A monitoring programme of collection escapes has been initiated in the domain (92 ha) of the Botanic Garden Meise. First, a nine-year vegetation survey of the (sub)spontaneous vascular plants was carried out from 2002 to 2010; it was followed by a systematic monitoring of the escaped neophytes during 5 years from 2012 on, recording several parameters such as their number and persistence. Up to 2014, a number of 202 species of collection escapes has been observed, which amounts to 2.5% of the taxa in open air collections. Of these, 57 species have not been reported before as a neophyte in Belgium. Some neophytes with invasive tendencies appear to come from outside sources rather than from the plants in the collections. A recent spreading of several thermophilic species has also been observed, some of which subsist long after the species have disappeared from the collections. Most of the escapes are ephemeral but some display strong invasive behaviour. The botanic collections seem to favour the appearance of escapes in some cases, such as the co-occurrence of both sexes of dioecious plants, and also by the presence of cultivated native species coming from colder areas than the local representatives. The hybridization and back-crossing of cultivated taxa with native species is an ongoing process which appears to occur simultaneously both in the botanic garden and on outside locations.