Arboretum Tree-Care Knowledge Goes to Africa to Save a Threatened Tree
High in a rocky, scrubby upland in southeastern Africa, an evergreen tree species driven to the edge of extinction by logging and fire is struggling to survive in patches of forest. Replanting, guided by the expertise of The Morton Arboretum, may be a key to its future.
The Mulanje cedar (Widdringtonia whytei) is critically endangered; one study estimated that only about 50 individuals are left growing in the wild. In recent years, foresters from the government of Malawi and a local conservation trust have planted hundreds of thousands of seedlings in the windswept, ravine-creased highlands of the Mulanje Mountain Forest Reserve, more than 6,000 feet above sea level.
But many of those trees are dying. “They seem to have trouble after they’ve been growing for a few years,” said Kurt Dreisilker, director of natural resources and collections horticulture at The Morton Arboretum.
He was one of a team invited to Malawi in 2019 through an initiative hosted by Botanic Gardens Conservation International (BGCI) to assess the replanting program and investigate why so many of the seedlings fail. Some causes were known: Illegal logging is common and, in one of the poorest countries in Africa, local people often set fires to clear land to look for fallen logs or drive game for hunting. Other wildfires start from cooking fires.
Scientists on the BCGI team were looking at soil and other factors. Dreisilker spotted something else: The seedlings were not being planted the way the Arboretum plants trees.
“Don’t plant trees too deep” is fundamental Arboretum advice, founded on decades of research and practical experience. Tree roots naturally spread out horizontally, just under the soil surface, where water and air can easily percolate down to them. If the roots are buried too far down, they can’t get air, and the tree will struggle and likely fail.
When the Arboretum’s horticulture staff plants a tree in its on-site collections, or when volunteers guided by its Chicago Region Trees Initiative plant trees in parks and parkways, they follow the same rules: Plant the tree so the root flare—the place where the tree’s trunk widens out into its roots—sits right at the soil surface, and the roots can grow just a few inches down. Make sure the roots spread out and don’t just circle inside the planting hole.
In Malawi, Dreisilker noticed, the standard practice was to plant the cedar seedlings in holes dug too deep, where the soil ended up settling and burying their root flares. So on a second visit in 2022, he suggested that local tree planters try it the Arboretum way, with the seedlings’ roots just below the surface.
The government’s forestry department was skeptical, but agreed to test the idea with 5,000 trees. Dreisilker conducted an impromptu tree planting class for the forestry crews, with a native Chichewa language speaker translating. “Even for people without horticultural training, it’s not hard to learn,” he said. “I was explaining the concept sentence by sentence, and even with an interpreter, I could see them getting it. They were leaning in.”
At the same time, the tree planters were also testing another idea that Dreisilker had suggested to help seedlings withstand wildfires: Planting them into the protection of shrubby thickets, rather than exposed areas.
Heartbreakingly, when he returned in 2024, many of the seedlings planted in the first trial had been destroyed by wildfires. Just being on Mount Mulanje was hazardous for the team. “We had fire charging after us,” he said. “There were close calls.”
Undaunted, Dreisilker worked with partners in Malawi to hold a workshop about the planting methods. As a result, in 2025, two nonprofits, the Mount Mulanje Conservation Trust and WeForest, committed to using the not-too-deep technique to plant 167,000 Mulanje cedar seedlings. Dreisilker’s team has also set up a small-scale trial to directly compare the growth of seedlings planted with this technique to that of trees planted deeply according to the government’s standard practice. The results may inform whether that standard is changed.
If a change helps more seedlings survive through both the dry and rainy seasons, it could make all the difference for this threatened species of trees.
Dreisilker also believes his experience on Mount Mulanje holds a larger lesson: that basic horticultural knowledge needs to be part of conservation efforts everywhere, not just in Malawi. He is working with the Arboretum’s Global Tree Conservation Program to find ways to spread the kind of good tree care advice anyone can get from the Arboretum’s Plant Clinic.
“There are so many tree-planting initiatives in the world today,” Dreisilker said. “My question is, are the trees planted well? Are they getting the proper care? What good does planting do if the tree dies in a couple of years?”
This is one example of the power of the broad, deep expertise that The Morton Arboretum has developed over a century. Combining its knowledge of tree care, science, and conservation gives the Arboretum a unique strength to address tree extinction, climate change, and other challenges facing trees. As a private, nonprofit organization, the Arboretum depends on support from its generous donors to spread that expertise around the world for the public good.
As Dreisilker puts it, “Proper tree horticulture, as we’ve been investigating it and practicing it at the Arboretum for more than 100 years, is as important in global tree conservation as it is in your neighborhood.”
