Cold composting: add, cover and wait
The method most yards can keep up: add scraps as they come, cover each addition with browns, keep the bin as damp as a wrung-out sponge, and wait. This chapter covers the routine, the moisture check, the browns you need on hand, winter, and the date to check.
- Start here: the essentials
- Add, cover and wait
- The squeeze test
- How long it takes, and what a cold pile does not do
- The browns supply
- Two bins in rotation, and tumblers
- Mixing is optional
- Winter and hot, dry summers
- The bin log
- Timelines, the squeeze test and mixing compared
- See it: Tomás and Hannah and David
- Practice: start the bin
- Your system
- Check your work
- Knowledge check and scenario
Start here Essentials
For a first pass, this chapter asks you to start a bin, add and cover your scraps, check the moisture once a week, and write down when to look for finished compost. If you chose a different method on your plan, or have no outdoor space, this chapter is optional: write "not for my setup" on the plan and go on.
- Start the bin with a layer of browns. Dry leaves, shredded plain paper or plain cardboard will do (add, cover and wait).
- Add each container of scraps and cover it with browns. Cover every addition so no food shows (add, cover and wait).
- Squeeze a handful once a week. Add water or dry browns to correct it (the squeeze test).
- Write the start date and a date one year out. The date is when to check for finished compost, not when it will be ready (how long it takes).
Minimum time: the first addition and one squeeze test, once; then each addition as it comes. Done when: the bin log has a start date, the first additions and one moisture check.
Add, cover and wait Understand
Cold composting is composting without trying to make the pile hot. You add material as it comes, cover it, keep it damp and wait. It is also called add-as-you-go composting, because you do not collect a whole batch first. Oregon State's guide describes an add-as-you-go bin with a hardware cloth floor to deter rats, mice and raccoons, and says it does not heat up enough to kill diseases and noxious weeds (Oregon State's EM 9475). It suits a household that has chosen low attention: the work is tasks you do when scraps come, plus a check now and then, and the cost is time. Convention
| Task | How often | What it does |
|---|---|---|
| Add scraps | When the kitchen container is emptied (Kitchen and collection sets this) | Gives the organisms their nitrogen and some water |
| Cover with browns | Every addition | Gives carbon and air, and keeps food from showing to animals and flies |
| Squeeze a handful | Once a week (this manual's convention; no source gives a frequency) | Tells you whether to add water or dry browns |
| Write a log row | With each addition or check | Records what went in and what the bin does |
| Mix the bin | Optional | Adds air and speeds breakdown (see mixing) |
| Check for finished compost | On the date you wrote, then when the signs show | Finished compost has the signs |
The first layer and the cover
Start with a layer of browns on the bottom. NC State's recipe layers four or five inches of browns with two or three inches of greens (NC State's AG-791). After that, each addition of scraps goes in and gets covered. EPA says to cover food scraps with four to eight inches of dry leaves or other browns (EPA's Composting at Home page). New York State's guide says to bury scraps under at least 3 inches of browns where flies or animals are a problem (NYSDEC's Home Composting Guide). We use one default: cover so that no food shows, with at least the 4 inches EPA gives as its low end if animals are near.
The cover and the ratio measure different things
EPA gives two rules that are easy to merge. One is a balance: two to three times as much browns as greens, by volume. The other is a cover depth. The ratio decides how much browns you need across the year; the depth decides how much one addition uses. The depth can use more. A bin with a surface of 2 by 2 feet (4 square feet), covered 4 inches (one-third of a foot) deep, takes 1.3 cubic feet of browns, or about 10 gallons, for one addition. That is a worked example of arithmetic, not a figure from a source, and it is larger than the ratio would give for a small addition of scraps. This is why the browns supply section tells you to log what you actually use.
The browns also keep the bin from going sour. Scraps are wet and rich in nitrogen, and without browns they pack down and smell (Choose your system explains why). Troubleshooting has the table for a bin that does.
