Thinking about starting a compost bin in your garden? Already got a bin but having a few issues? Here are 10 things you shouldn't be putting in your compost!
In New Zealand there are official standards outlining best practices for composting, ensuring safe and effective compost production (these are officially known as NZS 4454:2005).
The Ministry for the Environment (MfE) and regional councils also provide guidance on composting to prevent soil contamination and protect ecosystems.
Here are 10 things you shouldn't be putting in your compost and some alternative solutions for them.
1. Coal or Charcoal Ash
- Why? Contains sulphur and heavy metals that can make soil toxic. Coal and charcoal ash can be harmful to plants and microorganisms in the compost due to their long-term toxicity.
- Reference: Bay of Plenty Regional Council warns that coal ash should never be added to compost due to its contaminating effects on soil.
- Alternative: Dispose of coal ash in general waste; use small amounts of untreated wood ash sparingly in garden beds.
2. Weeds with Seeds or Invasive Plants
- Why? Seeds and roots from weeds like wandering willie (Tradescantia fluminensis) and convolvulus (bindweed) can survive and regrow when compost is spread.
- Reference: Biosecurity New Zealand warns against composting invasive weeds, as they can spread and outcompete native species.
- Alternative: Dry or burn invasive plants before composting or use a hot composting method (55–65°C) to kill seeds.
3. Treated or Painted Wood Scraps
- Why? Wood treated with copper, chromium, arsenic (CCA) or boron-based preservatives can leach toxic chemicals into compost, contaminating soil and harming plants.
- Reference: Ministry for the Environment (MfE) recommends treated wood be disposed of at hazardous waste facilities, not composted.
- Alternative: Dispose of treated wood through council hazardous waste collection points.
4. Glossy or Coloured Paper and Magazines
- Why? Many inks and coatings contain synthetic dyes, heavy metals, and microplastics that do not break down in compost.
- Reference: Auckland Council advises against composting coloured or glossy paper due to potential chemical contamination.
- Alternative: Recycle through kerbside paper collections.
5. Citrus Peels and Onions (In Large Amounts)
- Why? High acidity in citrus peels and onions can slow compost decomposition and harm beneficial microbes and worms. Large quantities may also cause pH imbalances in small compost piles.
- Reference: Compost New Zealand suggests using citrus and onions sparingly, especially in worm farms, where they can negatively affect worm populations.
- Alternative: Chop citrus peels and onions into small pieces and add them in moderation, or compost them separately in a well-balanced system.
6. Oily or Greasy Foods (Including Cooked Food Scraps)
- Why? Fats and oils coat organic matter, slowing decomposition and attracting rodents.
- Reference: Wellington City Council advises against composting fats and cooked food due to pest risks.
- Alternative: Use a Bokashi bin or council food waste collection services.
7. Weed Killers and Other Chemicals
- Why? Some herbicides, such as clopyralid and picloram, persist in compost for years, harming plants and soil microbes. NZS 4454 warns against composting chemically treated materials.
- Reference: Environment Canterbury highlights that some herbicides in compost can cause plant deformities and stunted growth.
- Alternative: Avoid chemical weed killers or use organic alternatives like boiling water or vinegar sprays.
8. Synthetic or Biodegradable Plastics
- Why? Even "compostable" plastics often require industrial composting conditions and may leave behind microplastics. NZS 4454 excludes plastics from acceptable compost materials.
- Reference: WasteMINZ confirms that most "compostable" plastics do not break down properly in home composts and should be disposed of through appropriate waste streams.
- Alternative: Check if a commercial composting facility accepts compostable plastics or avoid using plastics altogether.
9. Flax (Harakeke)
- Why? Harakeke fibres are slow to decompose and can tangle in composting machinery.
- Reference: Te Papa’s Harakeke Weaving Guide explains that flax is naturally resistant to breakdown and should be repurposed rather than composted.
- Alternative: Cut flax into small pieces or use it as mulch instead.
10. Diseased or Pest-Infested Plant Material
- Why? Plants infected with fungal diseases like powdery mildew, rust, or blight can spread pathogens through compost. Pest-infested plants (e.g., those with aphids, whitefly, or scale insects) can carry pests into your garden.
- Reference: MPI’s Biosecurity New Zealand advises against composting diseased plant material, as some pathogens and pests survive the composting process.
- Alternative: Burn, deep bury, or dispose of diseased and pest-infested plants through council green waste collections.
By following NZS 4454 guidelines and local council recommendations, you can ensure your compost remains nutrient-rich, contaminant-free, and safe for use in gardens and landscaping