The extensive and burgeoning literature on the productivity of urban farms and gardens is largely focused on measures of crop yield and resource use, with little offered to date on their contribution to social productivity and sustainability. This paper suggests that evaluation of urban agriculture should consider all types of resource consumption and productivity simultaneously. The research reported here used a citizen science approach to collect data from seven community gardens and one community farm in London, UK in the 2019 and 2020 growing seasons. The paper examines the many variables that impact the sites’ overall performance, highlighting the complex nature and relationship between the many benefits and outcomes of urban farms and gardens. Data are presented on crop yield, equivalent fruit and vegetable portions, input use (including water and fertilizer), journeys made to the garden by volunteers, social benefits, and social outreach. Results show very mixed levels of crop and social productivity, depending on the organizational structure and agenda of the various sites included in the study. With no clear pattern emerging, this paper suggests that the evaluation of citywide productivity, often based on projections of small data samples, may not be reliable. By ensuring that training opportunities for volunteers are made available, higher resource efficiency as well as higher productivity could be attained.