What to compost: greens, browns and what stays out

How much you make, what it is, and what your method cannot take. The chapter sorts materials into greens and browns, measures one week of scraps in gallons, and ends with a stays-out list on your plan.

Greens and browns: a working sort Understand

Choose your system introduced greens and browns. The sort is the working tool of this chapter, so here it is in full. Greens are wet, nitrogen-rich materials: fruit and vegetable scraps, coffee grounds, fresh grass clippings. Browns are dry, carbon-rich materials: dry leaves, straw, shredded plain paper, plain cardboard, untreated wood chips. EPA's lists follow this sort (EPA's Composting at Home page). The names are not a color code. Coffee grounds are dark brown and count as greens; a pile of green grass left to dry in the sun behaves more like a brown.

Behind the sort sits one idea, the carbon-to-nitrogen ratio (written C:N): how many parts of carbon a material holds for each part of nitrogen, by weight. Microbes use carbon as food and energy and nitrogen to build their bodies, so a pile needs both. Cornell's composting science pages give about 30 parts carbon to 1 part nitrogen by weight as the usual target at the start; NC State gives a range of 25:1 to 35:1 (Cornell's Compost Chemistry page, NC State's AG-791). Cornell says that below the target a pile loses nitrogen as ammonia gas, and above it the pile stays relatively cool. Physical principle

The table below is the one table of ratios this manual teaches. The numbers come from a 1991 compilation by Dickson and colleagues that both Cornell and NC State reproduce. Read each as a typical range, not a measurement of your leaves. Cornell adds that newspaper, corn stalks and straw break down slowly, so a higher starting ratio can be planned for them.

A ratio of 30:1 sounds like a recipe, but nobody weighs a kitchen scrap bucket. Two things turn the idea into a rule you can use. First, browns are light and bulky and greens are heavy and wet, so a volume rule stands in for the weight rule. EPA's rule is to add at least two to three times the volume of browns to the volume of greens. Convention Second, wetness matters more than the ratio. Cornell's mixing page says moisture is the more critical variable and advises erring toward a high C:N, which means a little more brown. The Go deeper section on mixing by weight shows why with Cornell's own example.

Our default is EPA's rule, starting at two parts browns to one part greens by volume and adding more browns if the bin is wet or smells sharp. The University of Vermont's extension gives the same range, one part greens to two or three parts browns by volume (UVM's Three Ways to Compost). Other sources give anything from equal parts to three parts; the comparison is under Go deeper, and The slow pile shows how to read the bin and adjust. Browns do a second job beyond balance: a layer of them over fresh scraps hides the scraps and soaks up moisture, and EPA says to cover food scraps with four to eight inches of dry leaves or other browns (The slow pile).

The scrap log: measure your own week

How much do you make? The honest answer for most households is "I do not know", and every size decision after this one depends on it: the kitchen container (Kitchen and collection), the bin (Site and bin), the browns you must store (the browns calculator) and a worm bin if you choose one (Worm bins). A scrap log is a daily record of the volume of food scraps you collect, kept for one week. Your own week beats any average, so we measure it.

The method uses a kitchen container and a measuring cup. Each day, before you empty the container or add a new day's scraps, note what went in and how much. There are 16 cups in a US gallon, so 4 cups is a quarter gallon and 8 cups is half a gallon. Pack the scraps loosely and the same way each day. The sheet takes gallons, with a cups column for small amounts, and adds the week's total for you (scrap log sheet).

• Record only what you would put in your method. If you throw eggshells out, they are not in the log. If you would put coffee filters in, they are.
• Write what went in, not only how much. The "what" column shows your mix: mostly peels, mostly coffee grounds, a lot of spoiled food. You use it again in the prevention section below.
• Log what your method cannot take in the notes column. Meat, dairy and cooked leftovers are not part of the volume, but they are part of the plan, because they need somewhere to go.
• Note an unusual week. Guests, a holiday or a week away change the total. If the week was not typical, log a second one.

For context only, here is an average. The Natural Resources Defense Council studied households in Denver, Nashville and New York, with one-week kitchen diaries, and reported in 2017 an average of 3.5 pounds of all discarded food per person per week (NRDC's city-level food waste report). That figure counts liquids poured down the drain and food that was edible when thrown out; 32 percent of the weight, by NRDC's breakdown, was inedible parts such as peels and bones, which works out to about 1.1 pounds per person per week by our arithmetic. It is a measure of all wasted food in three cities. It is not a forecast for your household and it is not a measure of what you could compost. We give no conversion from pounds to gallons, because no source we read gives one. Your log does.

