Brown and green materials make compost work
A working compost heap needs two types of material: browns (carbon-rich, dry things like leaves and cardboard) and greens (nitrogen-rich, wet things like grass clippings and food scraps). The pile breaks down fastest when you layer them roughly equally by volume. Browns alone decompose very slowly; greens alone turn into a wet, smelly mess. The balance between them is what creates the right conditions for microbes to do the work.
What goes in depends partly on what you're composting for—a hot pile that reaches high temperatures kills weed seeds and pathogens, while a cold pile takes longer but needs less attention. Most home gardeners use cold composting, which means being more careful about what you add.
Key Takeaways
- Greens include grass clippings, vegetable scraps, coffee grounds, and fresh plant trimmings; browns include dry leaves, shredded paper, cardboard, and straw.
- Meat, fish, dairy, oils, and diseased plants should never go in a home compost heap because they attract pests or spread disease to your garden.
- Coated or glossy paper, pet waste, and treated wood will not break down properly or may introduce toxins into your finished compost.
- Layering browns and greens in roughly equal amounts by volume keeps the pile from becoming too wet or too dry and speeds decomposition.
- A finished compost pile takes three months to two years depending on pile size, turning frequency, and whether you use hot or cold composting.
Greens: nitrogen-rich materials that speed decomposition
Greens are the wet, fresh materials that heat up the pile and feed the microbes doing the work. Grass clippings, vegetable and fruit scraps, coffee grounds and tea bags, fresh plant trimmings, and plant-based kitchen waste all count as greens. Crushed eggshells are fine too—they break down slowly but add minerals. Fresh manure from herbivores (horse, cow, rabbit) is a green, though it should be aged a few weeks before adding to a cold pile to avoid burning plants when you use the finished compost.
Avoid adding meat, fish, bones, dairy, oils, or fats—these attract rats, raccoons, and flies, and they take far longer to break down than the rest of your pile. Diseased plant material should also stay out of a cold compost heap; if you have a hot pile that reaches 140°F or higher, disease pathogens die, but most home piles do not get that hot.
Browns: carbon-rich materials that balance moisture
Browns are dry, woody, or fibrous materials that absorb moisture and create air pockets in the pile. Dry leaves, shredded newspaper, cardboard (with tape and labels removed), straw, wood chips, sawdust, and shredded paper all work. Avoid glossy or coated paper—the coating does not break down and may contain chemicals. Newspaper and plain cardboard are safe because the inks used in modern printing are soy-based and non-toxic.
Do not add treated wood, plywood, or wood that has been painted or stained—these contain chemicals that will end up in your finished compost and eventually in your soil. Pine needles and conifer leaves are acidic and break down very slowly, so use them sparingly if at all. Sawdust from untreated wood is fine, but sawdust from pressure-treated lumber or MDF board should never go in.
Materials to keep out of your compost heap
Pet waste (dog or cat feces) carries pathogens that survive cold composting and can contaminate vegetables you grow. Human waste has the same problem. Diseased plants, as mentioned, can spread disease to your next crop unless your pile gets hot enough. Weeds that have gone to seed will survive the pile and spread when you use the compost, so either remove seed heads before composting or use only a hot pile.
Synthetic materials—plastic, rubber, metal, glass—do not break down and should never go in. Charcoal ash from a grill contains chemicals and should stay out; wood ash from a fireplace is fine in small amounts (it raises pH), but do not add it if your soil is already alkaline. Coal ash should never be composted. Glossy magazines, treated paper, and anything with a plastic coating will not decompose.
How to layer your pile for faster breakdown
Start with a layer of browns about 4 to 6 inches deep on the ground—this lets air flow underneath. Add a layer of greens about the same depth, then alternate. Aim for roughly equal volumes of each, though the exact ratio is flexible; if your pile smells like ammonia, add more browns; if it is not heating up and breaking down, add more greens.
Chop or shred materials before adding them—smaller pieces have more surface area for microbes to work on and decompose faster. A pile that is at least 3 feet by 3 feet by 3 feet will heat up better than a smaller one, but even a small pile will eventually compost. Turning the pile every week or two speeds things up by introducing oxygen, but turning is optional if you are willing to wait longer.
Finished compost and what it looks like
Finished compost is dark brown, crumbly, and smells earthy—not like rotting food or ammonia. You should not be able to identify the original materials. A cold pile typically takes six months to two years to reach this stage, depending on pile size, how often you turn it, and the mix of materials. A hot pile that you turn regularly can finish in three to four months.
You can use partially finished compost in garden beds or as mulch, but fully finished compost is best for mixing into soil or using as a potting medium. If you pull out material that is not fully broken down, put it back in the pile or use it as a brown layer in a new pile.
Frequently Asked Questions
Can I compost citrus peels and onions?
Yes, citrus peels and onion skins are fine to compost. They break down slowly because of their tough outer layer, but they will eventually decompose. Chop them into smaller pieces to speed things up. Some gardeners worry that citrus slows decomposition, but the effect is minimal in a balanced pile.
What about bread, pasta, and cooked food?
Plain cooked grains and pasta are technically compostable, but they attract pests in a cold pile and can create a slimy layer that blocks air flow. It is safer to leave them out unless you have a hot pile or a sealed bin. Bread is the same—it will compost, but the smell and pest risk usually make it not worth it.
Can I put weeds in my compost?
Weeds without seed heads are fine. If they have already flowered or gone to seed, leave them out unless your pile reaches 140°F or higher—cold piles do not get hot enough to kill seeds, and you will spread weeds when you use the compost. Perennial weeds with roots are also risky because the roots may survive and regrow.
Is it okay to add wood ash from my fireplace?
Wood ash in small amounts is fine and adds potassium and calcium to your compost. Do not add it if your soil is already alkaline or if you have a lot of ash—too much raises pH too high and can lock up nutrients. Never add charcoal ash or ash from treated wood.
How do I know if my pile is too wet or too dry?
A working pile should feel like a wrung-out sponge—moist but not dripping. If it smells like ammonia or is slimy, it is too wet; add more browns and turn it. If it is not heating up and breaking down after a few weeks, it is too dry; add greens or water and turn it. The pile should feel warm in the middle if decomposition is happening.