Written by MNWR Aquatic Biologist Dominic Bachman/Photo by Dominic Bachman
On August 4th, 2025 a patch of invasive Purple Loosestrife (Lythrum salicaria) plants were discovered at the mouth of the Blitzen River where it meets the Malheur Lake wetlands. These plants were in about 3.5 feet of water. It is estimated to be about ¼ acres worth of plants ~180 individual plants. This is the very first time this invasive weed has been seen on the Refuge. Based on the size of the plants, there likely were some plants in this site last year that just didn’t get noticed. It is assumed that at least some of the plants sprayed has already produced some seeds. This invasive weed is known to occur in Harney County near Drewsy, Oregon, in Stinking Water Creek and and along the Malheur River in a few locations. As soon as the discovery occurred, the Refuge created a Pesticide Use Proposal for overwater spraying PUP Number R1-25-13570-005 and contracted with Harney County weed control specialists to treat the plants.
During late August we completed 15 hours of transects and reconnaissance looking for other locations with flowering loosestrife. We also floated the Blitzen River upstream for 15 miles looking for other occurrences. None were found. On 8/28/25 the County came out and sprayed the known plants from an airboat. We treated these plants with Garlon 3A applied at six quarts to the acre. On 9/18/25 we spot sprayed about twenty plants that were either resprouts or missed plants that became obvious after the other plants died from initial spraying.
In 2026 we will need to be vigilant regarding this aggressive species. We plan to do biweekly surveys starting in June in the known invasion area and conduct reconnaissance in other areas of Malheur Lake and the Blitzen River. Ideally, we will spray any plants long before they are able to seed. It appears they began to flower in late July and had seeds by mid- to late August.
