Milfoiled Again: Lessons Learned from one of the Slough’s Least Wanted Plants

Blog post by Thomas Meinzen, Program Associate

Last month, we introduced some of the aquatic plants that green up the Slough in late summer and early fall, capturing carbon dioxide, feeding wildlife, and filtering toxins from our water. While many of these plants have positive effects on the Slough ecosystem, there are some that spell trouble for our urban waterway. Aquatic plants are capable travelers—especially with the help of unwashed boats—and so many have found their way into our watershed from distant continents. By taking root in a new place, these plants often leave behind the predators and competitors that once limited their population. Without these limiting factors, such plants can become invasive species, taking advantage of ample nutrients to grow exponentially and dominate our ecosystem, leaving little space for other species.

Unfortunately, the Slough is home to several aggressive aquatic invasive species, including the infamous Ludwigia at Smith and Bybee Wetlands, parrot’s-feather milfoil, white water lily, and curlyleaf pondweed. One of the most daunting invaders is a feathery plant called Eurasian watermilfoil (Myriophyllum spicatum).

You might recognize Eurasian watermilfoil from aquariums, where its thick, feathery leaves and long stems were once used a decorative addition. Unfortunately, Eurasian milfoil is a remarkable survivor. If aquarium plants are flushed down the drain or—worse—dumped in a local pond or slough, milfoil can quickly take over, creating dense mats of vegetation which affect recreation, water conveyance, fisheries, property values, and more.  

Alex Staunch, photo courtesy Mosaic Ecology.

To learn about this problematic plant, I spoke with Alex Staunch, operations manager at Mosaic Ecology in Portland and an invasive aquatic plant expert. Alex has surveyed and studied Eurasian milfoil in the watershed, finding it to be one of the most widespread invasive aquatic plant in the Middle and Upper Slough. While Eurasian milfoil causes problems wherever it invades by outcompeting native species, monopolizing sunlight, and reducing water flow, Alex says that it poses particular problems for the Slough, a waterway designed to move water through and out of the area to reduce the risk of flooding. “Eurasian milfoil impedes water conveyance, meaning that it will actually hold up water and increase the chance of flooding,” Alex explains. “It also clogs the Slough’s ditch and levee system. When the pumps are on, everything gets dragged through, and unfortunately a lot of investment goes into raking this plant out.”

The Slough is but one of thousands of watercourses across the country plagued by Eurasian watermilfoil. Milfoil is such a widespread threat, Alex says, because it’s easily transportable—and really hard to kill.  Fragments of Eurasian milfoil left on boats or machinery can survive for days out of water and start a new population in the next place boaters visit. Milfoil can spread and regrow both from seeds and from tiny fragments of itself, so it’s essential that boaters and paddlers in the Slough and beyond thoroughly clean, drain, and dry their boats between each use. It’s far easier to stop milfoil from entering a lake or waterway by thoroughly cleaning boats, Alex says, than it is to remove the plant once it’s there.

In fact, Alex admits, “It is highly unlikely we will ever remove it from the Columbia Slough. Eurasian milfoil is likely there to stay.” Nonetheless, there are ways, he notes, to help reduce its presence and prevent it from spreading within our watershed.

Although isolated populations of milfoil can be treated through hand pulling, herbicides (in standing water), or bio-controls (for example, the use of milfoil weevils in Blue Lake), reducing the presence of invasive species like Eurasian milfoil across the watershed requires longer-term strategies. One of these is planting trees and shrubs along the Slough’s banks. “Increasing shade by establishing riparian buffers and associated canopy cover would help reduce invasive aquatic plant presence within the Columbia Slough,” reads a 2018 Multnomah County Drainage District report. Trees shade the water, which means milfoil gets less sunlight and has less energy to grow. The increased shade also lowers water temperature, promoting the health of native aquatic vegetation. Furthermore, tree roots intercept stormwater runoff and take up nutrients that might otherwise flow into the water and feed milfoil. Cooler, cleaner water gives milfoil just enough trouble that native and beneficial species have a fighting chance.

Unfortunately, large stretches of the Slough have minimal vegetation or tree cover on their banks. These areas are vulnerable to high amounts of stormwater runoff and higher water temperature, and they can quickly fill with dense clumps of milfoil and other invasive aquatic vegetation. The good news is that even a narrow riparian buffer of trees and shrubs along the Slough can help (learn more about riparian habitat in our Slough School vidoes!). Multiple agencies and organizations, including the City of Portland Bureau of Environmental Services, Multnomah County Drainage District, and the Watershed Council are working to increase canopy cover and healthy riparian buffers along the Slough. As trees and shrubs grow again on the Slough’s banks, clearer water and healthier fish populations can return.

Paddling past Eurasian milfoil on a sunny afternoon in the Middle Slough.

Much of the Slough passes through commercial and industrial lands where nutrient and chemical runoff can seriously impact water quality. Vegetation on the land and along the water plays an important role in intercepting and capturing that runoff before it can get into the Slough. Not only do natural areas buffer pollution, they also provide critical refuge for wildlife amid the pavement and buildings, not to mention providing a place for people working in the area to relax and recharge during breaks.

In an effort to address watermilfoil and make these industrial zones as healthy as can be, we are working to connect with businesses (take our watershed business survey here!) to increase riparian plantings and provide environmental benefits to the people who work in the watershed.

Next time you visit the Slough, keep an eye out for aquatic plants like Eurasian milfoil, and notice how the aquatic vegetation changes in stretches with less direct sunlight. Can you tell where trees and shrubs along the banks—especially along the south side—help keep the water cooler, and, over time, clearer? And if you’re bringing your kayak, canoe, or another boat out on the water, please make sure to clean, drain, and dry it carefully before and afterward! Thank you for helping keep our Slough safe from the next aquatic invasive!