Now I have no issue with people who want to avoid synthetic chemical application on their own plot, but a considerable disike of those who seek to impose their view on everyone else. Showing prospective tenants a plot I always check they know about couch, buttercup and bindweed.and the time required to manually remove them, aso that chemical control can be first year only, or individually targeted e.g. bindweed in soft fruit. Emphasis on personal choice and understanding weed control commitment.

We have no specific clauses abut chemicals as it's covered under not being a nuisance. e.g. don't spray on a windy day.
All site agent replies to Town Council consulting us have been along these lines, but I thought I'd research a bit further afield.
Does your site have any restrictions on weedkiller beyond the good practice ones?