The squeeze test
The squeeze test is how you check moisture without a meter. EPA's target is a pile as damp as a wrung-out sponge. Take a handful from inside the bin, not the surface, and squeeze it hard in your fist. Penn State's guide says to test at several depths (Penn State's Home Composting guide).
| What you see | What it means | What to do |
|---|---|---|
| No water, and the handful feels dry or falls apart | Too dry | Water lightly, a little at a time, and check again in a few days |
| A few drops come out | About right | Nothing; log "right" |
| Water streams out, or the handful is soggy | Too wet | Add dry browns, and keep rain out of an open bin |
The default is "a few drops". The University of Missouri says that if a few drops of water can be squeezed out, moisture is about right (University of Missouri's G6956), and NC State's bulletin says a drop or two. Test NC State adds that piles are most often too dry, which is worth knowing because a closed bin does not catch rain. New York State's guide notes that an enclosed bin may need more water for that reason. A source or two words the target as "no water" instead of "a few drops"; that wording is under Go deeper.
Write the result in the log's moisture column as dry, right or wet. If you correct it, check again within a few days, not within the hour; water takes time to spread through a bin.
How long it takes, and what a cold pile does not do
Plan on a year. EPA says a well-maintained pile is usually ready in about three to five months, and that left untended it may take a year; a cold pile you add to and do not turn is nearer the second. NC State gives one to two years for a pile left alone. Utah State says six months to two years without turning (Utah State's Backyard Composting in Utah). The sources span six months to two years, and a pile fed all year does not have one "ready date", because the top is newer than the bottom. Convention The timelines are side by side under Go deeper.
That is why Start here asks for a date one year out. It is a date to look, not a date of completion. Finished compost has signs: dark, crumbly, smelling of soil, no recognizable food. Finished compost teaches them, and also the germination test. Until the signs show, the bin is not finished, whatever the date says.
What a cold pile does not do
A cold pile makes compost, but heat is what reduces weed seeds and pathogens, and most cold piles do not get hot. Washington State's guide says slow composting is slower, cooler and less effective at killing weed seeds and pathogens (WSU's Backyard Composting, EB1784E). The University of Vermont says most compost piles do not get hot enough to kill seeds or pathogens (UVM's Three Ways to Compost), and LSU's guide lists, as a drawback of a static pile, that it does not kill weed seed or pathogens (LSU AgCenter's Backyard Composting). So the stays-out list for a cold bin includes weeds gone to seed and diseased plants (What goes in).
The times here are the sources' ranges, under their conditions. Your bin may take longer: what goes in, the size, the moisture and the weather all change it. A date on the plan is a date to look, and finished means the signs in Finished compost, not a date. A cold bin does not reliably deal with weed seeds or disease, so those stay out.
The browns supply
A cold bin runs on browns. Every addition needs them, and in most climates they arrive once a year, as fall leaves. The browns calculator sizes the supply. Its inputs are the gallons of scraps you make in a week (from your scrap log in What goes in), the ratio (2 to 1 or 3 to 1, inside EPA's two to three times), the weeks to cover (52 for a year) and the bag size (30 gallons is the default). Its output is browns a week, browns for the period in gallons and cubic feet (gallons divided by 7.48), and bags to store. Convention
Three things to know about the number.
- It is loose leaves by volume, not weight. Leaves settle in a bag and shrink as they dry, so the count of bags is an estimate. Adjust it after your first fall.
- It uses the ratio. It does not include the extra browns that a deep cover uses, which the previous section described.
- It starts from one week. A single logged week does not predict a year. Log again in summer and in the holidays.
Where browns come from, and how to store them, is Leaves, grass and yard; the materials inventory in What goes in lists what you have and when. If you run low, plain shredded paper and plain cardboard are browns (EPA's list), and What goes in says which papers stay out. A household that cannot get browns can send scraps to a drop-off instead of letting a bin go sour.
