How Long Does Compost Take to Be Ready?

The published times run from about a month for a hot, turned pile to a year or more for one nobody tends, and the sources do not agree on a single number for the middle. Each figure below belongs to the kind of pile it came from.

Illinois Extension lists the size of the pile, its materials and how finely they are cut, its moisture and how often it is turned as the things that change the time a pile takes. The chemistry behind them is in how composting works, from ratios to pile temperatures.

Frequent turning shortens the time, and the sources differ on when to start. Illinois Extension waits at least two weeks so the center can heat and then turns every 2 to 4 weeks, UC Master Gardeners turn after about three days in the 130 to 160°F range and at least weekly, and Oregon State restacks the pile when it starts to cool at 5 to 10 days.

A hot pile starts at about 1 cubic yard (3 by 3 by 3 feet) or more, with enough nitrogen-rich material in it to heat. Mix equal amounts of chopped greens and browns, add water, and check the center after a few days with a compost thermometer or by touch. The active range is 130 to 160°F, and heat above 140°F destroys most harmful pathogens and weed seeds, according to Clemson.

If the pile does not heat up, grass clippings, manure or coffee grounds add the nitrogen it lacks. When it starts to cool, usually after 5 to 10 days, pull it apart and restack it with the outside material in the middle and the middle material on the outside.

Cold composting

A cold pile takes scraps and yard waste as they come, with little or no turning. It never heats enough to kill diseases or noxious weeds, so diseased plants stay out, and weeds go in only before their seed heads form, never the kinds that spread by roots or rhizomes.

The slower pile has one advantage for vegetable beds. High temperatures in a hot pile kill certain fungi and bacteria that help suppress soil-borne diseases, and those beneficial microbes stay intact in slower compost.

Finished compost passes all of these checks together.

If a few clumps remain, hardware cloth with ½-inch squares screens them out, and the screened compost works as a 1 to 2 inch mulch.

Using compost before it is cured

Compost that has stopped heating still needs a rest before it touches tender roots. For seedlings or vegetable beds, Oregon State has it cure another 4 to 6 weeks, and Penn State has compost for a seed-starting mix sit an additional month or two so that microorganisms that could attack seedling roots leave the pile or die.

The wait protects germinating seeds, which are sensitive to the volatile fatty acids often present in uncured compost. Very immature compost can also carry ammonia and volatile organic acids that kill or injure plants.

Nitrogen is a second risk, since compost with a carbon-to-nitrogen ratio above 20, which is typical of woody composts, can tie up soil nitrogen in the first season, while well-composted material settles at 10 to 15. Compost used as mulch around established woody ornamentals can go on sooner.

A stuck pile is short of air, water, size or greens, and the symptom shows which.

To have compost for spring planting, Penn State starts the pile in fall while the garden is cleaned up for winter. The pile goes dormant over winter and starts to decompose again when air temperatures rise above 50°F, and a second pile started in spring supplies fall planting.

Warm summer temperatures speed up the bacteria and winter slows them, so Illinois Extension recommends stopping the turning after November to keep heat in the center of the pile.

Illinois Extension suggests keeping two piles, one holding finished compost ready for the garden and the other holding unfinished compost. For what finished compost does once it is in the bed, see the benefits of composting.