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 Secondary invasions in which nontarget invaders expand following eradication of a target invader commonly occur in habitats with multiple invasive plant species and can prevent recovery of native communities. However, the dynamics and mechanisms of secondary invasion remain unclear. Here, we conducted a common garden experiment to test underlying mechanisms of secondary invasion for 14 nontarget invaders after biological control of Ambrosia artemisiifolia in two consecutive years. We found secondary invasion for all tested nontarget invaders, but secondary invasiveness (change relative to natives) varied with species and time. Specifically, secondary invasiveness depended most strongly on phylogenetic relatedness between the target and nontarget invaders in the first year with closely related nontarget invaders being most invasive. By contrast, secondary invasiveness in the second year was mostly driven by functional traits with taller nontarget invaders or those with higher specific leaf area, or smaller seeds especially invasive. Our study indicates that secondary invasion is likely to occur wherever other invasive plants co-occur with an invasive species targeted for control. Furthermore, the most problematic invaders will initially be species closely related to the target invader but then species with rapid growth and high reproduction are most likely to be more aggressive secondary invaders.