Species that invade natural areas have shorter lag times than other naturalized ornamental species

Time lags between initial introduction and subsequent naturalization of non‐native species are infrequently quantified but are pivotal to address the dynamics of plant invasions. Key outstanding questions are the magnitude of these time lags, how they reflect variation in introduction effort, and whether they tend to be shorter for species known to be invasive. Using ornamental plant nursery catalogues from New Zealand spanning over 130 years, we examined lag times for introduced ornamental plants, contrasting species invasive in natural areas with other naturalized species and accounting for introduction effort. We estimated the date of introduction from the first record in nursery catalogues and calculated the lag time between the first record and the date of naturalization. Introduction effort was estimated using the frequency with which plants were sold across catalogues following introduction. Of the 1395 ornamental species in our nursery catalogue database, 340 (24%) had become naturalized by 2020. Lag times for all naturalized species averaged only 60 years and have not changed significantly over time. Ornamental species that invade natural areas had shorter lag times than other naturalized ornamentals, and this was not explained by the frequency with which a species was listed in nursery catalogues or its life form. New Zealand's world‐leading biosecurity regulations limiting the introduction of non‐native ornamental plants were only implemented in the last 30 years. Given an average 60‐year lag time, this indicates a substantial invasion debt. Therefore, a significant number of species currently grown in private gardens likely have the potential to naturalize and become invasive in the coming decades, particularly those ornamentals that are or have been widely sold. A better understanding of both the historical and contemporary ornamental horticulture market appears essential to disentangle drivers of plant naturalization and their impacts from historical trends and to correctly identify future high‐risk species.