed seeds through solarization or maintaining higher temperatures.

Diseased Vegetation

Fungal diseases, powdery mildew, blights, and other common plant illnesses can persist through backyard composting. Diseases require extremely high, sustained temperatures to fully deactivate. Unless your pile reaches over 160°F, do not compost obviously diseased plants. Prevent contamination by removing and destroying sick specimens before they infect healthy plant debris destined for compost.

Tree Leaves and Needles

Large quantities of tree leaves and needles acidify compost and create airflow problems from density. A few layers mixed with other ingredients are fine. But piles composed mainly of leaves or needles break down too slowly. Chop or shred them first before adding them to compost to prevent matting and compaction issues. Otherwise, spread leaves as mulch or incorporate them directly into the garden soil.

Corn Cobs and Stalks

The woody nature and odd shape of corn cobs and thick stalks resist decomposition in compost. They take up space for long periods, slowing down turnover time and nutrient recycling. Chop coarsely or grind corn waste into small pieces for compost piles. Or purposely allow large stalks and cobs to decay over seasons when used directly as garden mulch on beds.

Brambles and Rose Stems

Thorny vines, rose stems, and plants with woody canes prove difficult additions to closed compost systems. Their odd shapes and stiff structure prevent compaction, increasing air pockets. Unless chopped before adding, they slow the process and take longer to break down. Avoid including any quantity of woody bramble vines, rose bush stems, or canes to keep compost piles uniform.

Full Branches and Shrub Trimmings

Intact branches and thick shrub prunings tax small compost piles quickly. They resist natural breakdown and take up significant space over time. However, large amounts of limbs and brush work well in designated outdoor holding piles. Allow them to slowly decay over years into wood chips to side-dress garden beds. But avoid adding to mixed compost piles meant for regular turnover.

Rotting Food from Storage

Old potatoes, mushy peppers, moldy strawberries, or any notably rotten produce from cold storage will not decompose effectively in compost. Food waste in advanced stages of decay lacks the proper carbon and nitrogen ratio ideal for composting. Remove rotten food directly from yard waste pickup or municipal compost instead of attempting to integrate it into garden compost piles.

Perfumed Items

Scented items like dryer sheets, soaps, and fabric softener clinging to towels contain chemical fragrances and detergents that do not break down readily. Residual compounds will persist in the finished compost. They can later contaminate edibles or affect plant growth. Avoid adding scented paper, fabric, or rags with perfumes, dyes, whiteners, and other additives to compost piles.

In summary, keeping certain items out of backyard compost piles helps streamline the process and creates safe, nutrient-rich additions for your garden’s soil. Follow these guidelines for what not to compost as you work waste materials into a sustainable resource supporting healthy plants. Divert problematic ingredients to trash or specialized disposal to avoid future headaches or contamination.