Two bins in rotation, and tumblers
One bin that you fill and leave has a problem: the material at the bottom finishes while you are still adding to the top. Two bins solve it. Washington State's guide describes a two-pile system for slow composting in which fresh yard debris and scraps go into one pile and the smaller pile from the previous year sits beside it and goes to the garden (Washington State, above). New York State's guide describes the slower multi-bin method: add to the first bin until it is full, then to the second; the first finishes, and once it is empty you can fill it again.
- Fill bin A for about a year, adding and covering as in this chapter.
- When A is full, stop adding to it and start bin B. A keeps finishing.
- Check A for the signs (Finished compost) while you fill B. Use or give away what has finished, then empty A.
- When B is full, go back to A.
A tumbler runs on the same idea, in one drum, and sometimes with a second one. Oregon State says to rotate a tumbler daily, or at a minimum several times a week, and to use two tumblers if possible. The Regional District of Central Kootenay says that when the drum is full it should rest, about 6 to 8 weeks, and to turn it 1 to 4 times a week while you fill it (RDCK's tumbler composters factsheet). It adds that a 7.5 cubic foot model is generally large enough for a family of four with no yard, and that the contents are heavy to turn when full, so some people fill only two-thirds. RDCK also warns against a maker's claim of finished compost in under four weeks. We treat a tumbler as add-and-rest: rotate several times a week while you add, stop adding when it is full, then rest it. With one tumbler, scraps need somewhere else to go during the rest, which is a drop-off or a freezer container. Product
Mixing is optional
You can run a cold bin without mixing it. New York State says turning or mixing the pile twice a month adds air and speeds breakdown (NYSDEC's Home Composting page), and NC State says a pile left alone may take one or two years against several months if you aerate it weekly. Mixing buys speed and air. It costs a task each time. For a low-attention method, our default is not to mix; if you do, twice a month is New York State's figure. Preference
If an unmixed bin turns sour or smells of rotten eggs, the table in Troubleshooting gives the usual causes, which are too much water and too little air, and the first change is to add dry browns. Whether an unmixed pile stays aerobic is a question on which two sources read differently, under Go deeper.
Winter and hot, dry summers
Winter
Penn State says a pile goes dormant in winter and starts to decompose again when air temperatures rise above about 50°F. A bin in a freezing climate does not fail, but it stops. Three things from the sources help it through.
- Keep adding and covering. New York State's guide says that when you add food scraps in winter you pull back the top layer, add them, and cover them again.
- Insulate with browns. The guide says to build the pile larger as cold weather approaches and cover the top with a one-foot layer of leaves, or surround the bin with a one-foot layer of lightweight browns on each side and the top; bagged leaves or straw bales can be piled around the bin, and added to the browns stockpile in spring. The state's web page says to add a layer of browns around the bin for a frozen pile. Utah State says to increase the pile size or add straw.
- Know what to expect. The University of Missouri says little decomposition occurs in winter except in the center of large piles. A small bin may freeze through. The guide says tumblers can freeze, and Oregon State notes that small two-chamber tumblers are too small to insulate themselves in its winters (Oregon State's compost tumbler answer).
None of the sources we read for this chapter describes a small bin frozen solid for months. So the evidence for your bin is your bin: log the date it freezes, what you add, and what the squeeze test and the smell show when it thaws. A frozen handful will not squeeze, so skip the check until the thaw. The winter line on your plan can be as short as "keep adding and covering; bank bags of leaves around the bin; check at thaw".
Hot, dry summers
In a dry climate the problem reverses. The University of Arizona's guide says that in the hot summer months you may need to water the pile weekly, that covering it slows evaporation but also blocks rain, and that a shaded location helps reduce water loss (University of Arizona's AZ1632). Colorado State says moisture must be added in its climate (Colorado State's PlantTalk #1627), and Washington State's Whatcom County brochure says to water lightly in summer and cover in winter (WSU Whatcom's backyard composting brochure). The squeeze test is the check. The seasonal calendar (seasons) marks the tasks by climate.
