These are not included in the specification for the Materials Recovery Facilities (MRFs), which sort the recycling into different materials. They are not accepted because plant pots are made of a type of plastic that the MRFs don’t have a recycling outlet for, i.e. there’s nowhere for the MRFs to send them to where they can economically be made into new products. Some garden centres will take back previously used plant pots for re-use. Alternatively please include them in the general waste bin.