Most untreated wood can go in compost, but it breaks down slowly and works better in larger piles where it won't crowd out faster-decomposing materials

Wood belongs in compost, but with conditions. Raw, untreated wood chips, sawdust, and small branches will eventually decompose—they're carbon-rich materials that feed the microbes doing the work. The catch is that wood takes months or years to break down, depending on the size of the pieces and how active your pile is. In a small home compost bin, wood can take up so much space that it leaves less room for the materials that actually speed decomposition. In a larger pile or one you're actively managing, wood works fine as long as you follow a few rules about what kind of wood and how much.

The wood must be untreated. This means no paint, stain, sealant, or chemical preservative. Sawdust from a woodworking shop, chips from a chipper, and shavings from your own yard are all safe. Pressure-treated lumber, which is common in decks and fences, is not safe—the chemicals don't break down and will contaminate your finished compost.

Key Takeaways

  • Untreated wood sawdust, chips, and small branches decompose in compost but take much longer than leaves or grass clippings.
  • Never compost wood that has been painted, stained, pressure-treated, or sealed, because the chemicals do not break down and will contaminate your finished compost.
  • Hardwood takes longer to decompose than softwood, so pine and fir chips break down faster than oak or maple.
  • Chop or chip wood into small pieces before adding it; whole logs or large branches will sit in your pile for years without breaking down.
  • Wood works best in compost piles larger than three feet on a side, where it won't dominate the pile and slow the overall decomposition process.

Untreated wood that is safe to compost

Untreated wood in any form—sawdust, wood chips, shredded bark, small branches, or wood shavings—can go into your compost pile. This includes sawdust from a table saw or hand planer, wood chips from a chipper, and the wood shavings sold as animal bedding (as long as they haven't been treated with pesticides or fungicides). Cardboard and paper are also wood products and compost readily.

The key word is untreated. The wood must come from a source where you know nothing has been applied to it. Sawdust from a lumber yard or woodworking shop is usually safe if the shop doesn't use pressure-treated lumber. If you're not certain, ask the source directly. Branches from your own yard are safe as long as they haven't been sprayed with herbicide or fungicide.

Wood to keep out of your compost

Pressure-treated wood is the most common problem. Lumber treated to resist rot and insects contains copper compounds or other preservatives that do not break down in a compost pile. These chemicals will remain in your finished compost and can harm plants or contaminate soil. Pressure-treated wood is typically used for decks, fences, and outdoor structures and is often greenish or grayish in color, though not always—ask if you're unsure.

Painted, stained, or sealed wood should also stay out. Paint, wood stain, and polyurethane finishes contain chemicals that do not decompose. They will persist in your compost and end up in your garden soil. This includes plywood and particle board, which are glued together and may contain formaldehyde or other adhesives that break down slowly or not at all.

Treated wood shavings sold as animal bedding sometimes contain antifungal or antimicrobial treatments. Check the label. If it says the shavings have been treated for dust control or mold prevention, they are not suitable for compost.

Why wood decomposes slowly

Wood is mostly cellulose and lignin, compounds that take a long time for microbes to break down. Leaves and grass clippings decompose in weeks or a few months because they have less structural material. Wood, especially hardwood, can take a year or more to fully break down, even in an active compost pile.

The size of the wood pieces matters a lot. A whole log might sit in your pile for several years without changing. Wood chips an inch or smaller will decompose faster. Sawdust decomposes faster still because it has more surface area for microbes to work on. If you're adding wood to compost, chop or chip it into pieces no larger than half an inch.

How much wood to add

Wood should make up no more than 10 to 20 percent of your compost pile by volume. Too much wood crowds out faster-decomposing materials and slows the whole process. A good rule is to add wood gradually as you add other materials, rather than dumping a large pile of wood chips all at once.

In a small bin—under three feet on a side—wood can dominate the pile and prevent it from heating up and breaking down efficiently. In a larger pile or one you turn regularly, wood integrates better and decomposes alongside other materials. If you have a lot of wood waste, consider a separate pile just for wood, or use it as mulch on top of garden beds instead of composting it.

Speeding up wood decomposition

Smaller pieces decompose faster, so run wood through a chipper or shredder before adding it. Sawdust decomposes much quicker than chips, which decompose faster than branches.

Wood needs nitrogen to decompose efficiently. Add nitrogen-rich materials—grass clippings, food scraps, manure, or coffee grounds—alongside the wood. A ratio of about three parts carbon (wood, leaves, paper) to one part nitrogen (grass, food waste, manure) keeps decomposition moving. Without enough nitrogen, wood will sit in your pile for years. Turning your pile regularly also speeds decomposition. Every two to four weeks, move material from the outside of the pile to the center, where it's hotter and microbes are more active. This exposes wood pieces to more microbial activity and breaks them up as you turn.

Wood ash and charcoal—do not compost

Wood ash from a fireplace or wood stove should not go in compost. Ash is alkaline and can raise the pH of your compost too high, making nutrients less available to plants. A small amount occasionally won't harm a large pile, but regular additions will throw off the balance.

Charcoal from a grill should also stay out. Charcoal is treated with chemicals to help it light and burn, and it does not decompose in a compost pile. Both ash and charcoal are better left out of the system entirely.

Frequently Asked Questions

Can I compost wood from a pallet?

Only if you know the pallet is untreated. Many pallets are heat-treated (marked HT) and are safe to compost. Pallets marked MB have been treated with methyl bromide, a pesticide, and should not be composted. If you don't know the history of a pallet, it's safer to leave it out.

How long does wood take to fully decompose in compost?

Sawdust takes two to six months in an active pile. Wood chips take six months to two years. Small branches take one to three years. Whole logs can take five years or longer. Speed depends on pile size, how often you turn it, and how much nitrogen-rich material you add.

Can I use wood from diseased trees?

Yes, if the tree was diseased with a fungal or bacterial disease that affects that species. Compost piles heat up enough to kill most pathogens. However, if the tree had a virus or a disease that affects many plants, it's safer to avoid it.

Is it okay to add sawdust from my workshop?

Only if the wood was untreated. Sawdust from pine, fir, oak, or other untreated lumber is fine. If your workshop uses pressure-treated lumber, stained wood, or plywood, don't compost the sawdust from those projects.

Can I compost wood that's been sitting outside and is starting to rot?

Yes. Partially rotted wood decomposes faster than fresh wood because fungi and bacteria have already started breaking it down. Chop it into small pieces and add it to your pile.