Although plant invasions are often regarded as a significant threat to global biodiversity, current understanding of the vulnerability of tropical forests to invasion or the factors that lead to alien species becoming invasive in the tropics remains limited. Here, we synthesise available information on plant invasions in protected areas for the most ecologically important montane forests of East Africa. We undertake a hierarchical analysis to explore patterns across the entire mountain chain with those within an individual mountain block down to a single nature reserve. A common feature of the occurrence of alien trees in the Eastern Arc Mountains is the overwhelming importance of propagule pressure in the representation of species found colonising forests. The patterns observed emphasise the need for scientifically sound advice regarding not only the potential impact of an alien species on native biodiversity but also an assessment of which mitigation strategies might be most appropriate and highlights the research, control and social challenges of managing invasive agroforestry trees in the tropics.