The release of genetically engineered trees into the open environment is an internationally disputed issue. Using genetically engineered poplars as an example, the following report gives clear evidence that it is not possible to adequately control the risks associated with the release of such trees, either in terms of space or time. Poplars reproduce via pollen and seed as well as vegetatively via shoots, with seed and pollen capable of spreading over very long distances. Even single events resulting from long distance distribution can be of major biological relevance, as individual trees can produce millions of pollen and seeds every year. In future, it is expected that the consequences of long distance distribution will increase significantly because of climate change.