The basics: greens, browns, and water

Compost works because it mixes two types of material: greens (nitrogen-rich) and browns (carbon-rich). The microorganisms that break down your waste need both in roughly equal amounts—think of it as a 1:1 ratio by volume, though home composters rarely measure exactly. Greens are usually wet and smell fresh; browns are usually dry and smell earthy. Add water when the pile feels like a wrung-out sponge, not soggy.

The pile also needs air. If you're using a bin, turn it every week or two with a pitchfork or shovel. If you're using an open pile, turn it when you think of it. Turning breaks up compacted material and lets oxygen reach the microbes doing the work. Without air, you get anaerobic decomposition—which smells terrible and takes much longer.

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, dairy, oils, pet waste, diseased plants, or weeds with seeds, because they attract pests, spread disease, or survive the process.
  • A working pile should feel like a wrung-out sponge and smell earthy; if it smells like ammonia, add browns; if it's dry, add water or greens.
  • Finished compost takes two to six months in a hot bin with regular turning, or six to twelve months in a passive pile you leave alone.
  • Shred or chop large items before adding them so they break down faster and don't create air pockets that slow decomposition.

What to add: greens that speed decomposition

Greens are the nitrogen engine of your pile. Fresh grass clippings are the most common—add them in thin layers so they don't mat down and block air. Vegetable and fruit scraps (peels, cores, wilted lettuce, carrot tops) break down quickly. Coffee grounds and tea bags (remove the staple from tea bags first) add nitrogen and moisture. Plant trimmings from pruning, deadheading, or pulling weeds also work, as long as the plants weren't diseased.

Manure from herbivores—horse, cow, chicken, rabbit—is excellent if you can get it, though it can carry weed seeds if the animal ate them. Never use dog or cat manure; it carries pathogens that don't die in home compost. Grass clippings from a lawn treated with herbicide or pesticide should sit for a few weeks before composting, or skip them entirely if you want to be cautious.

What to add: browns that build structure

Browns are the carbon backbone. Dry leaves are the easiest source—shred them with a mower or by running over them with a leaf blower to speed breakdown. Shredded paper, cardboard (remove tape and labels), and newspaper all work. Wood chips and sawdust from untreated wood are fine, though they decompose slowly. Straw (not hay, which has seeds) adds bulk without compacting.

Avoid treated wood, glossy paper, and cardboard with plastic coating. If you're unsure whether wood is treated, don't use it—treated wood contains chemicals that won't break down and can contaminate your finished compost. Sawdust from pressure-treated lumber or plywood is a common mistake; stick to sawdust from a woodworker using untreated stock.

What never goes in: the no-compost list

Meat, fish, bones, and grease attract rats and raccoons and take months to decompose even in a hot pile. Dairy (milk, cheese, yogurt) does the same. Oils and fats coat particles and slow microbial breakdown. Pet waste (dog and cat feces) carries pathogens that home compost doesn't reach hot enough to kill. Human waste also belongs in a septic system or sewer, not a compost pile.

Diseased plants (powdery mildew, blight, rust) can survive the process and spread disease to your garden next year. Weeds with mature seeds will germinate in your finished compost. Glossy magazines, colored paper with heavy inks, and plastic-coated cardboard don't break down. Treated wood, coal ash (use only wood ash), and synthetic materials like rubber or plastic have no place in compost.

Citrus peels, onion skins, and garlic can slow decomposition and repel worms if you add too much at once—use them sparingly. Black walnut leaves and twigs contain a chemical called juglone that inhibits plant growth; compost them separately if at all. Pine needles are acidic and decompose very slowly; they're better left as mulch under acid-loving plants.

How to layer and balance your pile

Start with a 4- to 6-inch layer of browns on the bottom—this lets air flow underneath. Add a 4- to 6-inch layer of greens. Repeat until your bin is full or your pile is about 3 feet tall. Sprinkle water between layers so everything is moist but not waterlogged. If you're adding kitchen scraps regularly, bury them under a layer of browns each time so they don't attract flies.

If your pile smells like ammonia or rotten eggs, you have too many greens or too little air. Add more browns and turn the pile. If it's not heating up and smells musty, add more greens or water. If it's dry and not breaking down, add water or fresh grass clippings. A healthy pile should feel warm in the center when you turn it—that heat is the microbes working.

Speeding up the process with size and turning

Smaller pieces decompose faster than large ones. Chop vegetable scraps into chunks rather than leaving them whole. Shred leaves instead of layering them whole. Run a mower over cardboard before adding it. This increases surface area for microbes to colonize and prevents large air pockets that slow the process.

A pile that's turned weekly in warm weather can produce finished compost in two to three months. A pile you turn every few weeks takes four to six months. A pile you never turn (passive composting) takes six to twelve months or longer. If you don't have time to turn, passive composting still works—it just takes patience. In cold climates, decomposition slows dramatically in winter; many gardeners stop adding to their pile in fall and harvest finished compost in spring.

Knowing when your compost is ready

Finished compost looks like dark, crumbly soil and smells earthy. You should not be able to identify the original materials—if you can still see leaves or cardboard, it's not done. Pick up a handful; it should hold together loosely but crumble when you squeeze it. If it's still hot in the center, it's still decomposing.

Sift finished compost through a 1/2-inch screen to remove chunks that didn't break down completely. Return those chunks to a new pile to decompose further. Use finished compost to amend garden beds, pot up seedlings, or top-dress lawns. It won't burn plants the way fresh manure can, and it improves soil structure and water retention year after year.

Frequently Asked Questions

Can I compost bread, pasta, and cooked food?

Cooked food, bread, and pasta attract pests the same way meat does. Skip them. Raw vegetable scraps, fruit peels, and coffee grounds are safe; cooked versions are not. If you want to compost food waste without attracting rats, use a sealed bin with a tight lid and bury scraps deep under browns.

What if my compost smells like rotting garbage?

That smell means anaerobic decomposition—not enough air or too many greens. Turn the pile when ready and add dry browns like shredded leaves or cardboard. If you can't turn it, spread it out and let it dry for a few days, then rebuild it with more browns mixed in. The smell should fade within a week.

Can I use sawdust from my workshop?

Only if it's from untreated wood. Sawdust from pressure-treated lumber, plywood, or MDF contains chemicals that won't break down and will contaminate your compost. If you're not certain the wood was untreated, don't use it. Sawdust from a hardwood floor or furniture refinishing is also risky unless you know the finish wasn't toxic.

Do I need to add compost starter or microbes?

No. Microbes are already on your materials—in the soil clinging to vegetables, in grass clippings, in leaves. A healthy pile with the right moisture and air will heat up on its own. Commercial starters are not necessary and won't speed the process significantly if your pile is already balanced.

Can I compost diseased plant material if I let it get really hot?

Home compost piles rarely reach the sustained temperature (160°F for several days) needed to kill most pathogens. It's safer to throw diseased plants in the trash or burn them if local rules allow. If you want to compost them, keep that pile completely separate and don't use the finished product on the same plants that got sick.