By Sherri Roberts, PAg, Crops Extension Specialist, Weyburn
June 2022
At first glance, these two plants look very similar, but there is a big difference in their impact on the Saskatchewan environment. Purple loosestrife, originally a horticulture plant, is an invasive weed that prefers moist soil, has invaded wetlands, marshlands and waterways to the point of choking out natural vegetation. Fireweed is a native wildflower that inhabits drier upland areas and is often mistaken for purple loosestrife. This article will cover some of the simple ways to differentiate between these two species.
Flowers
Purple loosestrife flowers are attached directly to the stem and will have five to seven reddish-purple to magenta-coloured petals surrounding a small yellow centre.
Fireweed flowers are borne at the end of a long tubular structure with a short stalk and will have four magenta to pink petals and four narrower pink sepals behind the petals. The style will have four stigmas.
Stems
Purple loosestrife stems are angular with five or six sides and are hairless to slightly hairy.
Fireweed stems can also be reddish in colour but the stem is smooth and round.
Leaves
Purple loosestrife leaves are attached opposite from one another and directly to the stem. Lobes at the base wrap past the stem. Veins go all the way to the edge of the leaf.
Fireweed leaves are borne alternately on the stem and do not have basal lobes that go beyond their attachment point. Veins in fireweed leaves loop back in on themselves to create a continuous vein that runs parallel to the margins to the leaf.