Eurasian Milfoil, Gross Yet Useful

It’s nice when something gross turns out to be useful (like how sheep sweat, lanolin, makes chewing gum softer). So I’m happy to report, in answer to a reader’s question some time ago, that, yes, Eurasian watermilfoil harvested from area lakes can be used in helpful ways, including as a soil amendment, fertilizer and even mulch.

First, though, if you’re not sure what Eurasian milfoil is, it’s that stringy, slimy plant that those big, blue boat harvesters remove from the lakes every summer. First detected in Minnesota in Lake Minnetonka in 1987, it is an invasive aquatic species that has spread to waterways across the state. The plant produces thick mats on the surface of the water and tangled stems and masses below, making it difficult, if not impossible, to swim and boat enjoyably. It can also disturb aquatic ecosystems by displacing native aquatic plants.

Milfoil first invaded our waters in 1987.

Acres of watermilfoil are removed from Minneapolis’ Chain of Lakes annually by the Minneapolis Park and Recreation Board’s mechanical harvesters. The machines usually cut off the top 3 to 4 feel of the plant, which can grow up to 15-feet long (super spooky to swim through). If you walk the paths around the lakes, you’ve probably seen (and smelled) piles of it onshore or at the boat launch.

Too Much Trash

Rachael Crabb, the Park Board’s water resources supervisor, says that once the milfoil is harvested, it’s dumped in small piles in a designated spot to dry out. Though terribly unwieldy when wet, milfoil is much easier to handle once it shrinks and composts down a bit. Until recently, the Park Board stored drying milfoil at a site at Fort Snelling. This worked well for the forestry division because whenever they needed to add organic matter to a soil mix, they could just take some from the piles.

Milfoil harvesters at Lake Harriet.

The only problem, says Crabb, was—trash. In the lakes, all of that tangled milfoil acts like a magnet for all manner of garbage that winds up in the water in one way or another. Trying to separate the trash from the drying piles was time consuming and inefficient. So, last summer, the Park Board started looking at other options and found that the Minnesota Landscape Arboretum was happy to take the milfoil off their hands. “It’s a trek out there, but it’s the best option we have right now,” Crabb explains, adding that the first delivery is scheduled for later this year.

Soil Building and Mulching

As it happens, the arboretum has been using milfoil from the Lake Minnetonka Water Conservation District as a soil amendment for about a decade. “After it dries and turns into a nice compost, we usually use a manure spreader to put it on the research fields in November,” Peter Moe, director of operations and research, told me. In addition to supplying a bit of nitrogen, the milfoil makes a good amendment because the organic matter helps loosen the arboretum’s clay soil, improving its water-holding capacity and fertility.

In addition to being a good source of free organic matter, Moe says milfoil harvested from area lakes is desirable because it doesn’t have any weed seeds in it. “It’s such a valuable material, even if there is some trash to deal with, we would be very happy to have whatever the Park Board delivers,” he explains. “Our soils here in Carver County are very difficult to plant in if you don’t have enough organic matter, especially in the spring.”

It’s not a pretty sight, but Eurasian milfoil can be used as garden mulch (if you don’t mind the fishy smell as it dries). While researching this story, I talked with several gardeners who’ve tried it successfully. But here’s the thing, because it is an invasive species, it’s not a good idea for all of us to go running down to the lake to collect Eurasian milfoil for our gardens. Seeds and fragments could easily end up in waterways and spread the problem. In fact, Crabb pointed out that the interconnected storm sewer network could even transport milfoil to lakes that aren’t yet infested with it.

Additional Benefits

While it would of course have been best to keep Eurasian milfoil out of our waters, now that it’s here, it has proven to offer a few benefits. Even where there are thick milfoil mats on the surface of the lake, there are areas where native plants with low light requirements can grow intermixed with the milfoil stems below. Eurasian milfoil also helps make lakes clearer by keeping sediment settled out and using up dissolved phosphorous that would otherwise become food for algae.

And although native plant communities are always preferable to an abundance of Eurasian milfoil, thick milfoil growth can provide good habitat for young fish, as well as protection from large predators. “Milfoil has a more difficult time dominating where a diverse community of native aquatic plants is growing,” says Crabb, who, as part of her job, does aquatic plant surveys of Minneapolis Park and Recreation Board lakes. “We found 10 or 15 native species in Lake Calhoun a couple of years ago, so there are still a few areas in the lakes where native plants have been able to thrive, which is really amazing and a good sign that the lake is healthy even though we have Eurasian Milfoil,” she says.