The paper is based on a survey and investigates the functioning of historic gardens during the pandemic. The authors collected and analysed information on the impact of the pandemic on the behaviour of visitors, maintenance, and condition of cultural heritage assets, European historic gardens. Four aspects were considered particularly carefully: the situation of gardens during the COVID-19 pandemic, maintenance and care in gardens, virtual activity and communication, and financial consequences. The authors determined the conditions of the gardens and the problems they faced based on a survey completed by 23 managers of 31 historic gardens from June to August 2020 and then proposed a diagnosis. The paper presents the survey results. In general, visitor volumes tended to drop in 2020, which significantly affected gardens’ financial standing and contributed to workforce reductions. The garden condition and treatments were affected, as well. Reduced visitor volumes resulted in positive environmental changes. Among them were ecological succession, the stability of landscaped plants, increase in vegetation, improved biodiversity in the ground cover, and enhanced animal presence. Additional safety measures were implemented after the gardens were reopened to the public during the pandemic, mostly social distancing, and obligatory face masks. Less than half of the gardens had contingency plans, and 25% of the respondents were working to develop one. The analyses provided foundations to start working on a universal emergency strategy similar to procedures used for years for permanent collections at museums. Note that, being open public spaces and live museums, historic gardens were the first places reopened after the lockdown. Recommendations based on the study can contribute to the future safe functioning of historic gardens in other similar crises. The guidelines offer instructions, advice, and recommendations that form foundations of the development of a universal management model facilitating the preservation of historic gardens in good condition while exploiting their ecological potential.