Gnomes, plastic chairs, artificial Christmas trees, dog poo and watering cans are being thrown out with grass cuttings in Wales.
Councils are urging residents to stop shoving anything from the garden in with the recycling to reduce contamination.
Authorities said often people wrongly put fruit in their garden waste bin.
In Cardiff, the council is now considering fining people £100 if they fail to recycle properly.
Across Wales some councils are now charging people for green waste collections – including lawn cuttings and twigs which are composted – in a bid to balance budgets.
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But in a report, Cardiff council, which collects garden waste for free, said some people were putting “anything” in the green bin, including gnomes, shed parts and asbestos.
The council, which has a co-mingled recycling system, would join a number of other authorities in introducing warnings and large fines to those who fail to recycle properly.
Out of the 40,000 tonnes of recycling collected in the capital, nearly 10,000 are rejected and have to be incinerated due to not being rinsed, or being contaminated by non-recyclable products.
While 5% of the 18,000 tonnes of garden waste collected each year is contaminated.
Under the proposed changes those breaking rules will be warned by having a pink warning sticker placed on the offending bag or box, a letter will then be sent, and if rules continue to be broken they could be fined £100.
What are people putting in garden waste bins?
A number of councils told BBC Wales some of the biggest problems were people putting dog poo, fruit and soil in their green recycling bins.
The collections are only meant to be for twigs and lawn cuttings.
Other items found in the bins include:
- Watering cans
- Green plastic chairs
- Garden furniture
- Parts of shed
Ceredigion council said it recently had an artificial Christmas tree placed in the garden waste container at one of its household waste sites.
Powys council said that it used to see a “huge amount of contamination” in garden waste banks at community recycling sites, including building materials, broken fence panels, furniture, toys and plastic bags.
“Since we have moved to a fortnightly household subscription collection service we have experienced virtually no contamination in the garden waste collected,” a spokeswoman said.