Little comparative research has specifically used farmers’ understandings of agricultural weeds and herbicides usage as important indicators of their environmental decision making and behaviours. This paper proposes that “organic’ farmers”, already attuned to environmental ideas, may be more likely to have favourable understandings and behaviours towards agricultural weeds as an integral part of environmentally sustainable agricultural farming systems than “conventional” farmers. Using a behavioural approach, the ways in which farmers’ (situated in central-southern England) understandings influence their environmental behaviours were examined. Most “conventional” farmers’ fields were kept relatively weed-free through herbicide usage. This contrasted with “organic” farmers having less concern about removal of weeds (with their associated invertebrates and seeds) which they understood contributes significantly towards biodiversity and agricultural sustainability. A remarkably high 92 per cent of “organic” farmers were critical of “conventional” farmers’ using herbicides and pesticides, asserting that lack of pesticide and herbicide usage as core reasons for their sustainability. This contrasted with most “conventional” respondents who claim they used as few chemicals as practicable to minimise environmental damage to soil and water, while maintaining adequate crop levels. Nevertheless, such environmental understandings and behaviours may not always be indicative of any differences that may be found between those farmers commonly classified as “organic” and “conventional” in the UK as a whole.