Cities and municipalities often rely on contractors for upkeeping infrastructure and public spaces. In this paper, we investigate a novel crowdsourcing approach as an alternative. As a case study, we look at invasive plant detection and removal from public spaces with an app called Crowdsorsa, a gamified location-based crowdsourcing platform. Through a field study in northern Europe, we demonstrate that Crowdsorsa reaches multiple critically important goals: (1) it is a cost-effective solution for cities to handle certain maintenance tasks; (2) it provides citizens an opportunity to gain work experience; (3) it provides workers a low-threshold way to earn additional income; (4) it engages citizens to care for public spaces; and (5) it teaches workers about the environment they live in; among others. With these promising findings, we encourage more research into the use of crowdsourcing for increasingly many tasks in cities and municipalities.