Not everything belongs in your compost bin! Discover the 7 things you should never throw on your compost heap — from onion skins to garden weeds — and learn how to keep your compost healthy, balanced, and pest-free.
What should you never put in compost?
Avoid composting onion skins, diseased plants, invasive weeds, meat, oily
foods, pet waste, and chemically treated materials. These items can attract
pests, spread disease, or disrupt composting microbes, leading to poor-quality
compost.
Introduction: Composting Done Right
Composting is one of the simplest and most effective ways to
recycle kitchen and garden waste, turning it into nutrient-rich soil that
nourishes your plants. But not all organic matter belongs in your compost bin.
Some items can slow decomposition, introduce toxins, or even kill off
beneficial bacteria.
To make sure your compost pile stays healthy and productive,
here are seven things you should never throw on your compost heap — and what
to do with them instead.
1. Onion and Garlic Skins
Onion and garlic skins contain natural antibacterial
compounds that can disrupt the microbial activity essential for compost
breakdown. Their dry outer layers also decompose slowly, delaying your
composting process.
Composting tip: Use them sparingly in worm composting or bury small
amounts directly in the garden soil.
2. Diseased Plant Material
Adding infected plant leaves or stems can reintroduce fungal
diseases like mildew or blight into your garden. Most home compost heaps don’t
reach the temperatures needed (above 55°C/131°F) to destroy plant pathogens.
Composting tip: Dispose of diseased material in the trash or burn it
safely to prevent spreading plant infections.
3. Persistent or Invasive Garden Weeds
Weeds with tough roots or seed heads — such as bindweed,
couch grass, or dandelions — can survive and sprout even after composting.
Unless your pile is consistently hot, they’ll reappear when you use your
compost.
Composting tip: Let weeds dry completely in the sun before composting,
or make “weed tea” fertilizer by soaking them in water.
4. Meat, Fish, and Dairy Products
Although these are organic, they cause unpleasant odors and
attract rodents, raccoons, and flies. They can also encourage the growth of
harmful bacteria, turning your compost anaerobic (airless).
Composting tip: Use Bokashi bins or municipal food waste services for
animal-based scraps.
5. Oily or Greasy Foods
Oil and grease create a waterproof layer that suffocates
your compost and prevents air circulation. The result? A smelly, slimy mess
instead of rich, crumbly compost.
Composting tip: Avoid adding cooking oils. Wipe greasy dishes with paper
towels and compost the paper instead.
6. Pet Waste
Waste from cats, dogs, or carnivorous pets may contain
harmful pathogens like E. coli or Toxoplasma gondii, which can
contaminate garden soil and food crops.
Composting tip: Only compost herbivore manure (like from rabbits or
guinea pigs). For pet waste, use a specialized composting system.
7. Chemically Treated Plant Material
Grass or leaves exposed to pesticides, herbicides, or
chemical fertilizers can introduce harmful residues into your compost. These
chemicals can persist for months and harm beneficial soil organisms.
Composting tip: Only compost untreated, organic material. When in doubt,
discard treated grass clippings separately.
Bonus Tip: What You Should Add to Compost Instead
For a thriving compost heap, maintain a balance between “greens”
(nitrogen-rich materials like food scraps and coffee grounds) and “browns”
(carbon-rich materials like dry leaves, cardboard, and straw).
Turn the pile regularly, keep it moist, and let the microbes
work their magic. The result will be dark, earthy compost ready to boost your
garden’s fertility naturally.
Conclusion: Smart Composting for Healthy Soil
Successful composting isn’t about throwing everything in —
it’s about knowing what to leave out. By avoiding these seven common composting
mistakes, you’ll protect your soil, prevent disease, and create nutrient-dense
compost that strengthens your garden ecosystem.
Compost smarter, not harder — your plants will thank you.
References
- Royal
Horticultural Society (RHS) – What not to compost
- University
of Illinois Extension – Home Composting Basics
- Environmental
Protection Agency (EPA) – Composting at Home
- Fine
Gardening Magazine – Compost Do’s and Don’ts