The bin log
A bin log is the bin's memory. The bin log has one row per visit: date, bin, greens and browns added in gallons, moisture (dry, right, wet), smell (earthy, none, sour, ammonia, rotten) and notes. Leave the temperature blank for a cold bin. Use the same container each time so the gallons compare. The log serves three purposes in this chapter: it shows what you actually add and cover, which settles the browns question; it shows moisture over time; and it holds the dates you will need in winter and at the check. Record what you saw, and decide elsewhere.
Timelines, the squeeze test and mixing compared Go deeper
Timelines for an unturned or slow pile
| Source | Range | Conditions the source gives |
|---|---|---|
| EPA | About a year | Left untended; three to five months for a well-maintained pile |
| NC State | One to two years, or several months | Left alone; several months if aerated weekly |
| NYSDEC (web page) | 6 months to one year | Ready when dark and crumbly |
| NYSDEC (guide) | 4 to 12 months | The composting process; its poster says six months to a year |
| Utah State | 6 months to 2 years | Without turning; monthly turning, 4 to 6 months; weekly turning, 1 to 2 months |
| Missouri | 3 to 9 months | The "traditional" method |
| Minnesota | A year or more | An untended pile (University of Minnesota Extension's Composting in home gardens) |
| Washington State | Two years | A two-pile slow system |
The ranges overlap and none is measured on your bin. The shorter ones assume some turning.
The squeeze test: the exception
Most sources read here say a few drops should come out: Missouri, NC State, Colorado State, Texas A&M and New York State's web page. Utah State words it the other way: a handful is too wet if water can be squeezed out. Washington State's EB1784E says both, that you should not be able to squeeze water out of the mix and that if you can barely squeeze out a drop, the moisture is ideal. In practice the wordings meet near "a drop or two". If your local extension office words the target differently, follow it.
Mixing and emissions
A study of six single-family home composting units in Denmark, followed for a year, found that the units that were mixed weekly emitted the most methane and nitrous oxide, and the units that were not mixed the least (Andersen and colleagues, Technical University of Denmark). It is six units in one country. It is one reason this manual does not call mixing better, and it does not say what your bin would emit.
Do unturned piles stay aerobic?
LSU's guide says an unturned pile limits aeration and creates anaerobic conditions; Cornell's composting pages say some systems keep enough oxygen passively (Cornell's Compost Chemistry page); Utah State says well-built heaps suit no-turn composting. The difference may be construction, size and browns, which is why the smell is the check.
A three-bin cold rotation
New York State's multi-bin slower method continues "and so on" past two bins. With three, you fill A, then B, then C; by the time C is full, A has had two years to finish. It uses more space and more browns storage; it suits a larger yard.
See it: Tomás and Hannah and David See it
Tomás Reyes is a fictional person, and every fact about him here is invented for this manual. He rents a duplex with a small fenced yard in a northern city where the ground freezes from December to March. His bin is an enclosed plastic bin of about 11 cubic feet with a lid, in the back corner. His first scrap log was 1.5 gallons in a week. The silver maple filled 9 thirty-gallon bags with leaves in his first fall. He starts the bin on October 17 (an invented date), writes "next October 17" as the date to check, and leaves it as a cold bin. Meat, fish and dairy go in the trash.
Known: the bin, the 1.5 gallons, the 9 bags (270 gallons). His assumptions: his scraps stay near 1.5 gallons a week; he uses 2 to 1 browns, inside EPA's range; the bagged leaves stay dry. The numbers. 1.5 gallons a week at 2 to 1 is 3 gallons of browns a week, 156 gallons over 52 weeks (about 21 cubic feet), which is 6 thirty-gallon bags. At 3 to 1 it is 234 gallons, 8 bags. Under these assumptions his 9 bags cover both. If he covers each addition 4 inches deep, as EPA suggests, he may use more; his log tells him.
