What could be bad about a tall, tough, deer-resistant perennial that sports bright purple-pink flowers in mid- to late summer, doesn’t flop and attracts butterflies?
Plenty, if that plant is purple loosestrife (Lythrum salicaria) or any of its hybrids. L. salicaria was introduced to this country in the early 19th century and found favor with gardeners and beekeepers for its ease of culture and long flowering period.
In the mid-1950s, plant breeders crossed Lythrum salicaria with L. virginianum and L. alata. The resulting cultivars included: ‘Morden Pink,’ ‘Morden Gleam,’ ‘Morden Rose,’ ‘Dropmore Purple,’ ‘Pink Spires,’ ‘Purple Spires’ and ‘Rose Queen.’ Meanwhile, loosestrife was colonizing wetlands throughout the country due to its prodigious seed production and vegetative vigor. For decades, highly reputable nurseries were selling lythrum cultivars, declaring them sterile and avowing they would not contribute to the demise of our native cattails, sedges and bulrushes.
In the 1990s, horticultural scientists did research on the many lythrum cultivars being sold and found that they were fertile, producing viable seed and fertile progeny. Since that time, 26 states have classified Lythrum salicaria as a noxious weed. Many have included its hybrids in that category and five have prohibited it altogether. In Pennsylvania, lythrum and all of its cultivars are classified as noxious weeds.
Loosestrife crowds out native plants that wildlife utilizes for food, nesting and cover. Many of the cattail plants muskrats used to build their homes have been replaced by lythrum. Waterfowl, especially ducks, avoid wetlands where it predominates. Songbirds do not eat its seed.
Waterways colonized with lythrum have reduced water flow due to the plant’s dense root system and lush foliage. This process promotes the deposition of silt, degrading water quality and necessitating the dredging and cleaning of drainage ditches. Shorelines clogged with it reduce access to hunters and fisherman.
Lythrum is found in many traditional perennial borders and has been a pass-along plant for generations. However, the presence of vast purple swaths of the plant lining our waterways and filling our wetlands should be the impetus for responsible gardeners to dig up the plant and discard it.
The Pennsylvania Department of Agriculture recommends that lythrum no longer be planted. It is best controlled by digging it out and hand-pulling seedlings. If you are utilizing an herbicide, use only those approved for aquatic areas. Before applying any pesticide to Pennsylvania waters, a permit is required, issued jointly by the Pennsylvania Fish and Boat Commission and the Department of Environmental Protection. Contact the nearest regional office of either agency for more information.
Several excellent plants fill the void left behind. If you’re growing lythrum for its spiky flowers, consider Liatris spicata or gayfeather, Veronica longifolia ‘Pink Eveline’ or Veronicastrum virginicum ‘Pink Glow’. The veronicastrum will need a bit of shade, but the liatris and veronica thrive in full sun.
Plants with similar flower color but different form include swamp milkweed (Asclepias incarnata), Joe-pye weed (Eupatorium purpureum) and queen-of-the-prairie (Filipendula rubra). If you’re looking for a tall, elegant perennial, try Thalictrum rochebrunianum ‘Lavender Mist’. It tops out at a stately 6-8 feet tall, but its foliage is delicate and airy. It looks terrific paired with phlox, daylilies, monarda and coneflower and it blooms for 6-8 weeks in midsummer. All of these plants will enhance your garden and you won’t have to worry about harboring a noxious weed.
Carol Papas is a Penn State master gardener. Columns by master gardeners sometimes appear in place of the Garden Q&A by Sandy Feather, a Penn State educator.