Not all the beauty at the University of Minnesota Landscape Arboretum sprawls out on the ground — sometimes, to soak it all in, visitors actually need to look up. Since its founding in 1958, the Arboretum has blossomed into a home to nationally recognized tree collections. Over the decades, experts at the Arboretum have refined their approach to selecting, planting and maintaining the health of countless trees — all while advancing research on tree health in a changing climate.
Expertly curated and cared for
“Historically, the Arboretum has showcased the best plant performers in a northern landscape,” said Nick Kreevich, Curatorial Manager at the Arboretum. When selecting top performers, he and his colleagues are always sure to consider a tree’s bloom period, fall color, cold resistance, and resistance to pests and pathogens.
The Arboretum also seeks to widen diversity in its collections through the collection of wild germplasm and through collaboration with other botanical or research institutions and the University of Minnesota’s Department of Horticultural Science. Research at the Horticultural Research Center at the Arboretum has led to the introduction of many new ornamental plants proven to withstand Minnesota’s historical climate. These University of Minnesota-developed plants can be found throughout the Arboretum’s collections and identified by their maroon plant tag.
Along with a focus on plant breeding and research, the Arboretum serves as a seed bank to safeguard plant species that are either endangered or on the brink of extinction. The dual-ended aim of the Arboretum’s Curatorial Team is to preserve species suited to Minnesota’s climate and test others for their adaptability.
As the global climate shifts, the Arboretum’s collections are expanding to include plants from regions south of the Upper Midwest. “This could include looking at specific plant species and their known or historic ranges, and for those more southern species, focusing particularly on their northernmost range,” said Kreevich. The approach increases the chances of a plant’s success in Minnesota’s shifting climate because plants that naturally grow near the northern edge of their range are more likely to be better adapted to harsh winter conditions.
“We utilize resources such as botanical literature, UMN Extension and field experts to understand how plants have thrived not only in their natural ranges, but also within a landscape setting,” said Brandon Miller, Assistant Professor in the Department of Horticultural Science and Curator of Plant Collections at the Arboretum. “Our lab’s research focuses on resilient landscape horticulture; we incorporate findings from recent research and what we are learning from our colleagues to inform plant selection and siting.”
In recent years, extended drought has made it difficult to establish new saplings. UMN faculty, researchers, horticulturists and arborists use Geographic Information Systems (GIS), a mapping technology, to assist in monitoring and caring for each planting. GIS technologies coupled with field data can be used to assess factors such as light and soil conditions, the slope of a tree’s location, and a tree’s proximity to other collections and surrounding species diversity.
Visitors can meet Arboretum tree experts, play games, attend presentations and take home a free tree sapling at the Arboretum’s Arbor Day celebration on April 25. Photos by Phil Zumsteg (left) and Sara Pace (right)
Making way for healthy trees
Miller said maintaining a tree’s health involves a dynamic set of considerations, including correct planting depth, root maintenance, proper and routine pruning tactics, avoiding planting co-dominant leaders near one another, and not mulching above a tree’s root flare.
Eliminating unhealthy trees also helps the Arboretum’s tree population thrive. Decaying trees can pose safety hazards, and trees infected with pests or pathogens can harbor disease and spread it to healthy trees. Before removal, Arboretum experts consider the aesthetic impact of clearing a tree, whether the tree provides essential wildlife value, whether any current research projects are tied to the tree and whether its removal would impact educational or interpretive programming.
Kreevich hopes the Arboretum’s tree collections help visitors better understand the value of matching a tree to a landscape. “I would also hope that tree collections help people slow down and recognize species diversity,” he said. “For example, you can take a stroll through the Oak and Nut Collection and come across many species of oaks that have a range of diversity in their leaf shapes, bark, acorns, forms and more.”
From the Arboretum to your own backyard
Tips on planting a tree from Arborist Stephanie Swanson
Choosing the right species
Selecting what to plant is the first step, Swanson said.
Here are three key things to consider when making your selection:
1. The diseases your new tree may have to face.
Identify the pests and pathogens that commonly cause tree loss in your area and avoid planting anything that might be susceptible. UMN researchers have helped develop disease-resistant varieties and hybrids for species affected by oak wilt, Dutch elm disease and chestnut blight.
2. Your yard’s topography.
This includes water levels and sunlight exposure, since these factors greatly influence how fast and how well a tree will grow. Also, if your property has overhead lines that could block tree growth, make sure to plant a tree that can tolerate a lot of pruning over its lifetime.
3. How your new tree will affect your landscape.
The shade, nuts and leaves a tree produces can affect what grows beneath it and will determine seasonal cleanup and maintenance.
Planting your new tree
You can plant a tree anytime the ground isn’t frozen, but spring and fall are best. Fall is ideal because trees expend less energy on growth and flowering, reducing transplant shock and helping roots establish before winter.
Planting steps vary depending on whether a tree is container-grown, balled-and-burlapped or bare-root. But in general, Swanson recommends digging a hole twice the width of the root ball to loosen compacted soil and help new fine roots, which are crucial for water uptake, grow. The hole should be no deeper than the root ball.
Next, untangle and remove any roots that could grow to strangle, or girdle, the tree’s trunk, cutting off the water supply. Then, locate the root flare and ensure the first major root sits less
than an inch below the soil surface. If a tree is planted too deeply, its roots can girdle the tree. “Container-grown trees are especially prone to this,” said Swanson.
Lastly, fill in the soil and water it right away! This helps the soil settle, removing air pockets.
Gardener Danielle Foerster and former Curator of Plant Collections Dave Stephenson plant a poplar sapling. Photo by Nick Kreevich
This story was originally published in Arboretum Magazine, a quarterly magazine for members of the University of Minnesota Landscape Arboretum. View the full story
Cover photo by Michael Borg