Competitive Interactions and Rhizome Reproductive Capacity of an Invasive Plant, Garden Loosestrife (Lysimachia vulgaris L.)

Abstract

Garden loosestrife (Lysimachia vulgaris L.) is an invasive wetland plant that has spread throughout King County and Washington State. While garden loosestrife populations are limited, it is of concern to land managers because it is difficult to distinguish among other vegetation when not it bloom and it spreads quickly and aggressively through vegetative reproduction. A trial was conducted to determine the ability of a vigorous native perennial species, small-fruited bulrush (Scirpus microcarpus J. Presl & C. Presl), to compete with garden loosestrife. Small-fruited bulrush growth was not negatively impacted by garden loosestrife. Conversely, garden loosestrife shoot weight, root weight, and total weight were reduced by small-fruited bulrush, although its vegetative rhizome growth was not affected by competition. Since rhizome growth is the primary method by which garden loosestrife colonizes new sites, a second study tested three rhizome segment sizes (1, 2, and 5 cm) transplanted at three depths (0, 4 and 8 cm) to determine the ability of this species to establish from fragmented rhizomes. These trials were begun either in early- or mid-summer, and plants were allowed to grow for six weeks in each. While shoots were produced by rhizome segments of all lengths, the only rhizomes that produced shoots were those on the soil surface. There were no significant differences in growth based on fragment length, but rhizomes grew more when cut and grown in early-summer versus mid-summer. These findings will allow invasive plant managers to better plan for garden loosestrife control in sensitive wetland and riparian habitats.