What each method takes, and what stays out

Every method has a list of what it takes. The lists differ because the methods do. A backyard pile that stays cool does not break down meat and dairy cleanly, and it draws animals. A sealed bokashi bucket ferments them. An in-ground digester absorbs them into the soil. A drop-off program sets its own list. The table gives one default per row and the method it applies to.

The sources behind the rows are EPA and NC State's gardener handbook for the pile and worm bin, WSU Extension Kitsap County for bokashi, the University of Saskatchewan's trench page and Nebraska Extension's trench page for the trench, Banyule Council's green cone guide for the digester (an Australian council's guide to one product), and NYC Sanitation's drop-off page for the program example. Worm bins and Bokashi and burying cover bokashi, trenches and digesters in full. Convention

What stays out of every home method this manual teaches

Some materials stay out of every row above, for reasons that do not depend on the method. We state each reason as the source gives it.

• Dog and cat waste, and cat litter. The next section explains why.
• Plants treated with weed killer, and clippings from a treated lawn. Some herbicides (aminopyralid, clopyralid and picloram) can remain active in manure and compost, in hay and straw, and in grass clippings, sometimes for years (NC State's herbicide carryover publication). Leaves, grass and yard covers the check; here it is on the list so you know to ask.
• Diseased plants and weeds with seeds. EPA lists them to avoid, and Clemson says it is best to discard weed seeds and diseased plant material instead of composting them (Clemson's composting factsheet). The reason is heat: NC State says a cold pile does not destroy seeds and pathogens, and a hot pile has to pass 131°F for most pathogens and 145°F for most weed seeds. The hot pile covers piles that do. Convention
• Treated, painted or pressure-treated wood, plywood, and their sawdust. EPA says wood treated with chromated arsenicals should not be reused in products such as mulch and should not be burned in a home setting (EPA's page on chromated arsenicals). University of Illinois Extension advises avoiding sawdust from lumber treated with CCA (Illinois Extension's composting questions). The sources differ on whether pressure-treated wood is acceptable for building a bin; Site and bin covers that.
• Coal and charcoal ash. NC State says charcoal ash and coal are resistant to decay and may contain substances harmful to plants. Wood ash from untreated wood is a separate case: NC State says a handful or two may be added and that more may harm microbes and cause ammonia smells. Ash from treated wood, waste oil, plastics or garbage stays out (University of Wisconsin's wood ash guide).
• Glossy paper and produce stickers. EPA lists both to avoid. Tape and wax on cardboard go with them: EPA's add list is cardboard without wax, tape or glue.
• Tea bags with plastic. University of Maryland Extension says many tea bags contain plastics and advises composting only plastic-free ones, or emptying the tea into the bucket and putting the bag in the trash (Maryland's how-to page).
• Plastic, synthetic fabrics and anything labeled "compostable" that was not made for a home bin. See the labels section below.

Where the things your method cannot take go

For each material your method leaves out, the plan needs one of three answers: the trash, a program that takes it, or a second method that does. A household that cooks with meat and dairy, uses a backyard bin and has no program near it usually lands on the trash, and writes that down. A household that wants meat and dairy kept out of the trash looks at a drop-off or curbside program (Kitchen and collection), at bokashi (Bokashi and burying) or at an in-ground digester (Bokashi and burying). None of these is the right answer by default; each costs space, money or attention. Write the answer on the plan in the line called "Where scraps go that your method cannot take".

Dog and cat waste

Dog and cat waste, and cat litter, stay out of every method in this manual. That includes bokashi, trenches and worm bins, and it holds for a pile that gets hot. We give three reasons, each from a named source.

• Parasites in the waste can reach people through soil. CDC says cat feces can carry Toxoplasma and advises gloves when you touch soil that may be contaminated and handwashing after gardening. CDC's toxocariasis page says roundworm eggs from dog and cat feces can get into people's mouths through contaminated dirt or unwashed hands, and that severe cases are more likely in young children who play in or eat contaminated dirt (CDC on toxoplasmosis prevention, CDC on toxocariasis). CDC says people at higher risk for toxoplasmosis should talk to their healthcare provider.
• Extension services keep it out of food gardens and piles. Penn State Extension says pig, dog, cat and human waste do not belong in a vegetable garden, and the University of Maine's bulletin says to keep cat, dog and pig manure out of vegetable gardens and compost piles (Penn State's manure page, University of Maine Bulletin 2510). Every list in the sources we read, including EPA's, NC State's and Maryland's, excludes pet waste.
• Home piles do not reliably get hot enough. BPI limits pet-waste products to commercial-only certification in Canada because backyard bins do not typically get hot enough to kill the pathogens in pet waste (BPI's certification page). NC State's temperature figures for destroying pathogens apply to a pile that reaches them, and the University of Vermont's extension says most compost piles do not get hot enough to kill seeds or pathogens.

