What You (Really) Can’t Compost

I don’t know about you, but I find most “can’t compost” lists incredibly annoying.

They often include items like grass (I compost vast amounts of grass every year), weeds (an excellent source of compost material) and human waste (haven’t they read Humanure, Shit in a Nutshell?)

In fact, I often find it quite hard to think of organic materials that can’t or shouldn’t be composted under any circumstances. 

So in this article, I’ve taken a different tack; dividing items into what you really shouldn’t compost, and what you can compost but is either challenging or carries more risk. 

• Black walnut leaves (slows down plant growth)
• Ivy (resprouts like an alien monster)
• Coal (contains heavy metals and toxins)
• Gasoline, oil, petrol, lubricants (because your compost heap is not a Formula 1 pit stop)
• Treated wood products (harmful chemicals could leach into the compost)
• Ash from coal (contains sulfur and heavy metals; but wood ash is a great compost amendment)
• Styrofoam (won’t break down)
• Anything with plastic in it (breaks down into microplastics, you don’t want to be eating that in your homegrown veg!)
• Anything with metal in it (won’t rot down, could contaminate the soil)
• Grass/straw/plants treated with certain herbicides (while some will break down in compost, herbicides such as Aminopyralid or Clopyralid will supress plant growth)
• Invasive plants such as Japanese Knotweed (technically compostable, but if you get it wrong the results could be disastrous!)
• Paper or paper towels treated with chemicals 
• Large amounts of salt (can be damaging to soil and plants)
• Synthetic fabrics – won’t break down (wool or cotton are fine)

• Avocado pits and skins (can take a very long time to break down)
• Human waste (read Humanure: Shit in a Nutshell first!)
• Manure from carnivores (can contain pathogens. Best hot composted.)
• Dairy products (best hot composted, use in moderation)
• Fat/cooking oil (best used in moderation, and hot composted. However, studies show used cooking oil can be beneficial.)
• Meat (best hot composted or trench composted.)
• Fish and fish guts (tricky—read our guide first)
• Dead animals (best hot composted or trench composted. Bury deep!)
• Natural wine corks (slow to break down)
• Coconut shells (tough and fibrous – break up before composting)
• Bones (ask any archaeologist – they can take a long time to break down! However, human composting shows it can be done.)
• Egg shells (can take years to break down if not treated first).

• Diseased plants (because composting kills pathogens)
• Grass (but ensure you mix with brown materials)
• Weeds with seeds (but hot compost if you don’t want more weeds)
• Paper with ink on it (most modern inks are soy-based and safe. However, shiny/glossy paper is very tricky to compost.)
• Hair and pet fur (breaks down slowly but adds nitrogen)
• Natural fabrics such as cotton, wool, silk, hemp, linen (can take time, best shredded first)
• Plain cardboard and paper bags (great for both composting and vermicomposting, but shred first for best results.)
• Wine corks – the real ones, made from cork only! (Slow but will break down over time.)

• Compostable plastic bags (I’ve tried – turns into a slimy mess in the compost heap) 
• Bioplastics (as above)
• Compostable diapers and wipes (take ages in home compost)
• Most chewing gum (unless it doesn’t contain plastic)
• Pet waste bags labeled ‘biodegradable’ (again, need industrial composting conditions)
• Latex gloves and balloons (slower than a government paperwork request)
• Stickered fruit peels (because nothing says ‘natural’ like a tiny, indestructible piece of plastic on your banana)

Wondering about something not on this list?

Then ask yourself: 

• Has it recently been alive?
• Is it free from metals and plastic?
• Are you fairly sure it hasn’t been treated with harmful chemicals or herbicides?
• Is it on the regularly tweeted lists of “you can’t compost it”?

If the answer is yes to all of those, you can probably compost it!

• What You CAN Compost: From the everyday to the mundane, get more ideas for what to compost along with tips for how to compost each material. 
• Composting Myths Debunked: The Truth About ‘Non-Compostable’ Items: 7 Non-Compostable Myths Covered – Plus Edge Cases
• Carbon:Nitrogen Ratios: Learn the C:N ratio for common compostable materials, and use our calculator to work out the best mix.  
• What Can You Compost In Winter: Struggling for ideas for winter composting? Here’s 12…
• How Humanure Could Change The World: If we don’t turn up our noses first.