A Threat to Kansas’ Rivers, Lakes, and Wetlands Purple Loosestrife A Threat to Kansas’ Rivers, Lakes, and Wetlands Purple loosestrife is quarantined by the Kansas Department of Agriculture and is not available for sale in the nursery trade. A perennial, purple loosestrife is characterized as a profuse seed producer, flowering throughout the summer, making up to 2.7 million seeds per plant. It is distinguished by a square stem and purple flowers that have 5-6 petals. Noxious in neighboring states, purple loosestrife forms dense monocultures, threatening river, lake, and wetland ecosystems.