In July and August, with funding from the Arboretum Foundation, UW Botanic Gardens horticulture staff worked on a project to restore the lower stream channel connecting the two ponds in the Woodland Garden. The channel dates back to the original Olmsted Brothers design plan for the Arboretum, and it flows over and through carefully sited rocks and boulders. Most of the rockwork was installed in 1938.
In fall 2017, a large sweet gum tree (Liquidambar styraciflua), planted by the lower Woodland Garden pond in 1939, fell during a windstorm and caused extensive damage to the stream channel. Over the decades, the tree’s roots had grown under the channel and among the original rockwork. When the tree fell, the sizable root ball ripped up several large boulders and altered the adjacent stream channel and rockery. As a result, a portion of the stream bank began to erode where the stream entered the pond, adversely affecting the water quality.
Due to the substantial size of the root ball, the sensitive wetland location, and the steepness of the slope on which the tree fell (limiting access to heavy equipment), a relatively complex removal and restoration complex was required. In the fall of 2019, the Arboretum Foundation applied to the Hugh and Jane Ferguson Foundation for a grant to fund permitting, design work, rockery restoration, and restorative landscaping around the lower Woodland Garden pond. Our application was successful, and we received a grant of $8,500.
The hort staff worked on permitting and a restoration plan over winter and spring but had to wait until the stream dried up this summer before working on the downed tree.
“In June, we finalized a plan to tackle the tree removal,” says Curator of Living Collections Ray Larson, “using a platform over the creek to access the site with light equipment and build a silt barrier to keep debris out of the creek. Work began in late July and was completed in stages over three weeks.”
“This month [September], we’ll be selecting and placing additional new stone and cobble to restore the stream channel and eliminate erosion. Next month, we’ll complete the rockery restoration and bring in new soil to back-fill the slope.”
“I selected a new specimen tree, a Persian ironwood (Parrotia persica), to replace the sweet gum, and it will be planted in late October. I chose it for its vibrant and long-lasting fall color, attractive bark in age, ultimate size of 40 feet, and overall shape. Adjacent companion plants will be added after tree planting, with the goal of project completion by December.”
Part of the trunk of the sweet gum will be left in place to serve as a nurse log, providing food and habitat for Arboretum wildlife.
Our thanks to the Ferguson Foundation for their generous grant, which has helped us improve and enhance the local watershed within the Arboretum!
Persian Ironwood: A Tough Tree with Stunning Bark and Fall Foliage, Arboretum Bulletin, Fall 2015.