The winter. The bin freezes in January. He keeps adding and covering. It thaws and shrinks in April. He skips the squeeze test while it is frozen, and does one at the thaw. Whether to bank some bags of leaves around the bin is a choice New York State's guide offers for a bin that freezes, and he has not decided.
Open: whether 9 bags last the year (the calculator and his log; Leaves, grass and yard returns to it); whether the bin fills before next October (at 4.5 gallons a week, 82 gallons of bin volume gives an upper bound near 18 weeks, and shrinking makes it longer, so he writes the level in the log each month); what the thaw smells like (Troubleshooting if it is sour); and when his compost is usable (the signs in Finished compost, not a date).
Hannah Bauer and David Lin (fictional) own a house with a backyard and two sugar maples. Their scrap log was 4 gallons in a week. They get about 20 bags of leaves each fall. Their plan is two enclosed bins with hardware-cloth floors, used in rotation: fill one for a year, let it finish while filling the other. The 10-year-old empties the kitchen container on Mondays and Thursdays.
Known: two bins; 4 gallons in a week; about 20 bags. Their assumptions (Hannah's): a bag is 30 gallons; 4 gallons a week holds for the year; browns go in at 2 to 1. The numbers. Under these assumptions: 4 gallons a week for 52 weeks is 208 gallons of scraps, browns at 2 to 1 are 416 gallons, and the total is 624 gallons, about 83 cubic feet before the material settles. Twenty 30-gallon bags are 600 gallons, so the leaves cover the 416 gallons of browns. Open: the actual size of each bin (the pile calculator and a tape measure settle it); whether a bin holds a year of additions after settling (their log will show when it fills; EPA says a pile shrinks to about a third of its original size); the real size of their bags (they measure one); and whether they cover deep enough to keep the rats away (Site and bin has the checks).
Practice: start the bin Practice
Minimum: make the first addition, do one squeeze test, and write the start date and the check date in the log. Time: about 30 minutes, once; then each addition as it comes. Materials: your bin in place (Site and bin), a container of kitchen scraps, a supply of browns, the bin log, a pitchfork or trowel and a watering can or hose.
- Put a layer of browns in the bottom. A few inches of leaves, shredded paper or plain cardboard.
- Add the scraps and cover them. Measure both in gallons with the same container, and write them in the log.
- Squeeze a handful from inside the bin. Record dry, right or wet; correct it if needed.
- Write the start date and a date one year out in the log and on the plan's "first check for finished compost" line.
- Write the winter line on the plan, even in a mild climate: "keep adding" or "pause and use the drop-off".
Run the browns calculator. Enter your logged gallons, a ratio, 52 weeks and your bag size. Compare the bags with what you have stored, and write the answer in the browns supply line of the plan, with the date.
Your system Your system
Check your work Check your work
Criteria
- Does the log show a start date, a first addition with greens and browns in gallons, and one moisture check?
- Does every addition get covered?
- Does the plan show a date one year out, labeled as a date to check?
- Does the plan name where browns come from and how many bags or gallons are on hand?
- Does the plan have a winter line?
Worked example
Tomás's first row reads half a gallon of greens, a gallon and a half of leaves, moisture right, smell earthy, no temperature. His plan has October 17 as the start, "next October 17" as the check date, 9 bags of leaves stored, and a winter line: "keep adding and covering; skip the squeeze while frozen; check at thaw". It passes. His plan does not say when the compost will be ready, and does not say the 9 bags last the year, because neither is settled yet.
Common mistakes
- Treating the one-year date as a deadline. It is a date to look.
- Adding scraps without covering them. Food on top draws flies and animals.
- Running out of browns in spring. The leaves arrive once a year. Count the bags in fall.
- Adding water when the handful is merely cool. Squeeze first.
- Letting weeds that have gone to seed or diseased plants into a cold bin. It does not reliably deal with them.
What is still unresolved
When your compost is finished is settled by the signs in Finished compost. Whether your browns last is settled by your log across a year. Whether you keep up the routine is what Your compost system reviews.