Anchorage Daily News

In the same devious way a black widow entices and then kills her mate, purple loosestrife (Lythrum salicaria) woos with attractive spikes of pink-purple petals splayed around an amber center while slowly strangling and poisoning other native garden beauties.

Be warned: Wildflower seed mixes frequently include this hardy perennial, and less-sophisticated gardeners mistake it for noninvasive fireweed, swamp loosestrife, blue vervain and winged loosestrife.

THE OFFENDER: Purple loosestrife enjoys a fairly long growing season. Its square, woody stems rise 4 to 10 feet high with pairs of lance- or heart-shaped leaves alternating down the stalk at 90-degree angles.

The many individual flowers offer a bountiful nectar source for insects upon which the plants depend for pollination. A single rootstock on a mature plant can sprout as many as 50 flowering stems that produce 2 million to 3 million seeds per year. To make matters worse, water, wind, wildlife and humans easily disperse the seeds, which germinate the following season or lay dormant for several years before sprouting.

Purple loosestrife also readily reproduces through underground stems at about 1 foot per year. And don't be fooled by those "Guaranteed sterile" cultivars at local garden centers. They're actually highly fertile and cross freely with purple loosestrife as well as other native lythrum species.

As a medicinal herb, however, this invasive plant treats diarrhea, dysentery, bleeding, wounds, ulcers and sores.

NOXIOUS NOTORIETY: The highly invasive nature of purple loosestrife allows it to form dense, homogeneous stands that choke native plant life and limit waterfowl habitat.

PREVENTING THE SPREAD: Catch this pest while still in flower -- right about now. As flower petals start to drop, capsules of seeds, tiny as grains of sand, appear. Once this phase commences, remove the flowering spikes first by bending them over a plastic bag and cutting them off. By all means, avoid shaking seeds from the plant.

Watch drainage ditches or streams leading from heavily infested areas, as new purple loosestrife colonies are likely to become established there.

MORE FLOWER FOR YOUR BOWER: Most garden centers and seed distribution companies have voluntarily ceased selling purple loosestrife and its cultivars. Instead, they offer such environmentally friendly perennials as Blazing Star, Gay Feather, delphinium, false foxglove, lupine, lobelia, obedient plant, salvia, Siberian iris and spike speedwell.

TRASH AND BURN: With small garden infestations, pull, cut, mow or dig the plants and then double-bag in plastic. Larger infestations require more muscle and a glyphosate-type herbicide like Roundup. Burn the bundles or transport to the landfill. Avoid composting since seeds are still viable, and the thick, woody stems and roots decompose slowly.

Remember to thoroughly brush off clothes and equipment to prevent small seeds traveling to new areas.

BIG WILD WORRIES: For more information about purple loosestrife or other noxious weeds, log on to the Integrated Pest Management program at www.uaf.edu/ces/ipm or www.uaf.edu/ces/cnipm, call 786-6300 to report a sighting or drop off a sample at 2221 E. Northern Lights Blvd., Suite 118. Download a pocket Alaska weed guide at www.alaskainvasives.org.

J. Mark Dudick is a communications specialist at the UAF Cooperative Extension Service in Anchorage.

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