What to do with the waste instead is your city's or hauler's rule to state. Most households bag it and use the trash. Some programs say so explicitly: Burnaby's green bin page lists cat litter and dog poop bags among the items it does not accept (Burnaby's food scraps and yard waste page).

"Compostable" on a label

Plates, cups, forks, bags and takeout boxes marked "compostable" or "biodegradable" are among the most common questions about what goes in. A label is a claim about a test, and the test is not your bin. We define four terms.

• Compostable means the maker claims the item breaks down into compost in a stated setting, usually an industrial composting facility.
• Industrially compostable means tested at the heat of a commercial facility. BPI's commercial certification tests disintegration at 58 degrees C (about 136°F) for up to 84 days (BPI's certification page).
• Home compostable means certified in a test at lower temperatures. BPI's home certification, based on a French standard, tests disintegration at about 25 degrees C (77°F) for up to 180 days and biodegradation for 12 months. It requires commercial certification first.
• Biodegradable means the material can be broken down by microbes. The label does not say how fast, in what setting, or into what.

EPA lists compostable foodservice ware and compostable bags among the items to avoid in a backyard pile, because backyard piles generally do not reach high enough temperatures to fully decompose them. Convention TÜV Austria, the certifier behind the OK compost marks, says a product certified only for industrial composting should not go into the garden compost, and that a biodegradable product may be broken down by microorganisms without becoming good compost (TÜV Austria's OK compost HOME page). BPI, speaking of products certified for home use, says they may take 12 months or longer in practice (BPI's guide to composting certified products at home).

The Federal Trade Commission's Green Guides give marketers a rule for these claims. A marketer needs competent and reliable scientific evidence that an item breaks down into usable compost in a safe and timely manner in the setting named, and has to qualify the claim if the item is not suitable for home composting (16 CFR 260.7, compostable claims). An unqualified "degradable" claim is deceptive for an item entering the solid waste stream if it does not completely decompose within one year after customary disposal (16 CFR 260.8, degradable claims). That is a rule about what a maker may say, not a statement about what happens in your bin.

The default for a home bin follows from this: leave out items marked "compostable" unless the item is certified for home composting and you accept a year or more of waiting. Programs differ, because a program's facility sets its own rule. Portland's page tells residents that items labeled "compostable" belong in the trash, while Seattle accepts bags labeled compostable and Burnaby accepts only paper (Kitchen and collection, liners). Ask the program; do not read the label for the answer.

Ask your program If your food scrap program takes compostable packaging or bags, its posted list says so. If you cannot find a list, treat the item as not accepted until you hear otherwise.

The materials inventory: browns through the year

The scrap log measures what you make in the kitchen. The materials inventory lists everything else you have or can get: each material, whether it is green or brown, where it comes from, when in the year it arrives, about how much, where it is stored and, for anything from outside your own yard, what you know about herbicide (materials sheet).

The inventory exists because of a timing problem. Greens arrive all year from the kitchen. Browns do not: leaves come down in a few weeks in the fall, and a bin that needs two to three parts of brown for each part of green needs browns in June and February as well as October. A household that writes down when its browns arrive and where it stores them finds the gap before the bin does. The browns calculator turns your weekly scrap volume and the ratio into gallons of browns for the weeks you choose, and adds the number of 30-gallon bags that is.

• Yard browns: leaves, dried grass, spent plant stalks and twigs, untreated wood chips. A yard with trees may have more than the bin needs; a yard with none may have none.
• Household browns: shredded non-glossy, uncolored paper, plain cardboard without wax, tape or glue, and paper tea bags, from EPA's add list. A household with no yard can still keep a bag of shredded paper.
• Outside browns: a neighbor's leaves, a business's wood chips, bagged leaves, straw or hay. Outside materials carry the herbicide question from the stays-out list: leave the herbicide column on the sheet blank until you have asked, and see Leaves, grass and yard (herbicide check) before you use grass clippings, hay, straw or manure from somewhere you do not control.
• Greens beyond the kitchen: fresh grass clippings, garden trimmings, plant waste. These swing the ratio the other way and arrive in bursts. Leaves, grass and yard covers them.

