Scrutiny president Andy Sloan criticised the States' voluntary in-house ban on glyphosate, which it adopted a few years ago, despite the herbicide remaining in legal use for commercial purposes.
He claimed that change was made 'before workable alternatives were in place' and had led to 'widespread weed growth and general unsightliness of the public sphere' which was concerning locals and putting off visitors.
But the move was defended on pollution grounds by Environment & Infrastructure president Adrian Gabriel.
'Glyphosate is a scourge, and colleagues at the States Trading Supervisory Board, who are responsible for Guernsey Water, would confirm that,' said Deputy Gabriel.
'Expenditure on getting rid of it out of our water system is significant. I and my committee back the voluntary ban by States Works on putting glyphosate everywhere because our water catchment area is nearly everywhere.'
Complaints to parishes about weed growth had increased over the past 12 months, according to St Sampson's senior constable Adrian Dilcock. He said that he found it bizarre that he could hire private contractors, but not the States, to use glyphosate to keep public spaces clear.
The States' decision to discontinue the use of glyphosate coincided with real-terms reductions in money spent tackling roadside weeds.
'All the parishes are aware that E&I is not clearing roads like it used to,' said Mr Dilcock.
'We recently had flooding on Commercial Road and we had two residents basically pulling weeds out of the drains to stop their houses flooding.
'It is very hard to tell someone to trim their privet hedge when there is a four-foot Buddleia growing out of the drain.'
Deputy Gabriel did not want Guernsey Water to face more costs to clear water storage of the pollution caused by glyphosate.
But Deputy Sloan feared that ending the use of glyphosate prematurely could result in higher maintenance costs in the future.
Guernsey Water's quality risk manager Daffyd Griffiths said the risk of drought increased as more glyphosate entered the island's water storage.
'If a water catchment shows high levels of glyphosate, or any other pesticide, we are unable to collect from it as the current water treatment on-island would not be able to process it to an acceptable level,' he said.
Glyphosate pollution has prevented the use of water from the Vale Pond catchment area, costing the island about 250m. litres a year.
'Being unable to collect water in this way is necessary to protect public health, but reduces our ability to refill our reservoirs and reduces our drought resilience,' said Mr Griffiths.
National Trust president Mike Brown disagreed that the island was looking less tidy.
'Untidiness is subjective - what one person considers an untidy neglected patch of land may be viewed as a beautiful natural habitat for wildlife by another,' he said.
'Glyphosate is a powerful weedkiller which if used indiscriminately, or at all, has the potential to pollute our streams and eventually our water supply.'
The trust was supportive of the States' voluntary ban, he said, but felt that it could go further.
'As an organisation we do not allow the use of pesticides nor herbicides of any type on any of our land,' said Mr Brown.
'Our environment is fragile, it is under increasing pressure from the activities of man, so any assistance that can be provided to help it fight back must be welcomed - so definitely no relaxation of controls.'