Green areas perform many ecosystem services for increasing and supporting the quality of urban environment. Thus, most factors of people’s welfare depend on and are defined by preservation of greeneries, especially in urban areas with their dense population and intensive anthropogenic impact that create increased ecological risk. Risk is a qualitative or quantitative estimation of a hazard linked to undesirable consequences and losses. Risk estimation consists of several stages: risk identification, analysis, assessment. The paper is based on long-term field studies of green areas in several dozen settlements (predominantly urban). The field data was recorded in standard geobotanical descriptions and forms as well as in author’s specially developed ecological assessment charts and defect lists. The main reasons, tendencies and factors of degradation and destruction of green areas were analyzed. A green area’s inability to fulfill its functions was estimated as the main risk. Nine event groups or secondary risks were described in the structure of the main risk: destruction and loss of species diversity, its substitution, simplification of vertical and horizontal structure, diseases, infliction of harm, inhibition and loss of decorative value. Classification and estimation of the risks are proposed, the main factors that cause a green area’s inability to fulfill its ecological functions are described: construction works, works for housing and public utilities, car runovers, recreational activities, trampling down, damaging and littering. The calculation of probabilities for the risk factors was carried out and their patterns were defined.