The easiest way to get started composting is with kitchen scraps and the food waste we make while cooking. Home-cooked meals and a healthy compost pile go hand-in-hand and are the start of a healthy home garden. Ideally, all the food scraps we don’t use in our meals should become the organic matter that builds our soil.
Onions are a staple for most kitchens, and onion skins are likely to find their way into a compost bin. The larger onion scraps and sometimes fresh sprouts all contain plenty of organic materials for your compost heap. But unlike other veggie scraps, onions are odorous food scraps that might not be great for backyard compost.
Can You Compost Onions?
Cooked onions and onion scraps count as green material in a compost pile’s decomposition process. Onion waste and other acidic food scraps like coffee grounds and moist materials, like fruit, can turn a compost pile into an anaerobic compost pit. Most bacteria work best in a more alkaline compost as the breakdown of materials naturally leads to an acidic environment.
Plenty of brown materials like dry leaves, paper towels, and pizza boxes should be added to the compost to balance the onions compost. Onions should not be added directly to the garden soil as the smell can attract pests and increase the acidity around plants. Make sure household kitchen waste with a strong smell, like pet waste, meat, and entire onions, is buried deeply and allowed to mature into finished compost over several months.
Are Onions Safe For Vermicomposting?
Worms can break down food faster than a traditional compost pile. A worm bin is a great way to increase the rate at which fresh compost is created. These little guys have a special diet that can’t handle everything that soil microbes readily break down, and vegetables like onions can cause problems for worms.
Worms do not actually eat the food scraps we put in their farms. Although the waste materials pass through the worm and are discarded in an enriched form as vermicastings, the food is not being consumed. Instead, the worms are absorbing the countless bacteria that colonize every inch of the organic waste in a compost pile.
Why Onions Are Bad for Worms?
Many species of the genus Allium, like onions and garlic, have extremely powerful antimicrobial properties. Much like the antibiotics doctors prescribe to the sick, onions will destroy all bacteria, good and bad, that it comes into contact with. The antimicrobial characteristics of onions are attributed to saving countless lives during the black plague. Homes with halved onions set out to purify the rooms saw significantly less death from the world’s deadliest plague than those that were onionless.
The healing and bacteria-killing properties of onions and garlic are so pronounced that they are even fabled to ward off vampires and other creatures of the night with their mere aroma. While all of these attributes may seem positive, they wreak havoc on an environment designed to cultivate a perfect balance of bacterial activity. Worms eat the bacteria in our compost, and if that bacteria is dead, our worms will starve.
How To Render Onions Harmless to Worms
Not all onions are equal, and certain steps can be taken to reduce the antibacterial effects of onions. Red onions, for example, are some of the strongest onions in aroma and affect bacteria within a large area. Both contact and the gases coming off of onions will kill the microorganisms nearby.
Just like how we reduce the tear-inducing effects of onions, we can also dilute the negative aspects of onions, making them safer for worms to ingest. White onions and yellow onions are less potent for microbial eradication and will likely not cause much damage to worms. Likewise, peels and the outermost onion sections will produce fewer chemicals than the inner areas.
Soaking onions in water will help them not burn your eyes but will reduce the flavor. Rinsing your onion peels before composting them can wash off some of the bacteria-killing compounds. Letting onion scraps dry in the open air for a few hours or overnight before adding them to worm bins can also reduce the negative issues associated with onion bulbs.
Cooked onions and garlic will have far less of an impact on soil health than raw onions. Cutting the onion scraps into small pieces before adding them to the worm bin can also help reduce the acidity entering the worm bin. Selectively adding small amounts of onions with plenty of alkaline and worm-friendly materials can help worms break down alliums safely.
What Are The Risks of Composting Onions?
Composting onions can have risks similar to those found in worm bins. Onions can cause the pH balance to shift and build up larger anaerobic zones that lead to stench and pest problems. These areas of rot are especially prevalent when a pile is flipped, and the moldy onions are exposed to air. This is a recipe for bad fungus and nasty pests to ruin your compost.
Onions are very aromatic and may attract hungry pests. The gas given off from rotting onions smells like many things and can attract scavengers looking for a carcass as well as foragers knowledgeable in bulbs. If onion scraps are not buried deeply enough, these pests might drag them out of the pile and make a mess in your yard.
Onions can also cause undesired sprouting from within a compost pile. Backyard composts often do not reach the temperatures needed to kill off germinating seeds and sprouts. Most of the time, conditions in the compost piles don’t allow new growth however when onions are near, things change.
Onions give off high levels of ethylene gas which can cause nearby produce to sprout. Roots and vegetables like potatoes, garlic, and other onions can be tricked into growing instead of decomposing. The longer the onions have been composting and the smaller the size of the onion, the less gas it will produce.
Other Vegetables That Need to Be Carefully Composted
There are a few other roots and vegetables that need to be composted carefully. While in most cases, compost will break down any organic toxins present in our food waste, there can be immediate adverse effects if care isn’t taken. Check out the information below before composting these kitchen ingredients.
| Vegetable | Parts Composted | Adverse Effect |
| Rhubarb | Leaves | Highly toxic to humans and animals if ingested |
| Potatoes | Leaves and tubers | Toxic leaves and peels on tubers turn green and toxic in direct sunlight. |
| Garlic | Bulbs, peels, and tops | Kills microorganisms, insects, and bacteria in the soil |
Rhubarb
This is a common kitchen vegetable that is usually made into dessert dishes. Only the stem of rhubarb is consumed as both the leaves and roots are toxic to humans and animals. While the toxins in rhubarb will not leech into the soil and affect future plants, the raw leaves can be an issue.
If rhubarb leaves are not properly buried, they could end up being eaten by a pet or wild animal. Ingestion of the leaves causes bad sickness and could be fatal to some creatures. Make sure any leaves that are toxic are covered until the toxins break down sufficiently over several months.
Potatoes
A member of the deadly nightshade family, along with tomatoes and eggplants, potatoes have a nasty side. Potatoes are harvested from the soil and sold with no green parts attached. Unless your garden at home has had potatoes, you probably have never seen a potato plant, only the root. One reason for that is that the leaves are toxic.
Toxic leaves and stems are not the only reason potatoes need to be composted carefully; when exposed to direct sunlight, even the familiar brown of potatoes turns to moss green. This color change is an indicator that glycoalkaloids have increased and that the potato is now toxic to mammals. Bury all potatoes and their leaves and stalks deeply in a compost pit and make sure they are not exposed to sunlight or accessible to animals.
Garlic
Unlike onions, it is much more difficult to make garlic less potent. Rinsing and letting it dry out have some effect, but garlic will remain potent much longer than onions. Not only is the smell stronger, but garlic is an insecticide as well as an antimicrobial, killing the main decomposition agents in your compost pile
Too much garlic in compost will drastically slow the decomposition rate. It is much better to use garlic to make a natural insecticide, and then after all the potency is extracted, discard it with plenty of good bacteria-building waste deep into your compost heap.