loosest

n. (Bot.) (a) The name of several species of plants of the genus Lysimachia, having small star-shaped flowers, usually of a yellow color. (b) Any species of the genus Lythrum, having purple, or, in some species, crimson flowers. Gray. [ 1913 Webster ]

False loosestrife, a plant of the genus Ludwigia, which includes several species, most of which are found in the United States. --
Tufted loosestrife, the plant Lysimachia thyrsiflora, found in the northern parts of the United States and in Europe. Gray. [ 1913 Webster ]