Garlic mustard (Alliaria petiolata) is an invasive, understory, biennial herb that is a threat to North American forests. It suppresses native plant growth through allelopathic effects on mycorrhizae, leading to decreases in understory plant diversity. Garlic mustard’s own roots lack arbuscular mycorrhizae, possibly limiting its access to phosphorus. Previous field work by our lab has shown that garlic mustard is limited by phosphorus where it performs poorly. We hypothesized that phosphorus should not be limiting in soils where garlic mustard grows vigorously. A limiting nutrient greenhouse experiment was conducted to test the effects of nitrogen and phosphorus on garlic mustard size in soils from a vigorously invaded forest plot. Garlic mustard was germinated and planted in field soil and watered with reverse osmosis water for three weeks, and then was randomly assigned a treatment group (RO water, Nitrogen, Phosphorus, and Nitrogen and Phosphorus). Plants received treatments twice a week and RO water at the end of each week for four weeks. The length of the third emerged leaf was measured before treatments began, and the length of the fourth emerged leaf was measured at harvest. All shoots and four root systems from each treatment were harvested, dried, and weighed.
A two-way ANOVA showed significant effects of nitrogen on the third emerged leaf size (p = 0.043) and shoot biomass (p = 0.002). These results indicate that garlic mustard is not phosphorus limited, but nitrogen limited, although pre-treatment measurements suggests that plants placed in the nitrogen treatment group were significantly larger by chance. This contrasts with previous observations of decreased plant vigor and survival in soils significantly limited by phosphorus. These data could be useful in identifying invasion-prone areas and could help set priorities for the management of garlic mustard based on a site’s potential to support vigorous garlic mustard populations.