Can You Compost Onions?

Onions can be composted without problems, and concerns about onions harming a compost pile are largely unfounded. The sulfur compounds that give onions their distinctive smell do not harm the microbial community responsible for decomposition, and onion material, including skins, tops, root ends, and whole rotted onions, breaks down readily in an active pile.

The Earthworm Concern

Like citrus, onions are sometimes said to repel earthworms due to their sulfur content and pungent compounds. Earthworms in a worm bin may avoid fresh onion material and move away from it temporarily. In an outdoor compost pile this is largely irrelevant: earthworms concentrate in the cooler, more stable outer zones of a pile and are not the primary decomposers in any case. The temporary avoidance of one material in a diverse pile has no meaningful effect on decomposition.

The Smell

Onions added in large quantities to a compost pile will produce a sulfurous smell as they begin to break down. This is temporary and resolves as decomposition progresses. Burying onion material in the center of the pile and covering with a layer of dry brown material (cardboard or dry leaves) significantly reduces surface odors. A pile that smells strongly of onions is not in trouble, it is simply decomposing fresh allium material.

Onion Sprouting

Whole onions added to a compost pile may sprout before they break down, particularly in a cold or slow-moving pile. The sprouts are not a problem and will die back once the onion material decomposes. In a hot pile the sprouting is unlikely because the thermophilic temperatures destroy the plant tissue before germination occurs.

Onion sets (small bulbs intended for planting) should not be added to a cold pile if you do not want them to establish and grow. In a hot pile this is not a concern.

Best Practice for Adding Onions

Chop or quarter onions before adding them to increase surface area and speed breakdown. Add onion skins and scraps directly with kitchen waste. Large quantities of onions accumulated at once (for example, after processing a harvest) are best added in layers mixed with brown material rather than dumped in a single mass.