We investigated the impact of arbuscular mycorrhizae fungi on common cattails (Typha latifolia), a ubiquitous wetland plant species. The mycorrhizal relationship, which involves the exchange of fungal-acquired nutrients and plant-produced carbon, has been shown to elicit a range of physiological and biochemical responses in host plants. Growth, photosynthetic activity, biomass accumulation, and nutrition were compared between seedlings inoculated with viable mycorrhizal fungal spores and control seedlings given a sterilized inoculum. Plants were grown in inundated soils at three levels of phosphorous availability under glasshouse conditions for 11 weeks. The presence of arbuscules and hyphae was confirmed in all inoculated plants with levels of infection reaching 23.1 and 34.1%, respectively. Control plants were taller and had greater above- and below-ground biomass. Photosynthetic rates measured at week 11 of the experiment were significantly higher in mycorrhizal plants than in control plants. Mycorrhizal plants had higher concentrations of N, P, and C in their shoots and higher concentrations of N in their roots. Our results suggest that under greenhouse conditions, the fungus acts to reduce host plant growth despite increased mineral nutrition and photosynthetic activity.