If you’re wandering along the Bog Boardwalk at Green Heron Pond this spring, you might notice large brush piles off in the distance. These are the results of the Natural Resources Team’s hard work this winter, pushing back woody invasives, including European alder (Alnus glutinosa L. Gaertn) and both common and glossy buckthorn (Rhamnus cathartica and Frangula alnus) mixed in.
Unlike buckthorn, which is prevalent throughout the state, invasive European alder populations are a challenge unique to the Arboretum. The tree was intentionally introduced near Green Heron Pond in the ‘60s, where it was likely being evaluated for its cold hardiness. Although the trees were later removed in the ‘70s, they had already matured and spread into the wetland. Just like with many introduced plants, gardeners had no idea the extent to which the alder population would boom. The trees are prolific seed producers whose seeds disperse via wind and water. They also resprout rapidly in response to damage, such as tipping or cutting. Alder does exceptionally well in wet soils, surviving in standing water, and outcompeting native dogwoods and willows.
The boardwalk pre-management. A combination of woody species (primarily alder) overtook the former meadow. Photo by Don Olson
The Minnesota-native speckled alder (Alnus incana) is a valued part of the wetland ecosystem and is often confused with the European alder. It is also likely that the two species have hybridized, which spreads just as rapidly as the invasive and is even harder to tell apart from the native. One of the largest differences is that European alder can grow up to 60 feet tall, double the height of the native species. The extra shade prevents the growth of herbaceous wetland plants, such as asters and goldenrods, as well as the shrub understory. When an area becomes dominated by one species (forming a monoculture), fauna and other flora living there suffer.
Before the plant was managed at the Arboretum, a forest of alder (with some buckthorn sprinkled in) made its way up to the boardwalk’s edge, stifling the area’s biodiversity.
The Natural Resources Team has started restoration to open up the area. Buckthorn and alder are cut down and stumps are treated with herbicide. Without chemical treatment, the stumps can resprout, multiple stems popping up the following year where there was only one before.
The wetland is easiest to access in the winter when the water and muck have frozen, making it more walkable than it is during the summer. Each year, the Natural Resources Team, along with contractors, chips away at the wall of alder and buckthorn. The invasive brush is then piled so that it can dry out and later be burned.
Natural Resources Gardener Danielle Foerster cuts down an alder tree (left), photo by Nina Charlier. Burning of invasive brush piles (right), photo by Sarah Rademacher.
After several years of repeated management, the alder has been successfully pushed several hundred feet back from the boardwalk. The new pockets of sunlight brought a golden reward: the return of the marsh marigolds. “I love to see the marsh marigolds respond to the openings we create. I don't think we can ever have enough of them!” said Sarah Rademacher, the Natural Resources Team Lead, who has been managing the wetland around Green Heron Pond for more than three years.
Pockets of marsh marigolds return after alder and buckthorn removal. Photo by Sarah Rademacher
The hope is that as efforts continue, we will see more species return to the former sedge meadow. Danielle Foerster, a Gardener on the Natural Resources Team, has played a large role in the restoration progress. For her, the work has been extremely rewarding. She’s especially excited for what’s to come: “We can look at the areas we've already done and imagine what our new areas will look like — healthy native vegetation like marigolds, sedges, willows and much more!”
Make sure to visit the boardwalk in May to spot the marsh marigolds and admire the restoration progress.
If you spot European alder (or any other invasive plants), please report it via iNaturalist or EDDMaps to help agencies monitor the plant’s population across the state.
Cover photo by Sarah Rademacher
Nina Charlier has been serving as a Minnesota GreenCorps member at the Arboretum since October 2025. She assists the Natural Resource Management team, plant health specialist and arborists in their day-to-day care of the Arboretum grounds. She holds a B.S. in Ecology, Evolution and Behavior Biology from the University of Minnesota.