Effects of various manual removal methods on seed production of Alliaria petiolata

Background/Question/Methods   Garlic mustard (Alliaria petiolata) invasions are a threat to the diversity of understory forest communities. Manual removal is one of several approaches to eradication of targeted A. petiolata populations. Because of A. petiolata's reputed ability to set seed even after plants have been uprooted, recommendations for manual removal suggest that plants be bagged and removed from the site. We tested the hypothesis that A. petiolata can set seed after uprooted, and examined seed set associated with a variety of factors related to manual removal. In particular, we evaluated seed set of uprooted plants at three stages of phenology (flowering, early fruiting, late fruiting), two stages of root removal (removed, intact), and three treatments (hang, scatter, and pile). Seeds collected from treated plants were counted and then assessed for viability. An additional side study was conducted to determine if taproots left in the ground could resprout after removing all above ground vegetation. Groups of plants treated at four different phenological stages were monitored for regrowth. Data were analyzed using an interaction ANOVA.

Results/Conclusions   We found significant differences among treatments in seed set. For instance. plants receiving the hanging treatment produced a mean of 13.78 seeds per plant, significantly (P = 0.006705) less than plants receiving the scattering treatment (39.35 seeds per plant). Across treatments, removing taproots from pulled plants did not lower seed production. However, we did find that treatment type interacted with both phenological stage (P = 0.045471) and root removal (P = 0.010634) to produce a significant effect on seed production. The interaction between root removal and phenological stage did not have a significant effect on seed production. While manual removal techniques are impractical for large infestations of A. petiolata, these methods are useful for controllingl spread to neighboring areas by eliminating satellite populations. Results of our study have several important implications for managers. First, we found that plants pulled at any time before the initial stages of flowering are unable to produce viable seeds. These plants can be left on site and do not need to be bagged for disposal. Second, we found that A. petiolata taproots left in the ground after pulling were not able to resprout, flower, and set seed in the same growing season. Finally, our work suggests that once A. petiolata has reached the early fruiting stage (~50% of the inflorescence developing fruit), all pulled plants should be bagged and removed from the site.