Note two things on the sheet that no table can supply: how much your yard drops, and how much you can store. Both are yours to measure. An open pile of leaves outside is a store, but it is also a pile that is already decomposing, so write where each store is and in what container. The slow pile covers keeping browns through the season (browns).

Prevent first

The scrap log has a second use. It shows you what you throw out, and some of that should not have been thrown out. EPA's 2023 report on food waste management pathways ranks eleven ways of handling wasted food and finds that source reduction, donation and upcycling are the most environmentally preferable, because they can displace additional food production (EPA's From Field to Bin report). The ranking is called the Wasted Food Scale. The page we read does not rank home composting separately from other composting. The order matters for a household: buying and cooking less that spoils comes first, and composting handles what remains, including the peels, grounds and bones that no one would eat.

NRDC's three-city study is a reminder of the scale. It found that 57 percent of the weight of discarded food was typically edible, 11 percent questionably edible and 32 percent inedible. That is an average from three cities in a 2017 report, not your kitchen. Look at the "what went in" column of your own log: if it fills with whole produce that went soft or leftovers that sat, the lesson for that week is upstream. Learn Meal Planning covers it in its chapter on leftovers and using food up (Learn Meal Planning, leftovers and using it up). This manual does not repeat it.

See it: two scrap logs See it

Practice: log a week, list the browns, write the stays-out list Practice

Minimum: keep the scrap log for seven days, list your browns, and copy your method's stays-out row to the plan with a place for each item. Time: seven days of logging, then about half an hour once. Materials: your kitchen container, a measuring cup, the scrap log sheet, the materials sheet, the system plan, and the materials table.

1. Measure and log a week. Use the measuring cup before you empty the container each day. Enter what went in and how much, and write what you left out in the notes.
2. List every brown. One row per material on the materials sheet: yard, household and outside. Add when in the year it arrives and where you would keep it.
3. Look up your method in the materials table. Filter it by your method. Write the rows marked out into the "Stays-out list" line of the plan.
4. Write where the left-out items go. For each, enter the trash, a named program, or a second method in "Where scraps go that your method cannot take".
5. Ask about anything from outside your yard. Note the question and the date on the plan's rules checklist ("Asked what was sprayed"). Leaves, grass and yard covers what to do with the answer.

Your system Your system

Check your work Check your work

Criteria

• Does the scrap log show seven days and a total in gallons?
• Does the materials sheet list each brown, with when it arrives and where it is kept?
• Does the plan's stays-out list match your method's row in the table, with the reason for any item you added or removed?
• Does every item your method cannot take have a destination: the trash, a named program, or a second method?
• Does the plan say anything it cannot know, such as a program's list you have not read?

Worked example

Tomás's log totals 1.5 gallons for a week with one away day. His materials sheet has maple leaves (brown, October and November, 9 thirty-gallon bags) and notes that the lawn is untreated. His plan's stays-out list names meat, fish, bones, dairy, plastic, compostable-labeled items, glossy paper, ash and weed-killer-treated plants, and sends the first four to the trash. Noor's log shows 1 gallon. Her stays-out list is the program's posted list, and she writes the rice as "ask the program". Both pass; neither claims a total or a program rule it has not read.

Common mistakes

• Logging from memory. A week reconstructed on Sunday night undercounts. Measure each day.
• Counting only the clean scraps. If you leave out what you would throw in the trash anyway, the plan has no destination for it.
• Using an average instead of the log. The NRDC number is for all discarded food in three cities. It does not size a bin.
• Reading a "compostable" label as permission. The label describes a test, not your bin.
• Listing only the fall's browns. The inventory should have a row that covers June.

What is still unresolved

How much you make across a year is settled by the log and the second week. What a program takes is settled by the program's posted list (Kitchen and collection). Whether outside grass, hay, straw or manure carries persistent herbicide is settled by asking the supplier and by the test in Leaves, grass and yard. Whether your landlord, association or city allows a bin is settled in Site and bin.

Knowledge check and scenario

Checklist

• The scrap log shows a week's total in gallons.
• The system plan lists what stays out, and where each item goes instead.
• The materials sheet lists my browns and when they arrive.
• I read the mixing formula and the comparison of sources. (Go deeper; optional)