The Basic Rule: Browns and Greens in Balance

Compost works when you mix two types of material: greens (nitrogen-rich, wet things) and browns (carbon-rich, dry things). The ideal ratio is roughly three parts brown to one part green by volume, though your pile will still break down if you're off by a bit. Greens heat up the pile and speed decomposition; browns keep it from becoming a soggy, smelly mess and give microbes the carbon they need to work.

The easiest way to think about it: if something came from a plant and will rot, it probably belongs in your bin. The exceptions—and there are several—matter because they either won't break down, will attract pests, or will kill the microbes doing the work.

Key Takeaways

  • Greens include fresh grass clippings, vegetable scraps, coffee grounds, and plant trimmings; browns include dry leaves, shredded paper, cardboard, and wood chips.
  • Never compost meat, fish, dairy, oils, or pet waste, because they attract rodents, create odors, or introduce pathogens that home compost does not get hot enough to kill.
  • Diseased plants, weeds with seeds, and treated wood should stay out because they survive the composting process and will spread problems to your garden.
  • Shredding or chopping materials into smaller pieces speeds decomposition by weeks, especially for leaves and cardboard.
  • A working pile needs moisture like a wrung-out sponge and air mixed in; too wet or too packed will slow or stop the process.

Greens: The Nitrogen-Rich Materials

Greens are fresh, moist plant matter. Grass clippings are the most common—use them from an unmowed lawn or a mower without a bag. Vegetable and fruit scraps from your kitchen work well: carrot peels, apple cores, lettuce leaves, onion skins, banana peels, and coffee grounds (including the filter). Tea bags break down too, though remove any staples first. Plant trimmings from pruning, deadheading, or pulling weeds all go in, as long as the plants were healthy.

Fresh manure from herbivores—horse, cow, chicken, rabbit—is an excellent green and heats a pile quickly. Never use dog or cat manure; it carries pathogens unsafe for food gardens. Seaweed and kelp are greens if you have access to them. Even hair and nail clippings will compost, though they take longer than other greens.

A common mistake is adding too many greens at once. A pile that is mostly fresh grass or kitchen scraps will become compacted, airless, and anaerobic—it will smell like rotten eggs and slow to a crawl. Mix greens in layers with browns, or let grass dry for a day or two before adding it.

Browns: The Carbon-Rich Materials

Browns are dry plant matter. Fallen leaves are the backbone of most home compost piles—collect them in fall and shred them with a mower or leaf shredder to speed breakdown. Shredded paper, cardboard (remove tape and labels), and newspaper all work. Wood chips and sawdust from untreated wood are fine; avoid sawdust from pressure-treated lumber, which contains arsenic. Straw and hay break down, though hay may contain weed seeds—use straw instead if you can.

Dry grass clippings, plant stems, and wood ash from a fireplace or wood stove are browns. Pine needles and conifer needles take much longer to break down than deciduous leaves because they are waxy and acidic, so use them sparingly or skip them if you want faster results. Shredded junk mail, cardboard egg cartons, and paper towel tubes all belong in the pile.

The biggest mistake with browns is not shredding them. A pile of whole leaves will mat down and shed water like a roof. Run a mower over them, use a leaf shredder, or chop cardboard into strips. Smaller pieces mean more surface area for microbes to work on, and your finished compost will be ready weeks sooner.

What Never Goes in Compost

Keep meat, fish, bones, and any animal products out of a home compost bin. They attract rats, raccoons, and other pests, and they create strong odors. Dairy products—milk, cheese, yogurt, butter—do the same. Oils and fats, including cooking oil and salad dressing, will coat other materials and slow decomposition. Diseased plants should not go in because the pile rarely gets hot enough to kill fungal spores or bacterial pathogens; compost them separately or throw them away.

Weeds that have gone to seed will survive and spread when you use the finished compost in your garden. Pull weeds before they flower, or compost only the roots and stems. Treated wood, plywood, and particle board contain glues and preservatives that do not break down and will contaminate your soil. Glossy or colored paper, magazines, and anything with heavy ink should stay out. Pet waste from dogs and cats carries parasites and pathogens unsafe for vegetable gardens.

Avoid citrus peels in large amounts—they take a long time to break down and can slow the pile. A few peels are fine, but do not dump a whole bag in at once. Onions and garlic in large quantities can repel worms. Sawdust from treated wood, coal ash (use only wood ash), and synthetic materials like plastic or rubber obviously do not belong.

Getting the Moisture and Air Right

A compost pile needs moisture to function, but not waterlogging. The ideal texture is like a wrung-out sponge—damp throughout, but not dripping. If your pile is too dry, decomposition slows to almost nothing. If it is too wet, it becomes anaerobic and smells foul. In dry climates or seasons, water the pile as you build it. In wet climates, a cover or roof over the bin helps shed excess rain.

Air is equally important. Microbes that break down compost need oxygen. A pile that is packed too tightly or layered with only greens will become airless and stall. Turn the pile every week or two by moving material from the outside to the center, or use a compost aerator tool to poke holes through it. If you do not turn it, decomposition will still happen, but it will take two to three times longer—six months to a year instead of two to three months.

How to Layer and Build Your Pile

Start with a layer of browns on the bottom—shredded leaves or cardboard—to let air flow underneath. Add a layer of greens, then browns, then greens again, aiming for roughly three parts brown to one part green. If you have manure, a thin layer of manure counts as a green and heats the pile quickly. Chop or shred everything as you add it; smaller pieces decompose faster.

Water each layer lightly as you build, so the whole pile stays damp. Once the pile is built, cover it with a tarp or lid to keep rain out and heat in, but leave gaps for air. If you are adding to the pile gradually—kitchen scraps a few times a week—bury them under a layer of browns each time so they do not attract pests or smell.

A pile that is at least 3 feet by 3 feet by 3 feet will heat up enough to decompose quickly. Smaller piles take longer because they lose heat to the air. If space is tight, a tall narrow bin works, but you will need to turn it more often to keep it aerated.

Frequently Asked Questions

Can I compost bread, pasta, and cooked food?

Cooked food, bread, and pasta will compost, but they attract rodents and insects just like meat does. If you have a rat-proof bin with a tight lid and you bury them deep under browns, they may work. For most home setups, skip them. Raw vegetable scraps are safer and compost just as well.

What about glossy cardboard and pizza boxes?

Plain cardboard breaks down fine. Pizza boxes have grease on them, which slows decomposition and can attract pests. Tear off the greasy bottom and compost only the plain sides and lid. If the whole box is heavily oiled, throw it away.

Do I need to add compost starter or microbes?

No. Soil, finished compost, and plant material already contain billions of microbes. A handful of garden soil mixed into your pile will speed things up slightly, but a pile will decompose on its own. Commercial starters are not necessary and do not make much difference.

Can I compost sawdust from my workshop?

Only if it is from untreated wood. Sawdust from pressure-treated lumber, plywood, or particle board contains chemicals that will not break down and will contaminate your soil. If you are unsure whether the wood was treated, do not use the sawdust.

How do I know when compost is finished?

Finished compost looks like dark, crumbly soil and smells earthy, not like rotting food. You should not be able to recognize the original materials. It usually takes two to three months in a hot, well-turned pile, or six months to a year in a pile you leave alone. When it is done, use it in gardens, potted plants, or mix it into lawn soil.