3. Purple loosestrife (Lythrum salicaria)
Purple loosestrife was brought to North America by settlers in the early 1800s to plant in their flower gardens. It didn’t take long for this beautiful, but aggressive invader to escape. Today, this hardy perennial can be found degrading wetlands and marches across Canada. Some cultivars of purple loosestrife are available to buy at garden centres and some wildflower seed mixes contain purple loosestrife seeds, so make sure you check the labels before you buy. Some garden varieties of this invasive plant can cross pollinate with purple loosestrife producing viable seeds that will continue the invasion.